Archived

This topic is now archived and is closed to further replies.

AClass USA 230

Curved daggerboards in A-Class

Recommended Posts

I thought I would start this topic for a couple of reasons. First, I enjoyed the C-Class thread started by Steve Clark as it was very informative (and entertaining). It's interesting that Fred Eaton has decided to not try curved daggerboards (at least for now) while Steve is going to commit to trying them on his new yacht.

 

Curved daggerboards are becoming more prevalent in the A-Class. I bought an Ashby Sailcraft ASG3 that was delivered in January (with 5 other US bound boats)and this boat has curved daggerboards. I'll offer the following observations/thoughts for comment:

 

1. In addition to curved daggerboards, the ASG3 has the front beam moved further back than any other production A-Class I am aware of. This change in platform configuration led Glenn Ashby to develop a shorter foot (~1,677mm versus ~1,880 mm typical) and wider head (952 mm versus 775 mm in previous model)sail that in its first iteration did not perform as well as expected. In comparison, the Glaser design used by Lars Guck in the US has an 830 mm wide head, the current design Steve Brewin has been winning with in Australia has a 1,005 mm head. Glenn has now developed a new sail for ASG3 that has a shorter head (around 800 mm) because he believes it is too difficult to get adequate leech twist control with the larger heads. I tend to agree with that observation and believe you cross a threshold between 850-900 mm. Glenn has gotten better results with the new sail.

 

2. In trying to optimize the ASG3, we are dealing with three significant changes: the curved blades, the front beam being further aft, and the shorter foot sail designs. I feel we are making progress in determining what makes this boat go but it has been difficult trying to measure which of the changes contributes or detracts from the boat's performance especially considering the previous Flyer II is considered a very competitive platform.

 

3. In regards to sailing upwind with the curved boards, I don't think there is any significant negative effect on upwind performance on any of the boats using them currently. On the ASG3 it seems like I am competitive with my US fleet upwind in medium to heavy air most of the time and any speed issues are rig/sail related which we are currently working on. In light air upwind, we are making progress and learning that proper mast rake and weight placement seem to be more critical on this boat. In general, we are keeping our weight further forward.

 

4. In regards to sailing downwind with the curved boards, this is more complicated. Initially, I believed I could leave the boards down when wild thinging but what I found was I could not sail as deep doing this. I now believe in marginal wild thing, it may work best to have the leeward board fully down and the windward board raised at least half way. Once you are fully powered, I believe both boards up 1/3 to 1/2 is the right range in terms of competitive performance and handling. In light air with both hulls in the water, it makes sense to me to raise the both boards as much as possible just as we do with straight boards.

 

5. I believe the characteristics of a canted curved board are close to a straight canted board (i.e. canted implies the top of the daggerboard trunk is outboard of centerline on the deck while the bottom of the trunk is on hull centerline). I believe the curved board will have slightly more up force than the straight board in use.

 

6. If the fore/aft pitch axis of the boat could remain constant to where no "pitch" angle of attack is introduced to the daggerboards, I do not believe there is substantially any more drag on a curved or straight canted board versus a staight board (all board sections being symmetrical). Once the pitch axis changes, you should get additional forces generated from curved or canted boards in addition to the forces resulting from the angle of attack from the normal leeway of the boat. Thus, I think weight distribution/placement is more critical on a boat with canted curved or straight boards.

 

7. I did a comparison of the ASG3 to the beam and daggerboard placements of several other competitive boats including the Bimare XJ, Melvin A3, and Marstrom Mk V (attached). One of the observations I make is that the CE of the sail on the ASG3 is behind the CLR (or daggerboard location). This could imply that the daggerboard trunk needs to come back 6-12 inches. This is going to trialed by some European sailors with ASG3's so we are anticipating the results. It is interesting to see how this observation compares to the A3, XJ, and Mark V which have the CE of the sail at or close to the CLR.

 

In addition to the ASG3, the only other A-Class I have sailed with curved daggerboards is the Marstrom Mk V. The feel, handling, and performance of the boat are very good, better currently than the ASG3 in my opinion but I think the ASG3 can be optimized. The new Hall/Cogan Barracuda with curved boards sailed this winter and spring by Ben Hall looks very promising so far and we are looking forward to seeing the performance of the EVO HT with curved blades (sailed by Matt Struble) against what we anticipate will be stock A3's (sailed by Lars Guck and Pete Melvin) at our North American Championship this September. Struble, Melvin, and Guck are the top echelon of our North American class. The tier right below is pretty damn good and the new designs being sailed this year will certainly make it interesting!

 

Hope to hear productive feedback from all but especially from other A-Class sailors currently sailing boats with curved boards for their thoughts.

 

Bob Hodges

A-Class USA 230

ASGS VS A3 VS XJ VS MARSTROM.pdf

Share this post


Link to post
Share on other sites

Wow. Great thread and lots of issues raised.

 

IMO, the biggest issue with the ASG3 is that they changed so much it is hard to be certain of what has had the biggest effect. The sail is certainly an issue. However, I don't think it is the head size that is the problem because somebody has recut the sail, left the head shape the same and is now significantly faster. Also, as you point out, the Brewin is bigger in the head (that's the sail, not Stevie!!) but I cannot agree that you cannot control the leach. In fact, Steve says that it is exactly that twist control that has helped him become more competitive in bigger breeze, dispite being a little lighter. I think that the key to this is different styles of sail. The Ashbey seems to be a lot flatter overall while Steve seems to have gone for a very powerful sail that can be flattened when needed. Flat sails seem to me to be more sensive to leach twist issues It seems his sail have become the weapon of choice for many and I know a fair number of people who have changed to Steve's sail and will not be changing back at the moment. Of course, these things come in cycles so who knows what will happen next year. The only reason for so much comment on a thread about curved boars is to highlight that there are so many variables. The other factor to note is that Steve (and others) are now using a very different style of mast which would also make a big difference on what the leach does. I suspect that it is simply a case of looking at a rig as a complete package.

 

Anyway, to the matter at hand! Firts off, I am convinced that curved boards are the way to go. The problem is doing it right and I am not at all convinced that any are close to having them right yet! I suspect we will see quantum leaps in this area over the next few years.

 

Analysis of this is even harder because the various different curved board set ups are so different. Even simple things such as whether the boards should be symetrical or assymetric are unknowns. When I started studying this 4 months ago, I was convinced the boards has to be assymetric. Now I am thinking that either they should be symnetrical or it doesn't matter!

 

As you righty point out, position in the boat is a critical factor and AFAIK, there has been no sreious testing of this, meaning it is a bit hit and miss at the moment, even if it is being done theorectically by clever people! Look at how they kept changing the foil configuration on the BMWO tri.

 

One question asked of me is whether the boards really can produce worthwhile lift. For me, the most interesting discussion was with Landy who reported that on the short leg from the windward to spreader marks, he was struggling to keep the boat in the water (20knots of breeze). My take in this was that there was a combination of speed and lateral forces in play - as much lateral force as upwind combined with a significant speed rise - and that this led to excess lift. To me, this was encouraging because if one can get past the limits of lift needed, then it must be possible to come up with the right amount!

 

The next question is whether in exchange for the lift, is the drag too much. I get the impression that in some cases, it all equals out, which suggests to me that when we understand it more, we will see worthwhile benefit from curved foils. I think the most encouraging thing is that most don't see a decrease in performance!

 

I think it is a shame that Glenn has made so many alterations to the ASG3 in an attempt to get a competitive platform for the worlds. I believe he has changed a number of boats back to straight boards although I don't know if the foil position has been changed. And as you rightly point out, he has done a lot rig development. This is why i am interested in what you guys in the USA have to say.

 

Your comments on piching, position in the boat etc seem to me to be very relevent and I think this is one of the reasons why Steve Clark is going for a winged rudder. I also think we will end up with a system of being able to alter the lift on the foils as Steve is working on, but maybe without the level of complication!

 

One issue that I don't seem talked about is platform stiffness. With a lifting foil in each hull, I believe that this becomes quite important and I have been checking various boats in this regard. To date, I haven't found one that has the degree of stiffness I would want! That includes the Flyer 1 and 2, ASG3 and Tool. I suspect that one of the reasons some say teh Nikita is so fast is because it looks like it should have a very stiff platform and the new generation of boats coming out of Europe seem to be paying a lot of attention to this. I know that conventional wisdom seems to be that platform stiffness isn't as vital on an A as, say, a Tornado because there are no forestay tension issues, but IMO flex has 2 problems. First, it changes relative foil geometry which isn't great when you are trying to be precise and it can lead to different geometry on different hulls. Secondly, flex takes energy which is usually bad unless it is absorbing "bad energy", such as acting as a shock obsorber to reduce pitching. However, I have personally never sailed in a class where hulls/platforms can be too stiff! Steve Brewin says that the Flyer he won the worlds in was the stiffest platform he has ever sailed, so what does that say!

 

The final issue that needs to be considered is hull shape. As soon as you start to get lift from the foils, I suspect that there are implications for hull shape. Certainly, this is what has been seen in the Int14's. As I see it, as the boats become more pressed they get pushed into the water more and if you don't consider this in your design, then you have issues. However, with curved boards, as the boat get more pressed, they provide lift. Does this mean you don't need to worry as much about the hull being pushed down? And off wind, can curved boards make the boat safer. Wnlets on the rudder would do. This is why i intend to stick with my Flyer 1. It currently has non canted boards (canted hulls)and with good, high aspect ratio foils, there is little between Steve Brewin and myslef in our straight line testing, other than his vastly superior sailing skills which keep the boat moving forward better than I can at all times. I a breeze, it is clearly scarier than the Tools but I am hoping that curved foils will help a lot. I also have some weight to play with so I am looking at how to stiffen the platform as well.

 

I know this is a bit of a brain dump and that I currently don't have a curved foil boat. however, I will do before the start of the new Oz season. The fun is making the right choices! Which foils, where in relation to the beam should they go and do they need to be adjustable?

Share this post


Link to post
Share on other sites

With the sails getting bigger in the head, has anyone considered looking at higher mast heights? Only restriction is sail area in the rules. Don't think you can go that much higher due to the beam, but 0.5m could make a difference with higher hounds.

Share this post


Link to post
Share on other sites

With the sails getting bigger in the head, has anyone considered looking at higher mast heights? Only restriction is sail area in the rules. Don't think you can go that much higher due to the beam, but 0.5m could make a difference with higher hounds.

the mast counts in the total sail area. one guy did have a longer mast at the belmont worlds and had to recut his sail as it did not measure. on the subject of curved boards im not a numbers man but her are the facts i have witnessed over the last couple of years. glenn ashby on the asg3 prototype with strait boards flogs all of us at the 2009 then with curved boards was very slow at the nationals. every one i sailed against this season that has gel-tek curved boards was slower than previous years.Dave Brewer on the other hand sailing a mark two flyer with marstrom curved boards has shown some excellent height and speed. the bottom line for me is one day they might be the way to go but at this point in time they are far from perfect. we shall know more after the world champs coming up

Share this post


Link to post
Share on other sites

Meant was that mast height is not measurement restriction. Sail area is. Mast area does count as part of this, but technically you can go whatever height you want.

Share this post


Link to post
Share on other sites

Meant was that mast height is not measurement restriction. Sail area is. Mast area does count as part of this, but technically you can go whatever height you want.

 

May work, could also increase pitching. Don't know if you don't try it though.

 

Great thread BTW

Share this post


Link to post
Share on other sites

Thanks for the great replies so far. Here are my comments to your responses:

 

1. I agree that perhaps there were too many changes on the ASG3 to expect the boat to be a winner "out of the box" and I question my own logic in proceeding with purchasing one this early. But despite my frustration, I'm enjoying sorting out the details and learning a lot despite the fact that I might spend some more bucks on modifications.

 

2. I believe 80% of the performance issue we have experienced so far (at least for me) has been to find a sail design that works with the platform. Glenn has sent us a sail to try that he believes is a big step in the right direction. He got good results with it recently on an ASG3 with straight boards.

 

3. The curved daggerboards on the ASG3 were by my eye not great quality. While they appear to be strong enough for the job, they were not fair and the LE is too blunt. Lars Guck looked at mine and agreed with the observation that the LE needs work so I'm trying to figure out the best way to do that. By contrast, the Marstrom curved boards are like jewels and I am not surprised that Dave Brewer has gotten good results with them on the Flyer II. In fact, Glenn told me that in heavy air, it seemed like Dave had the best boatspeed of all the top Aussie sailors.

 

4. The curved boards on the Marstrom and Ben Hall's new Barracuda are longer (by about 4") and have more curve than the ASG3 boards but they are not canted in the hulls. The upwind performance of the ASG3 especially in med to heavy air suggests to me that a longer board is not needed.

 

I'll be training in a couple of weeks in Fort Walton Beach with Bob Curry and Randy Smyth. We should get an idea of how Glenn's new sail is going in addition to trying out some new ideas mentioned above.

Share this post


Link to post
Share on other sites

It's interesting that Fred Eaton has decided to not try curved daggerboards (at least for now) while Steve is going to commit to trying them on his new yacht.

 

Fred's first C was Patient Lady VI, which he bought from Steve. She had curved boards - they are even yellow in colour, true banana boards. See this link to an old SA cover for some pics of what happens when curved boards provide a bit too much lift.

 

 

Fred also decided to explore the full foiling C, which resulted in Off Yer Rocker. Clicky here for some stuff on her.

 

One of the things to keep in mind when comparing C and A is the cost of the thing sticking out of the boat. Dumping in an A is slow. Wiping out in a C and damaging the wing can remove you from the regatta and cost you an amount of money that would buy several As and fund a global A campaign for a few years - so the C decisions on boards have an added dimension to the normal L/D calculations, namely: wing_preservation=L/D*$$$^4

 

Cheers,

Giles

Share this post


Link to post
Share on other sites

It's interesting that Fred Eaton has decided to not try curved daggerboards (at least for now) while Steve is going to commit to trying them on his new yacht.

 

Fred's first C was Patient Lady VI, which he bought from Steve. She had curved boards - they are even yellow in colour, true banana boards. See this link to an old SA cover for some pics of what happens when curved boards provide a bit too much lift.

 

 

Fred also decided to explore the full foiling C, which resulted in Off Yer Rocker. Clicky here for some stuff on her.

 

One of the things to keep in mind when comparing C and A is the cost of the thing sticking out of the boat. Dumping in an A is slow. Wiping out in a C and damaging the wing can remove you from the regatta and cost you an amount of money that would buy several As and fund a global A campaign for a few years - so the C decisions on boards have an added dimension to the normal L/D calculations, namely: wing_preservation=L/D*$$$^4

 

Cheers,

Giles

 

Personally, I've sailed the ASG3 in 22-25 knots of breeze and downwind I've never noticed any tendency for the boat to lift clear of the water or any hint that it was headed in that direction. To be honest, the handling characteristics are a lot like my previous A3. Speed seems OK but still working on the depth via board height and sail selection.

 

It will be fun and interesting to see what Steve concludes when he gets on the water this summer.

Share this post


Link to post
Share on other sites

Personally, I've sailed the ASG3 in 22-25 knots of breeze and downwind I've never noticed any tendency for the boat to lift clear of the water or any hint that it was headed in that direction. To be honest, the handling characteristics are a lot like my previous A3.

The curved daggerboards on the ASG3 were by my eye not great quality. While they appear to be strong enough for the job, they were not fair and the LE is too blunt. Lars Guck looked at mine and agreed with the observation that the LE needs work so I'm trying to figure out the best way to do that. By contrast, the Marstrom curved boards are like jewels and I am not surprised that Dave Brewer has gotten good results with them on the Flyer II.

 

I wonder if these 2 comments are connected. I had a long chat with Landy about curved boards and he has had issues with the boat lifting out, although admittedly it is usually when powered up sailing to the spreader mark. He reported that he had to be very careful as the whole boat would pop out and I was under the impression he really could feel the lift in more normal downhill sailing in big breeze. He sailed the last worlds with Marstrom boards (assymetric) and is now sailing with symetrical Scheurer boards. It appears he cannot tell any real difference between them and to my eye, they both look sweet.

Share this post


Link to post
Share on other sites

Got the beam and daggerboard location numbers for the Barracuda, Nikita, EVO, Flyer II, and Bimare VR and expanded the comparison to the ASG3 (see attached).

 

It looks like a valid proposal to consider moving the ASG3 daggerboard trunks back 150-300 mm (6-12 inches) if you consider that all of the other boats compared have their sailplan CE's fairly well aligned with the CLR of the boat.

 

It will be interesting to see the kits at the Worlds and how they compare.

ASGS VS A3-XJ-MK5-CUDA-NIKITA-VR-EV0-FLYERII.pdf

Share this post


Link to post
Share on other sites

Interesting data, Bob. It looks like the other outlier is the Marstrom Mk V. Does it have the need for daggerboard movement as well? I haven't heard of such and am curious for your thoughts.

 

Bailey

Share this post


Link to post
Share on other sites

Interesting data, Bob. It looks like the other outlier is the Marstrom Mk V. Does it have the need for daggerboard movement as well? I haven't heard of such and am curious for your thoughts.

 

Bailey

 

Bailey,

 

I'm not a sail or boat designer so I have to look at things in a pretty basic perspective. As I said in my first post, I've sailed a Marstrom MkV and was very impressed by the balance and handling, it seemed quite neutral. So I am surprised of the difference I saw from the other boats. Glenn and Goran may have felt having the curved boards more forward would work better to lift the bows, don't know if that is true.

 

My ASG3 currently does not have aa balanced helm. Here's what I am dealing with. My ASG3 wants to turn to starboard (or clockwise). This results in lee helm on port upwind and weather helm on starboard upwind. It makes the boat hard to sail downwind (especially on port tack in lighter air). I've isolated the issue to the starboard rudder and have switched rudder blades with the problem remaining. I've concluded there is either a misalignment in the rudder head itself. a misalignment of the mounting of the rudder head, or a misalignment of the platform. I have reduced the turning tendency by sanding the inboard trailing edge of the starboard rudder blade. The aft 30% of the ASG3 rudder has a concavity to it so I may experiment with building it up slightly on the opposite side of the starboard rudder blade to see if I can dial the turning tendency down some more. I cannot evaluate helm and balance issues until I correct that.

 

Cheers,

 

Bob

Share this post


Link to post
Share on other sites

Interesting again! If you lift both rudders, can you get the boat to track straight and demonstrate the issue is in the rudders? I remember reading Simon's post about the quality of the curved boards on the ASG3 and wonder if that could somehow be related.

Share this post


Link to post
Share on other sites

Bob:

Small asymmetry in the rudders will cause the boat to turn.

Another possibility is that the boards are not aligned and as a result are working at different lift coefficients. A toed in board will generate more lift and may result in lee helm. This is counter intuitive, but if the boat is crabbing to windward ( making negative leeway) the flow into the bow will be from the weather side instead of the leeward side and this will make the boat want to turn down.

We have this as an institutional problem with asymmetrical boards, and it is highly speed related. At 12 knots or so we need to reduce the board area( by pulling them up a bit) in order to keep the hjelm balanced. If you go down wind with too much board, you get lots of lee helm.

 

With regard to board placement:

If I heard correctly, Marstrom has asymmetrical boards, so they will naturally be further forward than symmetrical boards.

It might be interesting to put a pair of Marstrom boards in your hulls and see what that is like.

Might require as much surgery as moving the boards..... So maybe not a good idea.

 

Lager rudders is a non surgical idea that will move the CLR aft.

 

If ideas from other development classes are acceptable to multi hull sailors, It is pretty common for us to build over sized daggerboard trunks in the canoes and then install "cassettes" that place the board where we want it. It is pretty handy to be able to change boards and their placement without having to rebuild the boat. My old "Greymatter" has had probably 6 different boards over the years, all of which have fit in the original hole in the boat. all be it with different pads and shims. You probably have enough weight to play with, the trunks are right here you would want correctors, so making a trunk that is 50-100% longer probably has no downside and gives you the flexibility to do many things.

You may also recall that the trunks on my DK-17 were bigger at the top than at the bottom so that I could experiment with canted boards by changing out the top plate.

Just some ideas.

SHC

Share this post


Link to post
Share on other sites

Interesting again! If you lift both rudders, can you get the boat to track straight and demonstrate the issue is in the rudders? I remember reading Simon's post about the quality of the curved boards on the ASG3 and wonder if that could somehow be related.

 

My test was to sail downwind in about 8 knots with both hulls in the water. Raised the port rudder and the boat wanted to turn to the right. Raised the starboard rudder and put the port rudder back down, boat tracked straight. I switched rudder blades in the rudder heads and got the same result. It has to be some alignment problem. Have checked the daggerboard alignment as best I can and that looks OK. I think perhaps it could be a gudgeon alignment issue or perhaps a problem in the rudder head.

Share this post


Link to post
Share on other sites

Bob:

Small asymmetry in the rudders will cause the boat to turn.

Another possibility is that the boards are not aligned and as a result are working at different lift coefficients. A toed in board will generate more lift and may result in lee helm. This is counter intuitive, but if the boat is crabbing to windward ( making negative leeway) the flow into the bow will be from the weather side instead of the leeward side and this will make the boat want to turn down.

We have this as an institutional problem with asymmetrical boards, and it is highly speed related. At 12 knots or so we need to reduce the board area( by pulling them up a bit) in order to keep the hjelm balanced. If you go down wind with too much board, you get lots of lee helm.

 

With regard to board placement:

If I heard correctly, Marstrom has asymmetrical boards, so they will naturally be further forward than symmetrical boards.

It might be interesting to put a pair of Marstrom boards in your hulls and see what that is like.

Might require as much surgery as moving the boards..... So maybe not a good idea.

 

Lager rudders is a non surgical idea that will move the CLR aft.

 

If ideas from other development classes are acceptable to multi hull sailors, It is pretty common for us to build over sized daggerboard trunks in the canoes and then install "cassettes" that place the board where we want it. It is pretty handy to be able to change boards and their placement without having to rebuild the boat. My old "Greymatter" has had probably 6 different boards over the years, all of which have fit in the original hole in the boat. all be it with different pads and shims. You probably have enough weight to play with, the trunks are right here you would want correctors, so making a trunk that is 50-100% longer probably has no downside and gives you the flexibility to do many things.

You may also recall that the trunks on my DK-17 were bigger at the top than at the bottom so that I could experiment with canted boards by changing out the top plate.

Just some ideas.

SHC

 

Steve,

 

When I isolated the issue to the starboard rudder, I switched the blades from rudder head to rudder head to see if the problem followed the rudder blade switch. It stayed the same so I believe there is some type of alignment issue. Ben Moon felt the starboard hull lower rudder gudgeon may be slightly off. I've checked the daggerboard alignment and it appears to be OK. I'm focusing right now on the gudgeons to see if that cures the issue.

 

Ben Moon and I have discussed a daggerboard trunk that can accept both curved and straight boards. That seems relatively easy. Ben wants to look at doing a trunk that accepts straight and curved boards and allows canted/non-canted positioning. I think its doable, not sure I want to go that far on my boat though.

 

Regarding placement of the trunk, again I am not a boat designer but it makes sense that the CE of the sailplan should be pretty much close to the CLR of the platform. The ASG3 seems to deviate from that guideline and I believe it may be a hinderance to its performance. The Spanish sailors were going to try moving the ASG3 trunks aft and Glenn was supposed to train with them in the next week so I am waiting on some feedback.

 

Fun stuff, see you Ohio in September? I'm working on some new pie recipes :P !

 

Bob

Share this post


Link to post
Share on other sites

I would doubt that the gudgeon's would have any effect. Unless it was WAY out of whack. There have been so many boats with so many messed up transoms that I would suspect that rudders that were actually 85% correct are in the minority. If you are sure about the boards being aligned, then I would think it might be that the boards themselves might not be the same. Try Ben's port board and see if you still have the same issue.

I am always amazed at how fussy some designs are to board placement while others seem to tolerate huge variations.

SHC

Share this post


Link to post
Share on other sites

Steve, tell me more about why asymmetric daggerboards would make sense for further forward. I didn't understand that part. Otherwise, you post made me realize just how tricky developing two hulls sailing together is compared to one. Creating a c-class from scratch has got to be extremely challenging but rewarding to get right. I suppose cats teach us more about what foils are doing that dinghies, or at least provide a different point of view on it.

 

My gut feel on Bob's boat is that it is not the rudders too, but I don't begin to be as experienced as either of you.

Share this post


Link to post
Share on other sites

I would doubt that the gudgeon's would have any effect. Unless it was WAY out of whack. There have been so many boats with so many messed up transoms that I would suspect that rudders that were actually 85% correct are in the minority. If you are sure about the boards being aligned, then I would think it might be that the boards themselves might not be the same. Try Ben's port board and see if you still have the same issue.

I am always amazed at how fussy some designs are to board placement while others seem to tolerate huge variations.

SHC

 

Blind man sees. I guess another way to test if it is the boards or the rudders would be to sail downwind in non-wild conditions (around 8-10 knots) and pull the daggerboards up to where they are not in the water at all. If the boat tracks straight, I'd have to rule out the rudders and focus on the daggerboards. Having curved boards makes the variables a bit more complex.

Share this post


Link to post
Share on other sites

Personally, I've sailed the ASG3 in 22-25 knots of breeze and downwind I've never noticed any tendency for the boat to lift clear of the water or any hint that it was headed in that direction. To be honest, the handling characteristics are a lot like my previous A3.

The curved daggerboards on the ASG3 were by my eye not great quality. While they appear to be strong enough for the job, they were not fair and the LE is too blunt. Lars Guck looked at mine and agreed with the observation that the LE needs work so I'm trying to figure out the best way to do that. By contrast, the Marstrom curved boards are like jewels and I am not surprised that Dave Brewer has gotten good results with them on the Flyer II.

 

I wonder if these 2 comments are connected. I had a long chat with Landy about curved boards and he has had issues with the boat lifting out, although admittedly it is usually when powered up sailing to the spreader mark. He reported that he had to be very careful as the whole boat would pop out and I was under the impression he really could feel the lift in more normal downhill sailing in big breeze. He sailed the last worlds with Marstrom boards (assymetric) and is now sailing with symetrical Scheurer boards. It appears he cannot tell any real difference between them and to my eye, they both look sweet.

 

Most, if not all, of the lift you get from curved or canted boards is because of the side force on the foil and resulting leeway of the foil though the water. Think for a minute about how you could get lift from a symmetric foil that has a zero degree vertical angle of attack (i.e., the leading and trailing edges of the foil are at the same depth). It has to be because the foil is being dragged sideway though the water (due to leeway) and acts like a kite to provide lift. Since the sideforce is directly proportional to righting moment, you have far less sideforce, and hence less lift, when you are going downwind sitting in the middle of the boat, compared to when you are fully trapezing upwind.

Share this post


Link to post
Share on other sites

Steve, tell me more about why asymmetric dagger boards would make sense for further forward. I didn't understand that part. Otherwise, you post made me realize just how tricky developing two hulls sailing together is compared to one. Creating a c-class from scratch has got to be extremely challenging but rewarding to get right. I suppose cats teach us more about what foils are doing that dinghies, or at least provide a different point of view on it.

 

My gut feel on Bob's boat is that it is not the rudders too, but I don't begin to be as experienced as either of you.

Dinghies are plenty fussy about what their foils do, and there has been much more attention paid to them than in most catamaran classes.

With symmetric dagger boards, you know that you will be making some leeway. This means that the lee side of the lee bow will be providing at least some of the side force. Duncan has, at times, declared this to be 25% of the total side force. so you have to take that into consideration when you place the boards.

With asymmetric boards, the goal is to be making 0 degrees of leeway. As a result, the bow does not operate at an angle of attack and will therefore not generate any side force. Therefore you move the dagger boards forward.

SHC

Share this post


Link to post
Share on other sites

that's one of the more interesting things I've read in a while. thanks for the insight.

Share this post


Link to post
Share on other sites

Most, if not all, of the lift you get from curved or canted boards is because of the side force on the foil and resulting leeway of the foil though the water. Think for a minute about how you could get lift from a symmetric foil that has a zero degree vertical angle of attack (i.e., the leading and trailing edges of the foil are at the same depth). It has to be because the foil is being dragged sideway though the water (due to leeway) and acts like a kite to provide lift. Since the sideforce is directly proportional to righting moment, you have far less sideforce, and hence less lift, when you are going downwind sitting in the middle of the boat, compared to when you are fully trapezing upwind.

I am not so sure! I am pretty confident that the reason why Landy was coming out on the spreader leg was more to do with the fore and aft trim of the boat than anything else. As you go around the top mark and accelerate, you step back so as to make sure that the bow doesn't get pushed under. This gives bow up attitude for most of the time but is needed to stay safe. Bow up gives the foils more AoA and therefore more lift. The answer? Don't step back on the bear away but that would be pretty risky!

 

I also struggle with your theory of sideforce, which is what I first thought was going on. However, most of the guys who should know what they are talking about in A's tell me that the real benefit of canted boards is downhill in breeze and that it makes no difference upwind. Now, they aren't rocket scientists but i tend to trust top sailors "seat of the pants" feel for this. And if that is what is going on, then curved foils would probably not be as effective as straight, canted ones. When it comes to the curved foils, I am absolutely convinced it is all about the AoA of the foil and there are various ways of adjusting that. Body weight is one answer (like the Moths set the AoA of their foils by varying where they sit) but there are a number of other things coming into play. The new DNA A class has a neat adjustment on the top of the case to allow you to set different AoA's but that is then fixed for the race. I like the idea that Steve seems to be pursuing (if I understand it correctly) where he is using winged rudders to alter the attitude of the boat and hence change the AoA of the curved boards. By being able to move the boards in the case, he can also adjust the total amount of lift. The other idea would be to make the system to allow you to change the AoA during races. I personally think that is achievable on an A, and look forward to trying it next season.

Share this post


Link to post
Share on other sites

Simon, you are spot on. The boat lift occurs due to fore and aft positioning. I have had the same thing happen as Landy, with both boards down reaching but still on the wire you can feel the boat getting a little flighty, then if you move aft (as you would when you come in off the wire) the boat still at speed wants to do some pretty crazy things, it has been kind of fun to play around with. I was sailing up wind the other day back to the club in about 20kns just cracked, flying a hull, it was puffy and the hull came back down, because i trapeze very low my reaction is to move aft a little on the platform (as i would when i am coming in for a tack) when the windward board grabbed the water the boat went bow up in a big way, when the board released the boat then did a strange thing, it nosedived forward while i was still on the wind and still on the wire. The boat does a similar thing through a tack but is quite fast once you get used to it. As you come in off the wire and back on the platform and the pressure comes off the rig the leeward hull comes out of the water even before the boat is head to wind.

Share this post


Link to post
Share on other sites

I would doubt that the gudgeon's would have any effect. Unless it was WAY out of whack. There have been so many boats with so many messed up transoms that I would suspect that rudders that were actually 85% correct are in the minority. If you are sure about the boards being aligned, then I would think it might be that the boards themselves might not be the same. Try Ben's port board and see if you still have the same issue.

I am always amazed at how fussy some designs are to board placement while others seem to tolerate huge variations.

SHC

 

Steve,

 

In regards to your last statement above, I've been reviewing the daggerboard locations of every current A-Class platform and comparing to the ASG3 (I've don't have the numbers for the Schuerer or your DK). The results I have posted earlier on this thread. The conclusion I come to is that the placement of the ASG3 curved daggerboard is in front of what I believe the CE of the rig is while all of the other designs appear to have the CE and CLR lined up together. Common sense and my understanding of most sailing theory texts agree with what the other designs are doing. Thus that is why I believe the Spaniards are making this modification to an ASG3 to trial with Glenn in the next week.

 

But then you take a look at BMW Oracle and Alinghi racing upwind in the AC (see attached pic) and what do you notice - the daggerboards are forward of the rigs! That seems to go against "logic" unless the actual CLR of each hull in the water is not necessarily in the vicinity of the daggerboard due to other factors in regards to the hull design or is this because they are using asymmetric foils (per your earlier post). Or does it really matter whether you align the platform CLR with the rig CE for "best" performance when there could be other factors that are more important (such as reducing the displacement of the leeward hull by optimizing the position of the lifting curved or canted daggerboard)?

 

Glenn told me that he tried several positions for the daggerboard of the ASG3 and while each position resulted in a different helm "feel", he could not measure any significant change in boat performance. An Alinghi designer told the Spanish that the ASG3 needs to have the daggerboard trunks moved back.

 

The ASG3 daggerboards are symmetrical in section shape.

 

Thanks for participating in this thread.

 

Bob

post-17917-127557601772_thumb.jpg

Share this post


Link to post
Share on other sites
But then you take a look at BMW Oracle and Alinghi racing upwind in the AC (see attached pic) and what do you notice - the daggerboards are forward of the rigs! That seems to go against "logic" unless the actual CLR of each hull in the water is not necessarily in the vicinity of the daggerboard due to other factors in regards to the hull design or is this because they are using asymmetric foils (per your earlier post). Or does it really matter whether you align the platform CLR with the rig CE for "best" performance when there could be other factors that are more important (such as reducing the displacement of the leeward hull by optimizing the position of the lifting curved or canted daggerboard)?

 

I don't pretend to know dick about boat/board design but I can relate my own personal experiences - the AC multis length/width ratio was much closer to 1:1 than the A-cat. When pushing downhill on a wider boat, such as a T, there is a noticeably bigger propensity for bow diving. Perhaps the lifting foils were placed more forward to keep the bows up due to their massive width.

Share this post


Link to post
Share on other sites

I don't pretend to know dick about boat/board design but I can relate my own personal experiences - the AC multis length/width ratio was much closer to 1:1 than the A-cat. When pushing downhill on a wider boat, such as a T, there is a noticeably bigger propensity for bow diving. Perhaps the lifting foils were placed more forward to keep the bows up due to their massive width.

 

That's a valid premise. The boards I am sure are an asymmetrical design, perhaps part of the equation is to provide the type of side force from the leeward board that will balance the helm as required.

Share this post


Link to post
Share on other sites

I don't pretend to know dick about boat/board design but I can relate my own personal experiences - the AC multis length/width ratio was much closer to 1:1 than the A-cat. When pushing downhill on a wider boat, such as a T, there is a noticeably bigger propensity for bow diving. Perhaps the lifting foils were placed more forward to keep the bows up due to their massive width.

 

That's a valid premise. The boards I am sure are an asymmetrical design, perhaps part of the equation is to provide the type of side force from the leeward board that will balance the helm as required.

 

Looking at more pictures of Alinghi and Oracle, Alinghi's mast is stepped on the front beam and the daggerboard is just in front of the front beam. Oracle's daggerboard is positioned at the front beam, the mast is stepped behind the front beam.

Share this post


Link to post
Share on other sites

In regards to your last statement above, I've been reviewing the daggerboard locations of every current A-Class platform and comparing to the ASG3 (I've don't have the numbers for the Schuerer or your DK). The results I have posted earlier on this thread. The conclusion I come to is that the placement of the ASG3 curved daggerboard is in front of what I believe the CE of the rig is while all of the other designs appear to have the CE and CLR lined up together. Common sense and my understanding of most sailing theory texts agree with what the other designs are doing. Thus that is why I believe the Spaniards are making this modification to an ASG3 to trial with Glenn in the next week.

My guess is that the rudders are working in tandem with the daggerboards to provide lift. On one extreme the Hobie 16 uses the rudders as daggerboards. To go fast you rake the mast as far back as possible and then trapeze in the back of the boat to get the CE as far back as possible.

 

The other similar design is the ARC 21 which has daggerboards in front of the front crossbeam and relatively large rudders compared to the daggerboards.

ARC21-102206-1.jpg

Share this post


Link to post
Share on other sites

It is a 3 dimensional problem not a two dimensional problem.

The boards actually want to be placed so that their CE is in line with the force vector off of the rig. When the rig and the boards are in the same vertical plane (as in a mono hull at 0 deg heel, you can line one up on top of the other on the outboard profile and everything will be OK. As the boat heels the CE of the rig will move to leeward and that will also help maintain the "correct" amount of weather helm.

Once they are separated horizontally, the boards needs to move forward in proportion to the beam because the force vector has to be forward of 90 degrees to centerline.

 

A further point is the size and loading of the rudder foil are also factors. You can have a very small daggerboard well forward as long as the rudder is big enough. Think of board less cats as the limit of this

 

I think that to disqualify rudder asymmetry, you have to sail with one rudder only.

 

The angle of attack on the boards is also effected by steering. It reduces as you bear away and increases as you head up. The curved foil makes the spikes more gradual than on straight inclined foils, and so it a useful compromise. But you should not be surprised if the leeward hull goes up when you head up

 

If I recall correctly, the Marstrom boards are asymmetric. This presents interesting problems all by itself because it is hard to have both foils operating in their low drag buckets at the same time. My first A Cat had asymmetric boards, and I had to pull the weather board after each tack. It was kind of a chore and made everything just that much harder to do. But there were times when the boat was deadly quick. In the normal course of events, foils limit amount of force they can generate by moving in the direction of their lift, thus reducing the angle of attack and lift coefficient. In most cases this means generating less leeway. Once there is another board generating a opposing force, then the board will create a higher force and the boat will start upward. The famous picture of PLVI flying her lee hull and tea bagging Blunted and Fredo is an example of exactly this phenomenon. So the Marstrom "leaps" are pretty logical, at least to my understanding of how these things work.

SHC

Share this post


Link to post
Share on other sites

A quick tangent - Steve, why are rudders often larger than daggerboards on the a-cat and other advanced boats? It seems to be a relatively long running theory as the Supercat 21 has the same feature.

 

Also, Steve are you saying that a boat with curved daggerboards won't lift well until both boards are in the water?

Share this post


Link to post
Share on other sites

I think that to disqualify rudder asymmetry, you have to sail with one rudder only.

 

The angle of attack on the boards is also effected by steering. It reduces as you bear away and increases as you head up. The curved foil makes the spikes more gradual than on straight inclined foils, and so it a useful compromise. But you should not be surprised if the leeward hull goes up when you head up

 

If I recall correctly, the Marstrom boards are asymmetric. This presents interesting problems all by itself because it is hard to have both foils operating in their low drag buckets at the same time. My first A Cat had asymmetric boards, and I had to pull the weather board after each tack. It was kind of a chore and made everything just that much harder to do. But there were times when the boat was deadly quick. In the normal course of events, foils limit amount of force they can generate by moving in the direction of their lift, thus reducing the angle of attack and lift coefficient. In most cases this means generating less leeway. Once there is another board generating a opposing force, then the board will create a higher force and the boat will start upward. The famous picture of PLVI flying her lee hull and tea bagging Blunted and Fredo is an example of exactly this phenomenon. So the Marstrom "leaps" are pretty logical, at least to my understanding of how these things work.

SHC

 

Steve,

 

Per Ben's post (AUS), we've seen the ASG3 "leap" a bit also. I've noticed the tacking thing that Ben references. I've not had the boat leap like Ben claims happened with his ASG3 or Landy has experienced with the Marstrom from the windward mark to the offset mark.

 

I'll be sailing with Bob Curry and hopefully Randy Smyth in a couple of weeks. We'll be trying out some of the ideas we are throwing out. Can't wait to get back on the water.

 

Bob

Share this post


Link to post
Share on other sites

But then you take a look at BMW Oracle and Alinghi racing upwind in the AC (see attached pic) and what do you notice - the daggerboards are forward of the rigs! That seems to go against "logic" unless the actual CLR of each hull in the water is not necessarily in the vicinity of the daggerboard due to other factors in regards to the hull design or is this because they are using asymmetric foils (per your earlier post). Or does it really matter whether you align the platform CLR with the rig CE for "best" performance when there could be other factors that are more important (such as reducing the displacement of the leeward hull by optimizing the position of the lifting curved or canted daggerboard)?

 

Actually the front of the BMWOR boards were just a few inches foreward, maybe 12" of the leading edge of the wing.

 

The CE was all over the place too with different sail configurations. Downwind, the rig would be raked back about 8 degrees to deal with all the lee helm from the A-sail up front. Rake could be tuned on the fly to make helm fit in the right range.

 

Also BMWOR obviously had assy boards, so the goal was also to have the platform go straight through the water, not have any leeway.

 

FYI

 

MC

Share this post


Link to post
Share on other sites

I hadn't considered some of the issues being discussed but they are exactly the same as seen on Moths when their foils pop out and you crash. The classic is when a newbie gets a big puff downwind, get scared as the speed goes up and the boat lifts higher out of the water. The instinct is to slow down by easing the sail but all that does is unload the rig and the boat pops straight out and you crash. The correct response is to sheet in and move forward You can feel that same rig unloading in gybes. Of course, the lack of vang on a cat adds to this greatly, so on the leg from top mark to spreader, you sail with lots of twist, thus unloading some of the downward component in the rig. The same happens on innitiation of a tack. Every significant ease of the sail will unload the rig and add to the lift.

Share this post


Link to post
Share on other sites

The boat does a similar thing through a tack but is quite fast once you get used to it. As you come in off the wire and back on the platform and the pressure comes off the rig the leeward hull comes out of the water even before the boat is head to wind.

Pulling the leeward bow out going into a tack is something we used to work on in tacking the Tornado, particularly in lighter winds. I imagine it makes the tack quicker and the boat spins on a tighter turning circle. Of copurse, that means the cranked tillers are probably no longer at the right angle, but were they ever ideal anyway or just a compromise!

 

It is stuff like this that convinces me that we are oinly just scratching the surface of what these foils can do. While the rules won't let us get airbourne, I still think there are some pretty significant performance gains to be found and it is one thing that attracted me to the class.

Share this post


Link to post
Share on other sites

>I have had the same thing happen as Landy, with both boards down reaching but still on the wire you can feel the boat getting a little flighty, then if you move aft (as you would when you come in off the wire) the boat still at speed wants to do some pretty crazy things, it has been kind of fun to play around with.>

 

The same thing happened in the '70s and '80s with surface piercing foils on cats: It was documented and even photographed in "FasterFaster" by Dave Pelly and other books on the foiling Tornadoes, specifically with "Icarus I- The Boat That Flies". At speed on foils- with a puff, this sudden high angle of attack occurs, which I call "leaping and crashing". My Hobie 18 with SP foils did this fairly often early on, happily crashing straight ahead but without pitchpoling. My wooden A-cat does the same thing occasionally, but now behaves much better with modified external SP foils. Easing the mainsheet then sheeting in again at the correct time saves the speed and the altitude- and the brown-staining of pants. Spilling wind? Yup- they they had to do that with "Off Yer Rocker" too.

 

So- it appears that curved-foil cats exhibit the same behavior. Wow- These boats are coming up out of the water, aren't they. Ah so desu ka.

Share this post


Link to post
Share on other sites

RUDDERS pulling one way.

Steve is almost certainly correct that the blades are asymmetric. Asymmetry near the trailing edge can be very small yet still have a large effect because it has good leverage over the centre of pressure. Check this by making a car bog pattern of the section say 3mm/1/8" thick and reversing it as a pattern on the other side.

 

The easiest cure is to chamfer the trailing edge. If the boat is turning to starboard then chamfer the port side trailing edge- ie the stbd side is undisturbed.

 

 

CURVED FOIL EFFECTS

Multihulls have a low top speed because their wetted surface area doesn't reduce with speed. The benefit of a foil is to lift the boat thereby reducing wetted area. If there is no control system the most stable configuration seems to be to lift the bow more than stern. ORMA60s had less volume in the bow than the modern A-class so foils added to their safety.

 

The foils are curved in ORMA 60s to act as a defacto control system, mainly. If they rise too high, they softly(!) ventilate and come back to earth. Alinghi did this in Genoa such there was only about 5m leeward hull left in the water at ~15deg heel.

 

The foils are curved in A-class because of the rule restricting maximum beam means a straight board can’t have as much effective cant angle.

 

The modern A-class hull is beamy enough that it has some dynamic lift effects that reduce wetted area at high speed. It clearly pays for this with extra drag in light airs due to increased wetted area, and in waves due to beam.

 

 

In summary,

- for high speed, want to lift the bow with a foil.

 

- on a wide boat the foil has to be further forward because it is approximately perpendicular to the apparent wind angle from the CE (as Steve says), so don't use BOR & A5 as a guide.

 

- if the foils are forward then the rudder must accept more of the sideforce. It works on paper but the higher rudder loading is more difficult to manage consistently in practice. Martin Fischer designed a boat like this for 2008 Euros.

Share this post


Link to post
Share on other sites

Well said, md234. I'm amazed to hear Alinghi lifted the bows so high. Can you tell me more about the theoretical advantages of narrow chord boards and larger chord rudders? It seems many a-cats have this kind of set up today.

 

Bob, what else did you learn this last weekend at Gulfport?

Share this post


Link to post
Share on other sites

Well said, md234. I'm amazed to hear Alinghi lifted the bows so high. Can you tell me more about the theoretical advantages of narrow chord boards and larger chord rudders? It seems many a-cats have this kind of set up today.

 

Bob, what else did you learn this last weekend at Gulfport?

 

Bailey,

 

I did not sail Gulfport last weekend but did some sailing on my own on Lake Pontchartrain. I had done a few hours work wet sanding both the daggerboards and rudders. On the daggerboards I was focused on sharpening and rounding out the LE and removing some humps and bumps along the chord. I think this was of benefit because the boards seem now not to be so "noisy" (I judge this going upwind on the wire and observing the weather board). The right turning tendency of the boat seems to be minimal now and all I have done there is focus on sanding the TE's of the rudders to correct.

 

I'll be sailing in Fort Walton Beach this weekend with Bob Curry and Randy Smyth. Both are great benchmarks obviously and I have the new Ashby sail to try out that Glenn developed specifically for the ASG3. Ben Moon tried the same sail last weekend in Gulfport and felt the boat's performance is significantly improved so I am looking forward to making my own observations measured against Randy and Bob.

 

Cheers,

 

Bob

Share this post


Link to post
Share on other sites

With some great input from a very good A-Class sailor, I believe the possibility that my boards on the ASG3 could have the wrong angle of attack. This sailor races a Marstrom Mk V and had to rake his boards back and add some LE toe-in to get what he believes is approaching optimum performance from the curved blades. Last night, I checked the fit of my blades in the trunk and there is actually 1/2" of movement. The forward motion of the boat is in the direction that by my understanding would create an angle of attack that is the opposite of what you are looking for. Time to try out some shims, stay tuned.

Share this post


Link to post
Share on other sites

Steve,

 

In regards to your last statement above, I've been reviewing the daggerboard locations of every current A-Class platform and comparing to the ASG3 (I've don't have the numbers for the Schuerer or your DK). The results I have posted earlier on this thread. The conclusion I come to is that the placement of the ASG3 curved daggerboard is in front of what I believe the CE of the rig is while all of the other designs appear to have the CE and CLR lined up together. Common sense and my understanding of most sailing theory texts agree with what the other designs are doing. Thus that is why I believe the Spaniards are making this modification to an ASG3 to trial with Glenn in the next week.

 

But then you take a look at BMW Oracle and Alinghi racing upwind in the AC (see attached pic) and what do you notice - the daggerboards are forward of the rigs! That seems to go against "logic" unless the actual CLR of each hull in the water is not necessarily in the vicinity of the daggerboard due to other factors in regards to the hull design or is this because they are using asymmetric foils (per your earlier post). Or does it really matter whether you align the platform CLR with the rig CE for "best" performance when there could be other factors that are more important (such as reducing the displacement of the leeward hull by optimizing the position of the lifting curved or canted daggerboard)?

 

Thanks for participating in this thread.

 

Bob

 

Location depends on what you are trying to achieve with the dagger board.

 

Traditionaly the dagger is placed roughly in line with the CE as you state above to provide lateral resistance and maintain the balanced helm feel.

 

With a curved board you are attmpting to generate some lift in addition to the lateral resistance. By placing a lifting component right at the CE you have created a potentialy very difficult platform to control. You have to maintain your ballance so the platform stays level as you will generate signifcant drag forces as the boat pitches around the center lifting point.

 

It is probably not possible on an A, but on the Ac boats they had a very narrow design window they were trying to meet. A forward mounted dagger can provide the lateral resistance as long as it is properly shared with the rudder and the characterisics of its mounting taken into account. With a sufficiently narrow performance and conditions you are targeting you can design the lift forces to ballance as well, so a more forward foil does not act like a teeter totter.

 

Matt

Share this post


Link to post
Share on other sites

Go back and read Steves post 31. Then, instead of viewing the relationship between CE and CLR in elevation (from the side) which is misleading, view it in plan. Looking down on the boat. As the beam of the boat increases the CLR must move forward.

Share this post


Link to post
Share on other sites

.. on the Ac boats they had a very narrow design window they were trying to meet. A forward mounted dagger can provide the lateral resistance as long as it is properly shared with the rudder and the characterisics of its mounting taken into account. With a sufficiently narrow performance and conditions you are targeting you can design the lift forces to ballance as well, so a more forward foil does not act like a teeter totter.

 

 

BOR had this yet-unexplained iteration, when they moved the mast step a full 2m forward - trying to eliminate too much load on the rudder or thinking they'd use the hard wing w/o head sails?

But then the boat took a decidedly bow-down tendency, and they hurried to bring it back to the original position

Share this post


Link to post
Share on other sites

Go back and read Steve’s post 31. Then, instead of viewing the relationship between CE and CLR in elevation (from the side) which is misleading, view it in plan. Looking down on the boat. As the beam of the boat increases the CLR must move forward.

 

I agree. Here is a plan view of a trimaran, with the heeling and pitching moments represented as virtual c.g. locations. This isn't exactly the same as the yawing moments due to lateral resistance, but I think you can still infer some of the lateral balance issues from it. The black lines are due to the forces on the sail rig, and you can see how the peak caused by the lift when closehauled points forward. (The center of buoyancy is shown in blue, but that's not relevant to this discussion.)

hullbalance.gif

 

You'd want the board positioned relative to that peak, the way the hydrofoil is located on the ama for the boat shown. The wider the boat, the more forward the board should be.

 

So I'm not sure you can draw a lot of conclusions about the daggerboard locations on A5 or USA 17 applied directly to an A cat. Those boats are also more efficient and would be sailing with smaller apparent wind angles. This will point their force vector more to the side, so you need to factor that in, too. It means their foils may not be as far forward as you might expect from their beam.

 

A VPP would help you figure out just where the aerodynamic force vector was pointing. You could use that to help position the foils.

 

Some rudder load would be beneficial. Since induced drag is proportional to the square of the depth of a foil, the way to minimize the induced drag when sharing the lift between two surfaces is to share them in proportion to the squares of their depths. In other words, the lift on the rudder should be approximately rudder_lift=total_side_force*rudder_depth^2/(rudder_depth^2 + foil_depth^2) and foil_lift=total_side_force*foil_depth^2/(rudder_depth^2 + foil_depth^2). I'm sure there are details of hull interference and boat trim, etc., that complicate the issue, but I think these relationships should be in the ballpark. They make intuitive sense to me - two identical foils would share the load evenly, but a deep foil with a short stubby rudder should take the vast majority of the load. And no matter what the proportion, there's always an advantage to carrying at least a little weather helm, but not too much. I think that agrees with experience.

 

One thing I'd like to do is to use a lifting line analysis to come up with the optimum planform shape for a curved board that has to provide a combination of side force and vertical lift. Can any one give me some candidate geometries to start with? Are curved foils pretty much circular arcs, or are they more J shaped? What would be most useful for designers, a catalog of planform shapes vs vertical lift/horizontal lift, perhaps?

Share this post


Link to post
Share on other sites

Tom,

>In other words, the lift on the rudder should be approximately rudder_lift=total_side_force*rudder_depth^2/(rudder_depth^2 + foil_depth^2) and foil_lift=total_side_force*foil_depth^2/(rudder_depth^2 + foil_depth^2).

 

It's quite a bit more complex because the leeward daggerboard is deeply end-plated by the hull, whereas the rudders and windward board aren't - the rudders are transom hung, and the windward board is not retracted. So one foil_depth should be 2xfoil_depth.

 

It seems easier to sail the boats with less than optimal load on the rudders just for handling reasons. Also having a relatively low base load gives more margin on stall and/or ventilation.

 

regards

Share this post


Link to post
Share on other sites

It's quite a bit more complex because the leeward daggerboard is deeply end-plated by the hull, whereas the rudders and windward board aren't - the rudders are transom hung, and the windward board is not retracted. So one foil_depth should be 2xfoil_depth.

 

It seems easier to sail the boats with less than optimal load on the rudders just for handling reasons. Also having a relatively low base load gives more margin on stall and/or ventilation.

Agreed. You can handle the hull endplate effect by using the effective span instead of the physical span. But I don't think the hull is a completely effective endplate, either, so I don't think its effective depth is fully twice its physical depth. I wish I had the means to calculate the foil, hull, and surface effects together.

Share this post


Link to post
Share on other sites

Just a quick thought regarding this balance and "jumping" phenomenon....

 

Would curved *rudders* be of any use whatsoever? I know the F16 class has had a lot of success with T rudders...

Share this post


Link to post
Share on other sites

Just a quick thought regarding this balance and "jumping" phenomenon....

 

Would curved *rudders* be of any use whatsoever? I know the F16 class has had a lot of success with T rudders...

 

 

I hope someone will try this idea !

Share this post


Link to post
Share on other sites

Just a quick thought regarding this balance and "jumping" phenomenon....

 

Would curved *rudders* be of any use whatsoever? I know the F16 class has had a lot of success with T rudders...

 

 

I hope someone will try this idea !

 

would a curve be hydrodynamicly favorable compared to a L shaped rudder. Presume T shaped rudders are out because of width issues.

Share this post


Link to post
Share on other sites

>...theoretical advantages of narrow chord boards and larger chord rudders?

In theory there is no advantage. The sizes are related to manoeuvring at slow speeds, and waves & gusts.

 

>would a curve be hydrodynamically favorable compared to a L shaped rudder

I don't think so. Read previous note about why O60s had curved foils.

With a control system, an L or T shape is probably better for many practical reasons.

Share this post


Link to post
Share on other sites

>...theoretical advantages of narrow chord boards and larger chord rudders?

In theory there is no advantage. The sizes are related to manoeuvring at slow speeds, and waves & gusts.

 

>would a curve be hydrodynamically favorable compared to a L shaped rudder

I don't think so. Read previous note about why O60s had curved foils.

With a control system, an L or T shape is probably better for many practical reasons.

 

yeah MD, I get that curved foils can be soft control systems and that that is why O60s went with curves despite costs. See the video of Banque Pop jumping out of the drink while 2 sail reaching. Very dramatic, curved foils or no.

 

But I was wondering about drag benefits of eliminating a vertex point in a submerged (leeward) L or T shaped rudder by substituting a J shaped rudder. I think I saw one of the LAC C class with L shaped rudders.

Share this post


Link to post
Share on other sites

>...theoretical advantages of narrow chord boards and larger chord rudders?

In theory there is no advantage. The sizes are related to manoeuvring at slow speeds, and waves & gusts.

 

>would a curve be hydrodynamically favorable compared to a L shaped rudder

I don't think so. Read previous note about why O60s had curved foils.

With a control system, an L or T shape is probably better for many practical reasons.

 

yeah MD, I get that curved foils can be soft control systems and that that is why O60s went with curves despite costs. See the video of Banque Pop jumping out of the drink while 2 sail reaching. Very dramatic, curved foils or no.

 

But I was wondering about drag benefits of eliminating a vertex point in a submerged (leeward) L or T shaped rudder by substituting a J shaped rudder. I think I saw one of the LAC C class with L shaped rudders.

 

 

 

O60s shall not be compaired with A-cat, curved foils and rudders will act very differently due to the factor

of 3X in lenght , the damping has lot lower resonance frequency in the O60s and need therefore no stab at the rudder to control pitch movement. (refer. small helicopter has rotor stabs, big ones do not need)

Share this post


Link to post
Share on other sites

Bob Curry and I had a very productive weekend of sailing this past weekend in Fort Walton Beach. Unfortunately, Randy Smyth was on the west coast racing.

 

Bob was sailing his Marstrom Mk V with curved daggerboards. He used a new Landenberger mainsail on Saturday and a new Skip Elliot design on Sunday. The Landenberger had a moderate head (around 800-820 mm I think) and a straight leech. The Elliot had a larger head and a hollow leech similar to Steve Brewin's latest designs.

 

I was using a new Ashby mainsail that Glenn sent to me and Ben Moon. The head width was around 800 mm, the foot was shorter for the ASG3, and the leech had a bit more positive roach to compensate for the shorter foot. The sail had about 20-25 mm more luff curve than his previous 2008/2009 sails and setup on the Fiberfoam mast it looked to have more depth (the sail looked very similar to the Landenberger in section shape).

 

We sailed all weekend in 5-8 knots of breeze. I have sailed against Bob a lot in light air and he excels in making a boat go fast in those conditions. The Mk V was sailed to 3rd overall at the 2009 WC and it is a very good boat in light air. My goal was to be competitive against Bob by the end of the weekend. To cut to the chase, I was very happy to achieve that goal. As expected, the ASG3 is harder than the Mk V to sail in light air because you have to stay pretty far forward. We switched boats on Saturday and I was immediately impressed with how forgiving the Mk V was to the conditions (light air with a lot of powerboat chop). Once I figured out weight placement, I was able to pretty much match Bob upwind and downwind. We even had some runs where the ASG3 seemed to have an edge and I've never had that happen in the conditions we had since I got the boat in January.

 

Regading the point of this thread, we believe we did learn some things about our curved blades. The first is to experiment with having the weather blade up partially sailing upwind in light air (non-trapeze conditions). We did a couple of runs and did not see any disadvantage and we want to explore that more to see if there is any advantage. Downwind, we pulled the boards up max all the time for the light air runs (just keeping the enough down to fill the trunks). We did get some breeze up to 10-12 knots on Sunday to where we could fly a hull downwind. We found that having the leeward board up 4-6 inches and the weather board up max worked best. This combination seemed to provide the lateral resistance and lift benefit (to reduce the displacement of the leeward hull) from the leeward board while minimizing drag from the weather board (since it is really just "along for the ride").

 

We are both going to look at the angles of attack of our boards. The ASG3 has some wiggle room in the trunk and I am going to play around with board rake and toe in to see if I can see any improvement. Hope to report on that after our next session. Stay tuned.

Share this post


Link to post
Share on other sites

Great update, Bob. Sounds like you've come a long way with the boat from your original posting.

 

Ben and I believe the Ashby sail is a HUGE step forward for the boat. I was getting concerned that major mods could be in order but I don't think that is the case now. I am sure we will see more iterations with curved versus straight and/or canted boards but for sure we are going in the right direction. My conclusion at this time is we have a tool to work with that is probably more biased for performance in breeze but that I now believe can show competitive pace in light air.

Share this post


Link to post
Share on other sites

 

From the DNA's website, the new A class made in The Netherlands,

a pretentious statement:

With the curved boards we enter a whole new area of performance. On a race course the boat is in medium conditions 3-5 percent faster than the old boats.Full text below:

 

Wednesday, 16 June 2010

 

<A href="http://dnacat.blogspot.com/2010/06/understanding-curved-boards.html">Understanding curved boards

2010 is year of the curved boards.We are beginning to understand why curved boards work and how the boards should be setup.More lift is als more drag so there is an optimum.Sometimes the boards can lift the boat completely out of the water, especially in a tack or gybe, which can be a bit scary, though overall control of the boat is much better.What we noticed is that while doing the wild thing we reach higher endspeeds and have more consistent speeds.The boat can be pushed harder and we even see people standing in the trapeze while doing the wildthing.Upwind the boat feels lighter and can handle more power. The boat foots very well but can be pointed if needed.With the curved boards we enter a whole new area of performance. On a race course the boat is in medium conditions 3-5 percent faster than the old boats.My prediction is that in the 2010 Worlds at least 80 percent of the top 10 will have curved boards.

CU in Cesenaticoat 10:12 Posted by Arno Terra 0 comments icon18_email.gif icon18_edit_allbkg.gif Labels: curved boards, Worlds 2010

 

 

Share this post


Link to post
Share on other sites

From the DNA's website, the new A class made in The Netherlands,

a pretentious statement:

With the curved boards we enter a whole new area of performance. On a race course the boat is in medium conditions 3-5 percent faster than the old boats.Full text below:

 

Wednesday, 16 June 2010

 

<A href="http://dnacat.blogspot.com/2010/06/understanding-curved-boards.html">Understanding curved boards

2010 is year of the curved boards.We are beginning to understand why curved boards work and how the boards should be setup.More lift is als more drag so there is an optimum.Sometimes the boards can lift the boat completely out of the water, especially in a tack or gybe, which can be a bit scary, though overall control of the boat is much better.What we noticed is that while doing the wild thing we reach higher endspeeds and have more consistent speeds.The boat can be pushed harder and we even see people standing in the trapeze while doing the wildthing.Upwind the boat feels lighter and can handle more power. The boat foots very well but can be pointed if needed.With the curved boards we enter a whole new area of performance. On a race course the boat is in medium conditions 3-5 percent faster than the old boats.My prediction is that in the 2010 Worlds at least 80 percent of the top 10 will have curved boards.

CU in Cesenaticoat 10:12 Posted by Arno Terra 0 comments icon18_email.gif icon18_edit_allbkg.gif Labels: curved boards, Worlds 2010

 

 

NACRA is making similar statements about curved blades on their new F-20. I think all of us using them believe the potential is there but we still have a LOT to learn about optimization. Steve's use of the boards in the LAC and the A-Class WC will be very insightful.

Share this post


Link to post
Share on other sites

I have been following the DNA in particular for some time and find it particularly interesting. The word from Europe is that it is a rocket ship and that it really impressed at Cesenatico first time out. However, while curved boards are an obvious feature, there are a number of things that seem to make the boat very different.

 

First, as much as I can tell from pictures, some line drawings etc, it appears to me that the hullshape is very different from what we are used to seeing.There are 2 factors to this. First is underwater shape, which has more volume than, say, a flyer and means the boat should be able to be pushed harder downwind. But the real difference is that the hulls seem very big, with lots of topsides and beam. This plays into one of the things I believe is really important. The whole platform is designed to be incredibly stiff. They have paid a lot of attention to this. The boat is pre-preg carnon-nomex. By having a large hull, that lends itself to good strength. Then they have spent a lot of effort in how the beams attach to the hulls. Again, this is something seen with the Nikita but which i think most boats have an issue with.

 

I have commented before on my suprise as to how much twist you get on most of the top platforms (at least the ones I have checked!). It is interesting that Bob Baier commented in a recent interview, saying

 

In my opinion, what is crucial here is simply the honeycomb construction. Here you have much greater stiffness than on a foam boat. Although I am no boat builder, I notice this in the steering. On a stiff boat, one has the feeling that the boat is moving as a single unit. The hulls do not work against one another, rather they move synchronously through the waves, which is how it should be. The DNA ... takes the same approach and takes it further with high-profile, solid cross-members, showing the direction in which this could go.

 

So, yet again, we see many variables add up to what appears to be a great package. The real question is which element has the biggest impact. I am convinced by the need for a stiff platform when using any foils designed to give lift and I expect platform stiffness to become a major theme going forward. When I commented before that I thought we would see hullshapes develop to go with curved foils, I had thought we would see thinner hulls relying on the foils to keep them out of trouble. The DNA seems to go the other way - fatter hulls that are safe when needed but whichhave foils to lift and therefore improve hullshape in other conditions.

 

Then there is the issue of what shape the actual c foils are and in that regard, we have no idea how they compare with what is out there at the moment. This is where it is going to get fun. Somebody needs to take a single platform/rig and compare all the foils. Of course, as suggested above, that will only work for foils of the same type as the position of assy foils would probably by different from symetrical ones. For instance, at the moment there are at least 3 curved foils I would like to try in my boat, but I cannot afford it!

 

The final thing of note for me is that the DNA has a really good way of adjusting the amount of lift from the curved boards MJC_2385.jpg While it does still need to be set up properly, it does seem to make a lot of sense. All that's missing is a few numbers next to the case so one can start building a picture of what is going on.

Share this post


Link to post
Share on other sites

From The Daily Sail

 

It seems that at least for the M20 straight daggerboards are better than curved!!!

 

Today two teams challenged the TNG Round Texel Record. Currently Herbert Dercksen (NED) and Mark Bulkeley (GBR) hold the record. They managed to round the island of Texel in 2 hours, 7 minutes and 2 seconds last year. Dercksen now tried to improve this time with an extra wide M20, just like his English opponents William Sunnucks and Oli Egan. The only difference was that Sunnucks and Egan sailed with a straight daggerboard, whereas Dercksen and Barney opted for a bent daggerboard. It turned out that Sunnucks and Egan had an edge over their rivals, as they could sail more closely hauled. They were one mile ahead at the lighthouse, when their mast suddenly broke. "One moment we were sailing with 13 knots speed on the flat water and the other moment we were suddenly swimming." Sunnucks took the Zwitserleven Round Texel line honours in 2009 and will do everything he can to join the race again on Saturday. "If necessary, we will use a wooden pole as the core", he says determinedly.

 

Dercksen and Barney arrived after 2 hours, 57 minutes and 27 seconds. "It was heavy, let me take a seat", sighs Dercksen. "The waves were enormous. A couple of times we took off from the water

 

 

Share this post


Link to post
Share on other sites

Somebody needs to take a single platform/rig and compare all the foils.

 

Luc can switch between straight or curved daggerboards on his new Scheurer G6.

Landy can change the position of the curved daggerboards on his new Scheurer G6.

 

flojo

Share this post


Link to post
Share on other sites

Somebody needs to take a single platform/rig and compare all the foils.

 

Luc can switch between straight or curved daggerboards on his new Scheurer G6.

Landy can change the position of the curved daggerboards on his new Scheurer G6.

 

flojo

This isn't quite the issue I am interested in tackling. I will probably fit my boat with a case that can take both straight and curved boards. However, the issue I am not sure I can tackle, due to financial constraints, is trying the different boards that arte out there. Even if I limit myself to symetrical boards, for no other reason than wanting to only have one plate case position which takes out a variable, I would like to be able to try a variety of curved boards. The immediate list would be the boards used by Scheurer, DNA, ASG3 and Nikita, assuming I could buy all those. However, there is no way I could afford that! And on top of that, you need a tuning partner who is consistant and very patient.

 

So, as somebody who can only just afford a set of curved foils, which should I go for? I have a pretty decent Flyer 1 and all of the rest of the gear (mast, sails, foils etc) are top class. I intend to put in new cases to take both straight and curved foils as I have a good set of straights. I get one shot! What do I do?.......................other than wait until after the Worlds and see if we are any clearer. Even then, if, say, Glenn cleans up with straight boards, does that tell us that curved boards aren't as good as straight?

 

HELP! I am confused...........

Share this post


Link to post
Share on other sites

From The Daily Sail

 

It seems that at least for the M20 straight daggerboards are better than curved!!!

 

Today two teams challenged the TNG Round Texel Record. Currently Herbert Dercksen (NED) and Mark Bulkeley (GBR) hold the record. They managed to round the island of Texel in 2 hours, 7 minutes and 2 seconds last year. Dercksen now tried to improve this time with an extra wide M20, just like his English opponents William Sunnucks and Oli Egan. The only difference was that Sunnucks and Egan sailed with a straight daggerboard, whereas Dercksen and Barney opted for a bent daggerboard. It turned out that Sunnucks and Egan had an edge over their rivals, as they could sail more closely hauled. They were one mile ahead at the lighthouse, when their mast suddenly broke. "One moment we were sailing with 13 knots speed on the flat water and the other moment we were suddenly swimming." Sunnucks took the Zwitserleven Round Texel line honours in 2009 and will do everything he can to join the race again on Saturday. "If necessary, we will use a wooden pole as the core", he says determinedly.

 

Dercksen and Barney arrived after 2 hours, 57 minutes and 27 seconds. "It was heavy, let me take a seat", sighs Dercksen. "The waves were enormous. A couple of times we took off from the water

 

I'd expect the curved boards to be less efficient upwind; given similar lengths / depths etc.

 

Same Will's mast broke and they could not have compared round the back of the island down wind.

Share this post


Link to post
Share on other sites

Hi SimonN

 

Dont be confused, just go racing a lot, get 100% comfortable (or at least 95%) with your current setup (you only have had the boat for some month)and then start experimenting (I now it is also a lot of fun to use all day long at work to think on how to make the boat go faster). Just a thought.

 

I go to the worlds next week and will have a look at all the nice new stuff, but till now I havnt been convinced about the superiority of boats with curved boards - as any ohter boat doing the right thing out on the course can take you close to the front of fleet, so ill start with focus on that...

 

have fun

 

Lars

Share this post


Link to post
Share on other sites

Hi SimonN

 

Dont be confused, just go racing a lot, get 100% comfortable (or at least 95%) with your current setup (you only have had the boat for some month)and then start experimenting (I now it is also a lot of fun to use all day long at work to think on how to make the boat go faster). Just a thought.

 

I go to the worlds next week and will have a look at all the nice new stuff, but till now I havnt been convinced about the superiority of boats with curved boards - as any ohter boat doing the right thing out on the course can take you close to the front of fleet, so ill start with focus on that...

 

have fun

 

Lars

Lars

 

That's why I have been out training with Steve Brewin, Chris Cairns, Scott anderson and others plus getting lots of time on the water. I am now 100% comfortable with the current set up (others have sailed the boat and confirm what I believe about it) and I know what I have to do next. However, IMO, the biggest mistake a sailor can make when trying to get up to speed in a new class is to sail with inferior gear to the top guys. It gives you a totally false sense of progress. I therefore know the program I need to do this winter, other than more and more sailing. However, with everybody away, there is only so much one man sailing you can do. Anyway, one of the big ones for me is to do with the c/b's and plate case. I have been using modern, high aspect boards but at the moment, they are fitted into a case that is far too big (designed for the original boards) and which are padded out with foam. And the boards aren't canted. It is altogether unsatisfactory. I need to do something perminent that works. It seems really silly to do that work now, only to then redo it in a few months. I also have limited tme for major alterations! I will wait until after the worlds, but effectively if I am going to do anything before the Australian nationals, I need to do it in July!!! I am therefore collecting as much data as I can before the worlds and then evaluating everything straight after. The biggest issue as i see it is that the people I expect to be in the top 10 would be there if they sailed with curved or straight boards. What we need is somebody who has a good track record who normally sails with one type of board (straight or curved) to change and for there to be a noticable change in his performance. Some say we saw that with the DNA, but one regatta doesn't prove anything.

Share this post


Link to post
Share on other sites

Hi SimonN

 

It makes better sense if you have to change your case anyway - but your are still on a difficult path of trying to take a fast lane to the front of the fleet. I looking forward to hear what your conclusion will be, there is for certain a lot of possibilties.

 

Lars

Share this post


Link to post
Share on other sites

Damn, that is sweet looking.

Share this post


Link to post
Share on other sites

DNA is a very good looking boat. Will be interesting to see how it performs on the big stage.

So the big secret is out on the DNA blog. Stevie Brewin has a DNA (with curved boards) for the worlds which I had been sworn to secrecy about since Easter (just imagine me managing to keep that quiet!!! lol). I am not certain, but I believe he also has access to a Tool like the boat he has sailed all year, just in case he doesn't like or cannot get used to the DNA. Steve has a week of sailing to get to know the boat.

 

It seems to me that almost all the main contenders for the worlds will be using curved boards, with Glenn being the obvious exception. Reports of the DNA say it is a total rocket ship, although I guess we will all know soon enough if this is true, and they have a well proven driver in Stevie. The Nikita is obviously well represented by Bob and the Scheurer has Landy and Luc.

 

The next few weeks should be very interesting. I get the feeling we are now at the point that the age of the curved board has well and truly arrived and I suspect irrespective of the results at the worlds, curved boards will be the thing to have.

Share this post


Link to post
Share on other sites

DNA is a very good looking boat. Will be interesting to see how it performs on the big stage.

So the big secret is out on the DNA blog. Stevie Brewin has a DNA (with curved boards) for the worlds which I had been sworn to secrecy about since Easter (just imagine me managing to keep that quiet!!! lol). I am not certain, but I believe he also has access to a Tool like the boat he has sailed all year, just in case he doesn't like or cannot get used to the DNA. Steve has a week of sailing to get to know the boat.

 

It seems to me that almost all the main contenders for the worlds will be using curved boards, with Glenn being the obvious exception. Reports of the DNA say it is a total rocket ship, although I guess we will all know soon enough if this is true, and they have a well proven driver in Stevie. The Nikita is obviously well represented by Bob and the Scheurer has Landy and Luc.

 

The next few weeks should be very interesting. I get the feeling we are now at the point that the age of the curved board has well and truly arrived and I suspect irrespective of the results at the worlds, curved boards will be the thing to have.

 

There probably has never been so much new technology at an A-Class WC as this one in quite a few years. The DNA is one nice looking package. I like the design brief to build a minimum weight boat that is stiff. The biggest departure to me are the flat bottom sections since a semi-circular bottom section has been the norm for quite some time. It would make sense that this could be quite quick in the breeze but will it be sticky in light air?

 

Regarding stiffness, I wonder if you would get any benefit from installing Dyneema stays between the hulls on other boats to limit flexing?

Share this post


Link to post
Share on other sites

Regarding stiffness, I wonder if you would get any benefit from installing Dyneema stays between the hulls on other boats to limit flexing?

I don't know about the idea as such, but I do know I spent a couple of hours yesterday trying to identify why and what allows my boat to twist. I did a very simple test - with the transoms on the ground you lift one bow and measure the difference in height the 2 bows move. On my boat, it seemed different for each side! The only conclusion I came to is that I need to build an accurate jig in which to measure the twist as for much of the time I thought there was more twist from 1 hull than the other. I thought it by starting with the boat on chocks at the front, I also thought I would be able to see exactly what was moving to create the twist (my daughter lifting the boat for me). Despite a fair amount of twist, I really couldn't work it out. Accurate measurement is needed! Then I can look at what stiffens up the platform......................

Share this post


Link to post
Share on other sites

Interesting idea on additional stays. Alinghi likely has the most experience on this. They used carbon tubes in compression above the boat and rigging in tension below it. That boat appeared very stiff.

 

It seems like the trick is to add reinforcement that is not in the horizontal plane of the beams if we want maximum effect, but I remember Steve Clark talking about the forestays and what they do. If you could tie in some rigging below the beam (but that's not so good for low drag through the water), and work with the mast's rigging already, you might have something with minimum additional structure?

Share this post


Link to post
Share on other sites

Regarding stiffness, I wonder if you would get any benefit from installing Dyneema stays between the hulls on other boats to limit flexing?

I don't know about the idea as such, but I do know I spent a couple of hours yesterday trying to identify why and what allows my boat to twist. I did a very simple test - with the transoms on the ground you lift one bow and measure the difference in height the 2 bows move. On my boat, it seemed different for each side! The only conclusion I came to is that I need to build an accurate jig in which to measure the twist as for much of the time I thought there was more twist from 1 hull than the other. I thought it by starting with the boat on chocks at the front, I also thought I would be able to see exactly what was moving to create the twist (my daughter lifting the boat for me). Despite a fair amount of twist, I really couldn't work it out. Accurate measurement is needed! Then I can look at what stiffens up the platform......................

 

Simon,

 

I've done the same test on every A-Class I own and it always seems like one hull lifts more than another (at least by eye). My previous boat was an A3 and we ended up with glued beams on it. When you lifted one bow though, the other bow still lagged behind a bit but the boat felt very stiff going through the water and Pete (Melvin) had designed the boat with a large almost square front beam (almost like a Tornado beam but in carbon construction obviously). On my new ASG3, what I notice more (with the front beam about 220 mm further back than the A3 front beam) is not the bows moving up and down as I sail through chop but more the hulls compressing in as the boat sails into waves upwind (don't notice much flex at all downwind). There is simply a longer cantilever on this boat. I know Glenn does not worry about this at all and my advice to you is not to worry over it.

 

I agree that stiffness is good for the path towards 100% optimization but we sailors can never sail our boats at that level consistently so the reality is to be in the ballpark which most of the better builders are hitting.

 

Cheers,

 

Bob

Share this post


Link to post
Share on other sites
I don't know about the idea as such, but I do know I spent a couple of hours yesterday trying to identify why and what allows my boat to twist. I did a very simple test - with the transoms on the ground you lift one bow and measure the difference in height the 2 bows move.
Are you doing this with the mast up? If it is, are the stays all equally tight? The four stays and the mast are what gives it its rigidity. It's like an inverted version of Alinghi's stay system.

 

 

 

Share this post


Link to post
Share on other sites

If you want to stiffen your A Cat platform, tighten the rig.

The king post truss system created by the mast and it's stays dominate all other factors such as stern beam S bending, torsional rigidity of the hulls and torsional rigidity of the main beam.

The problem with tightening the rig is that most A cats are not set up to accommodate much of any rig tension and the mast won't rotate properly.

This can be solved by better hounds hardware, but no one has really pursued that route.

 

SHC

Share this post


Link to post
Share on other sites

On my new ASG3, what I notice more (with the front beam about 220 mm further back than the A3 front beam) is not the bows moving up and down as I sail through chop but more the hulls compressing in as the boat sails into waves upwind (don't notice much flex at all downwind). There is simply a longer cantilever on this boat. I know Glenn does not worry about this at all and my advice to you is not to worry over it.

 

I agree that stiffness is good for the path towards 100% optimization but we sailors can never sail our boats at that level consistently so the reality is to be in the ballpark which most of the better builders are hitting.

 

Cheers,

 

Bob

Bob

 

I don't like disagreeing with somebody of Glenn's great knowledge and proven skills but I think he is totally wrong on this issue. Every bit of movement in the hull is energy being used up and I m 100% certain that I can feel it on the A's I have sailed. To somebody who comes from a high performance dinghy background, I find it truly shocking. Moreover, from development work I was involved with when coaching T's, I came to realise that platform stiffness was a factor and not just because of the jib. We noticed that the stiffest platforms were better downwind in waves (when forestay tension wasn't a factor). Poor Stevie Brewin had the msifortune to hear me bang on about this during an 12 hour each way road trip and when he got home, he got reports from somebody else on how much better a stiff, prepreg carbon-nomex felt, saw the results of the DNA and ordered one impediately. During our conversation, he remembered that the Flyer he won the worlds in, which he says is one of the fastest A's he ever owned, was bt far the stiffest and did sail more in the way I suggested a stiff boat would sail.

 

Going on from there, as soon as you add the extra forces of the curved boards, I believe platform stiffness becomes a key factor. Going forward, this debate will either be killed off very fast in the next 2 weeks or it will run and run. If Glenn wins the worlds, some might still say platform stiffness isn't such a factor but if Glenn doesn't win, I expect to see continued focus in this area, with prepreg carbon-nomex boats the norm (with curved boards) and those of us who cannot afford new boats looking closely at ideas such as the one Steve mentions, which I like a lot!

Share this post


Link to post
Share on other sites

The problem I see with Steve's point (which of course is standard practice on C-cats, but coupled at least in my time to a Lazy Susan at the hounds) is that it's too effective: meaning it takes up all the racking load, inevitably adding a lot of compression to the mast.

 

But forgive the slight OT: isn't something similar (i.e. tensionable ama stays) used on tris, mainly ORMAS? Or could it be a fore and aft bridle on the shrouds?

Share this post


Link to post
Share on other sites

Simon, I too have come from the dinghy and skiff back ground among other things and the difference in hull stiffness through laminate schedule is far more important in a hull that has larger unsupported panel area such as a 505 or I14 than it does in a cat especially an A cat. While i don't disagree that a stiff platform is important i think that there is more important factors relating to platform stiffness. As Steve C pointed out rig tension is the most important. I also think tramp tightness and even tramp material has huge affect. While i don't discount the fact that every bit counts the shape of the modern A cat hulls allow them to be very stiff on their own especialy the boats with smaller area of flat pannels such as the A3 and Barracuda. I believe Ben Hall/ Peter Cogans's new boats may have a more technical laminate schedule than most however i also think they have removed most of the internal frames and their hulls are super stiff. Yes there will be a difference from the old kevlar hulls, Flyer and earlier to the new hulls but i think any of the new carbon hulls it would be hard to tell much of a difference. I think the biggest factor relating to laminates is durability. I think the main construction stiffness control in a modern A is in beam size and beam to hull attachment. The trend on the new boats having as much gluing surface of beam to hull is a huge factor. These are just some of my thoughts and it would be intresting to hear from some of the builder themselves.

Share this post


Link to post
Share on other sites

When we started building the LR2 and then the LR3 I wanted as stiff a platform as possible. Panel stiffness is directly related to core thickness so I used 9mm Corecell and 6 oz. carbon skins. We have great panel stiffness. When it came to beam attachment I used a four step approach: first was to wrap a piece of 6 oz. carbon under the beam and up onto the hull; the second step is to gussett from beam to hull with a 2" wide strip of carbon cut on the bias; third step is to glue 9 oz. carbon uni straps around the beam and onto the hull; the fourth step is to bog a fillet from beam to hull and laminate a final wrap of 6 oz. carbon over the whole area. The platform has very little rack and our third boat weighed 150 lbs at the 2007 Worlds. Details and photos at:

LR2ACATS : LR2 A-Class Catamaran

John Lindahl @ LCD

Share this post


Link to post
Share on other sites

Lots to talk about!

 

First off, I thought I had worked it all out. Steve had explained that cuved boards and position is a 3D problem and I got that. I worked out where the boards need to go in my boat. Then he convinces me that rig tension will stiffen up my boat and I have designed a really neat system to allow as much tension as I want and still rotate. I am even beginning to convince myself that the compression loads on the mast will, when set up right, make rotation an even more effective depowering tool. Then I read the news from the worlds.........

 

So, now I need to consider canting the rig. It seems to make sense to go with the curved foils. As has been reported, the rig is providing a significant proportion of the downforce, which is why the boat leaps up in tacks and gybes. Reduce that downforce, by canting the rig, and you don't need the foil to work as hard for the same amount of lift, or you get more lift for the same drag. Just when I thought this was getting easy............

 

The first thought is that I suspect this adds to the 3D problem that Steve was talking about, as canting moves the centre of effort. So, unless I am mistaken, if you cant the rig you either have to move the boards forward or rake the rig more. Have I got that the right way around?

 

Then there is the issue of how to cant and I cannot wait to see the photos. With the amount of rig tension most of us carry, this shouldn't be too much of a problem but when one considers that i have been looking at increasing rig tension to stiffen my boat, I could be in trouble.

 

And while on platform stiffness, I just want to come back on comments from Aus (Ben?).

 

I think the main construction stiffness control in a modern A is in beam size and beam to hull attachment. The trend on the new boats having as much gluing surface of beam to hull is a huge factor.
I think this misses the real issue and is a blind alley. Steve sums it up well when he discussed the stern beams s bending, torsional rigidity of the hulls and the main beam. These are not issues of gluing surface of beam to hull. In fact, I suspect that in some cases, that can be counter productyive as it simply shifts the problem from one place to another.And I should also add that I don't see any real isuses with panel stiffness as such.

 

For me, the issue is that one needs to treat the whole platform as a single monocoque. As such, I am more interested in the torsional stiffness of each hull as a complete unit. I believe that I have now identified that this single factor is contributing to over half the undesirable bend in my platform and this is why pre-preg crbon-nomex boats have the potential to be so much better.. Next, there is the issue of using round tubes for the beams. I particularly like the way that the Nikita and DNA use profiled sections to specifically increase torsional stiffness in these components. I think the days of the round beam are numbered.

 

Finally. I think one needs to pay very close attention to the way the beams join the hulls. Again, I like the thinking behind the DNA, which looks at ways of making the whole thing far more one unit, rather than gluing together 4 seperate parts. To me, the ultimate will be a platform that is built as a whole, with far more attention being paid to how the beams integrate into the hulls to work as a complete monocoque construction.

Share this post


Link to post
Share on other sites

Lots to talk about!

 

So, now I need to consider canting the rig. It seems to make sense to go with the curved foils. As has been reported, the rig is providing a significant proportion of the downforce, which is why the boat leaps up in tacks and gybes. For this discussion, how is the rig producing downforce and where is the downforce acting?

Reduce that downforce, by canting the rig, and you don't need the foil to work as hard for the same amount of lift, or you get more lift for the same drag. Just when I thought this was getting easy............

 

The first thought is that I suspect this adds to the 3D problem that Steve was talking about, as canting moves the centre of effort. So, unless I am mistaken, if you cant the rig you either have to move the boards forward or rake the rig more. Have I got that the right way around? When you cant the rig, why is it changing the rake assuming you are referring to fore and aft rake?

Then there is the issue of how to cant and I cannot wait to see the photos. With the amount of rig tension most of us carry, this shouldn't be too much of a problem but when one considers that i have been looking at increasing rig tension to stiffen my boat, I could be in trouble.

 

And while on platform stiffness, I just want to come back on comments from Aus (Ben?).

 

I think the main construction stiffness control in a modern A is in beam size and beam to hull attachment. The trend on the new boats having as much gluing surface of beam to hull is a huge factor.
I think this misses the real issue and is a blind alley. Steve sums it up well when he discussed the stern beams s bending, torsional rigidity of the hulls and the main beam. These are not issues of gluing surface of beam to hull. In fact, I suspect that in some cases, that can be counter productyive as it simply shifts the problem from one place to another.And I should also add that I don't see any real isuses with panel stiffness as such.

 

For me, the issue is that one needs to treat the whole platform as a single monocoque. As such, I am more interested in the torsional stiffness of each hull as a complete unit. I believe that I have now identified that this single factor is contributing to over half the undesirable bend in my platform and this is why pre-preg crbon-nomex boats have the potential to be so much better.. Next, there is the issue of using round tubes for the beams. I particularly like the way that the Nikita and DNA use profiled sections to specifically increase torsional stiffness in these components. I think the days of the round beam are numbered.

 

Finally. I think one needs to pay very close attention to the way the beams join the hulls. Again, I like the thinking behind the DNA, which looks at ways of making the whole thing far more one unit, rather than gluing together 4 seperate parts. To me, the ultimate will be a platform that is built as a whole, with far more attention being paid to how the beams integrate into the hulls to work as a complete monocoque construction.

 

I agree with these points about construction, but it could drive the price of the boats up significantly. A couple of years ago, a Spanish builder came out with the Balance design which seems to have disappeared(?). It had the construction approach of attaching the beams to the hulls inside the hulls to make the entire boat one intergrated structure.

Share this post


Link to post
Share on other sites

For this discussion, how is the rig producing downforce and where is the downforce acting?

The rig is a wing. Tilt the rig, the lift still comes off the rig at the same angle, relative to the rig. It's how windsurfers are able to pull the rig over on top of themselves and sail boards that would otherwise sink. Moths do it as well.

 

When you cant the rig, why is it changing the rake assuming you are referring to fore and aft rake?
You can do a system that doesn't change the rake, but what i am suggesting is that either the rake or the position of the centreboard needs to change. It's the 3D problem that Steve describes above. I am uncertain but I think that as you cant the rig, the force vector off the rig changes. However, I intend to keep the foil where I was going to put it and simply play with rake.

 

I agree with these points about construction, but it could drive the price of the boats up significantly. A couple of years ago, a Spanish builder came out with the Balance design which seems to have disappeared(?) It had the construction approach of attaching the beams to the hulls inside the hulls to make the entire boat one intergrated structure.

I am not convinced it will have a major impact on costs. I believe that this is how the DNA attaches its beams and it isn't more expensive. It needs clever design, and if I ever find the money or backer, I have a way of building an A that will produce a very stiff platform, take less time to build, solves all the problems and totally integrates the beams into the whole boat. :D

Share this post


Link to post
Share on other sites

 

This can be solved by better hounds hardware, but no one has really pursued that route.

 

SHC

 

steve, can you show us what kind of mast hounds work best?

Share this post


Link to post
Share on other sites