stinky

DC Designs

Recommended Posts

I once asked my senior hydrodynamics lecturers about the transition between the different modes. He would comfortably say that the flow detaching from an immersed stern marked the transition from displacement mode. Beyond that, there was nothing except shades of grey. What he tried to explain to me was that - if a hull is not in displacement mode, then it is planing.

 

The phrases semi-displacement and semi-planing have evolved to deal with those hulls which are optimised to exceed hull speed, but not be subject to hydrodynamic trim and so tend to be somewhat slender - such as catamarans.

 

Now what that has to do with an IC is completely up for conjecture...

 

One thing I never knew (and became too embarrased to ask) is - what is the difference between spring and rocker? Aren't they both describing curvature of the profile?

 

And finally, does anybody here know about the Rainbow class? Off memory those were also double-handed planks similar to the Skate but about 16'?

Share this post


Link to post
Share on other sites

IMO a formula board gets lots of lift off the long flexible fin at planning speeds, its really a very different sensation and dynamic to a dinghy. Does make me wonder if flexible daggerboard could be worth trying, bit like the curved cat boards but its got to flip its curvature on each tack.

Share this post


Link to post
Share on other sites

Power to weight canoes vs sailboards. I meant to say in my last post that power to weight ratio will not determine the speed which will be determined by the power to resistance ratio. Power to weight will give acceleration.

Share this post


Link to post
Share on other sites

Shooting from the hip slightly here - I think spring refers to the longitudinal curvature of the keel, and has its roots in traditional wooden boatbuilding where the keel is the first part of the vessel to be set up, and the rest of it is built around it. The keel will have less "rocker" or curvature than the planks built out from it, as described by buttocks or diagonals.

Rocker I think should only refer to surfboards and the like, where the longitudinal profile of the centreline is pretty descriptive of the entire longitudinal form of the board and various aspects of the handling and performance. Presumably the term came from putting a board on a flat floor and measuring how high the nose and tail were, and that the board coud be rocked back and forth on its bottom.

A "proper" boat can have a completely flat rocker, but loads of curvature in buttock and diagonals.

For displacement (or more accurately, non planing) craft, sectional area curve and traditional hydrostatic measures are king.

For planing craft, "rocker" is generally completely straight in the areas of the boat that are wet during planing. Plan shape, deadrise, wetted area, trim angle etc are more widely used to predict performance.

Share this post


Link to post
Share on other sites

The Jig:

post-738-065981200 1326816979_thumb.jpg

Pretty nifty looking artifact.

I hope we can build cool boats out of it.

Turns out the foam I have is not the foam I thought I had, so there will be a brief delay.

SHC

Share this post


Link to post
Share on other sites

That's very cool, Steve, and quick too!

 

How do you build the transom? It looks like the jig just stops flat.

Share this post


Link to post
Share on other sites

The transom is all forward of the last station.

So you cut at an angle according to taste, in this case 34degrees to center line.

SHC

Share this post


Link to post
Share on other sites

If not concerned about IC rules: Does the cut aid, or would the boat be as good or a touch better with the transom full width?

 

As estimation?

Share this post


Link to post
Share on other sites

The transom is all forward of the last station.

So you cut at an angle according to taste, in this case 34degrees to center line.

SHC

 

Thanks for the explanation, Steve, makes sense. One other question, if you were to press foam into the jig to shape it out, how would you keep it in shape? Are you attaching it to the jig somehow?

Share this post


Link to post
Share on other sites
If not concerned about IC rules: Does the cut aid, or would the boat be as good or a touch better with the transom full width?

I doubt there's one answer. I can readily imagine circumstances itn which it would be better and circumstances in which it would be worse.

 

On a quite different topic, I keep mulling over rudder foils, but I suspect unless you have a forward raking rudder the limitations imposed by the need to get the tiller across behind the sailing seat and to tack without tripping over the tiller means the cg would be a bit far forward.

Share this post


Link to post
Share on other sites

"For planing craft, "rocker" is generally completely straight in the areas of the boat that are wet during planing. Plan shape, deadrise, wetted area, trim angle etc are more widely used to preict performance. "

Yes but from moth hull experience a straight keel line aft of half way also makes a markedly faster boat in "forced", "transition" or "Semi diplacement" modes. It was worth a couple of knots of top speed over more rounded hulls. Definately not planing by any definition, bow down, no satagnation point under the hull, no lifting above static waterline, but lots of spray off the bow. Cat hulls do not seem to be going that way, maybe because of manoeverability particularly tacking? No excuse for canoes.

 

Steve, nice work on the mold. Is 34 deg a compromise between Chris' 45 deg rule limit and a more conventional canoe shape? At least you have all options open.

Share this post


Link to post
Share on other sites

Growing up in San Diego;(the cliffs at point loma), I surfed a Greenough spoon for years(68-72), most fun boards I ever kneed on! Always wondered how that would work on a dinghy, now that my brother is phd in naval eng, will have to ask him same ?. How small is our world.

Share this post


Link to post
Share on other sites

On a quite different topic, I keep mulling over rudder foils, but I suspect unless you have a forward raking rudder the limitations imposed by the need to get the tiller across behind the sailing seat and to tack without tripping over the tiller means the cg would be a bit far forward.

 

If the seat is that far back, just cross in front of the seat and raise the tiller high enough that it can go over the seat.

Share this post


Link to post
Share on other sites

Yes the foam will be secured to the jig.

There are various methods, hot melt glue, double sided tape, tooth picks, wire ties and of course screws. The exact technique will be developed and refined as necessary. There are enough tools in the quiver.

I am toying with various possibilities as I get more comfortable with how much we will have to munch on things. It may be that a variety of techniques will be best, one size seldom fits all and if you listen carefully, the part will often tell ypou how to build it.

SHC

Share this post


Link to post
Share on other sites

Growing up in San Diego;(the cliffs at point loma), I surfed a Greenough spoon for years(68-72), most fun boards I ever kneed on! Always wondered how that would work on a dinghy, now that my brother is phd in naval eng, will have to ask him same ?. How small is our world.

 

 

B)

 

Please?

 

Ran across George down in Santa Barbara when I was a kid- my grandparents lived down there. His inspiration helps keep my particular brand of creative lunacy going, although I only talked to him a few times. Ever see his windsurfers? They NEEDED a stainless skeg....

Share this post


Link to post
Share on other sites

Yes the foam will be secured to the jig.

There are various methods, hot melt glue, double sided tape, tooth picks, wire ties and of course screws. The exact technique will be developed and refined as necessary. There are enough tools in the quiver.

I am toying with various possibilities as I get more comfortable with how much we will have to munch on things. It may be that a variety of techniques will be best, one size seldom fits all and if you listen carefully, the part will often tell ypou how to build it.

SHC

 

Thanks Steve. I was in awe of Big D's AC when he dropped in at Park City last Summer.

Share this post


Link to post
Share on other sites
The problem with those tests was that we were unable to pull the boats from their center of effort, so they behaved pretty badly at high speed, trimming nose up by stupid amounts

Mind you that's an interesting observation (to me at least) in itself. I presume you weren't ballasting aft, so doesn't that indicate a good deal of dynamic lift from the front end of the boat? How does a C hull (you surely must have tried) behave when towed in a similar manner?

Share this post


Link to post
Share on other sites

No. All it indicated is a moment. To decide if it is lift at the front or suction at the stern we need to know whether the C of G moved up or down.

Share this post


Link to post
Share on other sites

Some random comments from someone who likes the design discusion but doesn't do ICs or DCs

 

So on the issue of the Sailboards from the 1990s, Much of the stuff you are seeing is marketing crap. I was sailing a lot of those boards back then an the primary issue was more like the snow-ski market than anything remotely related to boating. Namely they wanted you to buy a new board roughly every 2-3 years so there was an incentive to make things SEEM to have been improved but they really weren't.

 

Missing from your pictures are images of what was called a D-2 or Division 2 board. The shapes for the D2 are very similar to what you guys have designed for optimized canoies

I've got one in my back yard from the 1990s called the Crit but here is a video of a more modern one

 

 

http://www.exocet-original.com/rs-d2-elite.php

 

The thing about the D2s vs standard planing boards was that they were clearly faster up to about 10-12 knots. At which point they started having stability problems compared to planing boards. Furthermore planing boards from about 10K onwards are not actually being sailed flat, Instead you have all your weight on the weather rail essentially canting the weather chine into the water to generate increasd lateral resistance. This is something that turns out to be fast on 49ers and Musto Skiffs as well. If you can sail with 3-5 degrees of "up bubble" (ie windward heel, you climb out from boats sailing flat.

 

Now add into that sailboard equation that ideally you are carryign enough sail area to "overcant" the rig about 15-20deg to weather, this in effect generates a vertical lifting force that helps reduce wetted surface and thus drag, and you are really looking at very different hull dynamics issues than what you get with a boat that doesn't have that lift.

 

 

Now on the issue of hull shapes that are "outside planing theory" I recollect reading somewhere that the Gugeon [sp?] Bros of West System fame in all their multi-hull experimentation came up with an optimized L/W ratio of 23:1 for drag minimization. Remember that the displacement "rule of thumb" is based on the idea of a "wave train" that has its two peaks at the bow and the stern. And for traditionally shaped hulls this is the case.

 

But if you have ever watched a 100' barge being towed, there is a bow wave and a stern wave that are very distinct even at less than "displacement maximums". So in part what is going on is not so much a wave propogation (In My Ignorant Opinion) but rather the classic issue of "boundary reflection" IE when there is a discontinuity in propogation resistance, you will always have some of the "signal" reflected. IE in essence it is a second "bow wave" but the "bow" in this case is the water behind the hull.

 

So the idea of the "pintail shape" is that of "energy recovery". You can see this in fluid dynamics of pulling various slug shapes through a tube: the energy in displacing the water in the bow is recovered in the stern and thus drag is minimized. Problem is that it doesn't really work for planing hulls because the "dynamic lift" is accelerating the water vertically away from the hull even at the aftmost part of the hull and thus there is no energy recovery. You would think this would make the pintail a dodo of the shortboard world. But AGAIN there is more than meets the eye here. Remember that short boards are gybed while still planing, So what is important is the ability to have a "rail" that has a nice "smooth carve" - and pintails do this better than square tails.

 

One more note- the boards don't actually "like" flexible fins. The big move was to go to carbon fins, and you could tell the difference very quickly: non carbon fins "felt mushy" and put a carbon fin on and you could point higher and steer more cleanly. I once had the experience of going out on my carbon fin and wondering why it felt so mushy. And then I came off a wave and SNAP the fin was gone. Apparently I had delam at the hull joint that was progressively failing

 

So the point that a hull is "planing" when there is separation from the stern is I think apt. It is interesting to note that the mega-tri Groupama and some of the Mini's actually have damns to increase this separation: P9200241 http://forums.sailinganarchy.com/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=164876

 

 

And I think the reason that the Gugeon 23:1 formulation works is that they typically are dealing with water in the 45-70 deg range, and 23:1 is optimimal for that viscosity. I think that if you sail in colder water you might need to go longer -(some of the moths have found that as they get below 40 deg real cavitation becomes a problem for a class of foils.

 

 

One past point Phil about artiulating cassetes with rudder T foils. You could put the foil at the bottom the way the Moth's do, then as long as your cassette was shy of the bottom of your hull you could theoretically retract the rudder above the keel line. BTW not all of the I14s are cassette areticulated. particularly the older ones that had to be converted. The use a vertical SS rod down the blade that has a compression spring in the bottom to push it "up". Then they use a lever at the top to push down on the rod via a control line that runs to a microblock on the tiller.

 

At the bottom the rod connects into a balljoint in the leading edge of the foil and the pivot is aft of that.

Share this post


Link to post
Share on other sites

I would have thought the water leaving the transom cleanly wasn't indicative of anything other than that the boat is moving fast enough for the flow to detach from the transom. If that's planing then Lasers plane upwind.

Share this post


Link to post
Share on other sites

I would have thought the water leaving the transom cleanly wasn't indicative of anything other than that the boat is moving fast enough for the flow to detach from the transom. If that's planing then Lasers plane upwind.

Well but "detatched flow at the transom" means that you are no longer captive of the wavelength... I think the point the poster made was that this is "semi-displacement" mode.

 

BTW the point about the importance of trim is interesting because I distinctly remmember how weight forward was fast upwind in 49ers up to about 10knots of boat speed, which in theory is planing mode already.

Share this post


Link to post
Share on other sites

I would have thought the water leaving the transom cleanly wasn't indicative of anything other than that the boat is moving fast enough for the flow to detach from the transom. If that's planing then Lasers plane upwind.

Key point on Lasers "planing" upwind if our definition is weak enough.

 

Perhaps from the standpoint of high-performance sailing, it's reasonable to insist that a really major reduction of wetted area occur before we use the "planing" term.

 

Now there will still be no exact standard for how much reduction is "really major" but at least there are some situations that clearly don't qualify, such as Lasers upwind.

Share this post


Link to post
Share on other sites

Well but "detatched flow at the transom" means that you are no longer captive of the wavelength...

 

If you've got a big bow wave in front of you, from your DLR and your half angle of entry, then the stern no longer being supported by the stern wave doesn't in and of itself reduce the work of pushing through that bow wave.

 

What does aid is that per unit time, a mass of water is being accelerated downwards such that it exits the transom cleanly. At lower speeds, the force resulting from accelerating this mass of water is not so much and doesn't reduce displaced volume much.

 

At high speed, and assuming transom immersion is an appropriate amount, it is a large force and does reduce displaced volume to an extent that can be very beneficial.

Share this post


Link to post
Share on other sites

Well but "detatched flow at the transom" means that you are no longer captive of the wavelength...

 

If you've got a big bow wave in front of you, from your DLR and your half angle of entry, then the stern no longer being supported by the stern wave doesn't in and of itself reduce the work of pushing through that bow wave.

 

What does aid is that per unit time, a mass of water is being accelerated downwards such that it exits the transom cleanly. At lower speeds, the force resulting from accelerating this mass of water is not so much and doesn't reduce displaced volume much.

 

At high speed, and assuming transom immersion is an appropriate amount, it is a large force and does reduce displaced volume to an extent that can be very beneficial.

 

Good Summary. Flow separation at the transom happens well before there is any significant lift. In fact, the point at which flow becomes fully separated from a deeply immersed transom is around the point of maximum pre-planing sinkage.

Share this post


Link to post
Share on other sites

Well but "detatched flow at the transom" means that you are no longer captive of the wavelength...

 

If you've got a big bow wave in front of you, from your DLR and your half angle of entry, then the stern no longer being supported by the stern wave doesn't in and of itself reduce the work of pushing through that bow wave.

 

What does aid is that per unit time, a mass of water is being accelerated downwards such that it exits the transom cleanly. At lower speeds, the force resulting from accelerating this mass of water is not so much and doesn't reduce displaced volume much.

 

At high speed, and assuming transom immersion is an appropriate amount, it is a large force and does reduce displaced volume to an extent that can be very beneficial.

 

Good Summary. Flow separation at the transom happens well before there is any significant lift. In fact, the point at which flow becomes fully separated from a deeply immersed transom is around the point of maximum pre-planing sinkage.

Well the words "significant" is problematic to my engineering brain in that "significant" means different things to different people. Which is why I suspect that the definition of "planing" for "low spring" hulls is so hard to define. I think that you start to get benefits quite early even before there is "significant" displacement.

 

And I'm not convinced from the above explanation that the fully separated flow isn't a siignificant factor in drag reduction even before "planing" occurs.

 

Again I'm mostly going on what I've experienced sailing "pseudo-canoes" aka D2 Longboards vs "pin tail" and "swallow tail" shortboards, driving 49ers and Mustos as well as 5ohs and 420s and Lasers and the occaisional J24 at full plane.

Share this post


Link to post
Share on other sites

Well the words "significant" is problematic to my engineering brain in that "significant" means different things to different people. Which is why I suspect that the definition of "planing" for "low spring" hulls is so hard to define. I think that you start to get benefits quite early even before there is "significant" displacement.

Agreed that such wording is vague. But when something varies over a range, trying to break it into two or three parts will tend to be vague, unless there are sharp transitions. Which often there are not.

 

But if using the word "significant," then so long as there is no significant displacement reduction, I don't think there's any significant drag benefit from the dynamic lift, either. Certainly no significant reduction in wetted area, if no significant reduction in amount of water displaced.

 

Of course if the boat design has transom still immersed at lower speeds, then yes, breaking free of that will give a significant benefit. But not if the static transom immersion was a value that would be appropriate for both speeds. I would see it more as paying a penalty below that speed from having excessive immersed transom, then a benefit suddenly appearing at that speed.

 

Other than, correct immersed transom for the speed is always a good thing, with this amount being zero or near zero at low speed and increasing somewhat as speed increases.

Share this post


Link to post
Share on other sites

Some random comments from someone who likes the design discusion but doesn't do ICs or DCs

 

So on the issue of the Sailboards from the 1990s, Much of the stuff you are seeing is marketing crap. I was sailing a lot of those boards back then an the primary issue was more like the snow-ski market than anything remotely related to boating. Namely they wanted you to buy a new board roughly every 2-3 years so there was an incentive to make things SEEM to have been improved but they really weren't.

 

Missing from your pictures are images of what was called a D-2 or Division 2 board. The shapes for the D2 are very similar to what you guys have designed for optimized canoies

I've got one in my back yard from the 1990s called the Crit but here is a video of a more modern one

 

 

http://www.exocet-original.com/rs-d2-elite.php

 

The thing about the D2s vs standard planing boards was that they were clearly faster up to about 10-12 knots. At which point they started having stability problems compared to planing boards. Furthermore planing boards from about 10K onwards are not actually being sailed flat, Instead you have all your weight on the weather rail essentially canting the weather chine into the water to generate increasd lateral resistance. This is something that turns out to be fast on 49ers and Musto Skiffs as well. If you can sail with 3-5 degrees of "up bubble" (ie windward heel, you climb out from boats sailing flat.

 

Now add into that sailboard equation that ideally you are carryign enough sail area to "overcant" the rig about 15-20deg to weather, this in effect generates a vertical lifting force that helps reduce wetted surface and thus drag, and you are really looking at very different hull dynamics issues than what you get with a boat that doesn't have that lift.

 

 

Now on the issue of hull shapes that are "outside planing theory" I recollect reading somewhere that the Gugeon [sp?] Bros of West System fame in all their multi-hull experimentation came up with an optimized L/W ratio of 23:1 for drag minimization. Remember that the displacement "rule of thumb" is based on the idea of a "wave train" that has its two peaks at the bow and the stern. And for traditionally shaped hulls this is the case.

 

But if you have ever watched a 100' barge being towed, there is a bow wave and a stern wave that are very distinct even at less than "displacement maximums". So in part what is going on is not so much a wave propogation (In My Ignorant Opinion) but rather the classic issue of "boundary reflection" IE when there is a discontinuity in propogation resistance, you will always have some of the "signal" reflected. IE in essence it is a second "bow wave" but the "bow" in this case is the water behind the hull.

 

So the idea of the "pintail shape" is that of "energy recovery". You can see this in fluid dynamics of pulling various slug shapes through a tube: the energy in displacing the water in the bow is recovered in the stern and thus drag is minimized. Problem is that it doesn't really work for planing hulls because the "dynamic lift" is accelerating the water vertically away from the hull even at the aftmost part of the hull and thus there is no energy recovery. You would think this would make the pintail a dodo of the shortboard world. But AGAIN there is more than meets the eye here. Remember that short boards are gybed while still planing, So what is important is the ability to have a "rail" that has a nice "smooth carve" - and pintails do this better than square tails.

 

One more note- the boards don't actually "like" flexible fins. The big move was to go to carbon fins, and you could tell the difference very quickly: non carbon fins "felt mushy" and put a carbon fin on and you could point higher and steer more cleanly. I once had the experience of going out on my carbon fin and wondering why it felt so mushy. And then I came off a wave and SNAP the fin was gone. Apparently I had delam at the hull joint that was progressively failing

 

So the point that a hull is "planing" when there is separation from the stern is I think apt. It is interesting to note that the mega-tri Groupama and some of the Mini's actually have damns to increase this separation: P9200241 http://forums.sailinganarchy.com/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=164876

 

 

And I think the reason that the Gugeon 23:1 formulation works is that they typically are dealing with water in the 45-70 deg range, and 23:1 is optimimal for that viscosity. I think that if you sail in colder water you might need to go longer -(some of the moths have found that as they get below 40 deg real cavitation becomes a problem for a class of foils.

 

 

One past point Phil about artiulating cassetes with rudder T foils. You could put the foil at the bottom the way the Moth's do, then as long as your cassette was shy of the bottom of your hull you could theoretically retract the rudder above the keel line. BTW not all of the I14s are cassette areticulated. particularly the older ones that had to be converted. The use a vertical SS rod down the blade that has a compression spring in the bottom to push it "up". Then they use a lever at the top to push down on the rod via a control line that runs to a microblock on the tiller.

 

At the bottom the rod connects into a balljoint in the leading edge of the foil and the pivot is aft of that.

 

 

If you're referring to post 2177, there's a linky to Div 2 boards which proves your point.

 

So what's in your backyard? 650? D2? I had both. 650 was a cool but strange board in many ways, but along with the Davidsson made some sort of point about a dinghy shaped D2 board.

Share this post


Link to post
Share on other sites

There are a lot of things on a Laser a lot worse than the hull shape, after all its a lot like a small FD, forerunner of most planing dinghies. But they do not plane upwind. Downwind though a laser does have a pretty gentle transition but they still really tell you when they are planing and when they are not.

 

If you want another definition of planing my take is that its when the whole boat and not just the bow, is well above its static waterline, usually accompanied by clean transom separation and a lot of spray and foam each side amidships. There is usually an increase in apparent stability since the narrow bow is proportionally unweighted and the wider stern and midship area is carrying the boat. The bow is not necessariy raised if the boat has little spring (rocker, banana, keel curviture or whatever other tern you want to use.)

 

Back to my original argument for narrow hulls like cats, moths and some new rule ICs (and D2 boards now they have been mentioned), there is now apparent lifting of the boat and significantly less spray, even at speeds when similar (blunter) boats are obviously planing. This seems to happen long before Geogeon's 1:23 ratio is achieved, moth hulls are more like 1:10, new ICs are more like 1:7. Even stumpy 12ft skiffs plane bow in the water when two sail reaching, and go a lot faster than displacment theory says they should. So maybe power and weight have a big influence as well as L/B ratios.

Share this post


Link to post
Share on other sites

And I'm not convinced from the above explanation that the fully separated flow isn't a siignificant factor in drag reduction even before "planing" occurs.

 

It wasn't an explanation. By fully separated I mean the point, as speed increases, at which there is no longer any water contacting the transom. At this point there is often still a lot of turbulent water flowing back towards the transom. On a resistance curve, the point at which flow separates is one of (if not the) major peaks, and the resistance is higher than it would be for a hull of the same length with no transom immersion. None the less, the flow is fully separated at this point so separation itself is not an indication of planing.

 

The reason there is a peak in the resistance curve at this point is that you have just lost the benefit of resistance reduction due to the water pressing against the transom (pressure recovery), but you haven't yet gained the benefit of negative pressure reduction, which is the benefit afforded by an immersed transom at higher speeds.

Share this post


Link to post
Share on other sites
Instead you have all your weight on the weather rail essentially canting the weather chine into the water to generate increasd lateral resistance.

I find it extremely hard to believe lateral resistance is anything to do with it without serious evidence due to a couple of empirical oberservations

1) Sailing at any speed we are vastly over centreboarded anyway, because the board has to be big enough to get out of a tack. So how an earth is the horrendously inefficient hull going to take up serious work from the foil which is a good number of orders of magnitude more efficient and lightly loaded.

2) The RS300 in the UK, a sort of developed 70s moth shape without signficant chines just loves being sailed heeled to windward upwind.

3) and come to think of it so does my Nethercott, which doesn't have a lot to offer in the chine department either.

Share this post


Link to post
Share on other sites

post-2679-053068200 1326971686_thumb.jpg

A Div 2 board I made a few years ago - rounded all the way to the transom.

 

Moths were doing windward heel before the foiling thing in the late 80's, and my cherub, 29er and IC all sail best upwind with windward heel.

 

Cherub sailing at intermediate speeds upwind, there was a noticeable wake ( and speed ) difference heeling one way or the other.

leeward heel made a deep V shape hole from the lee chine, and windward heel a much flatter wake and a shallow V from the windward chine ( and was faster )

Share this post


Link to post
Share on other sites

post-2679-053068200 1326971686_thumb.jpg

A Div 2 board I made a few years ago - rounded all the way to the transom.

 

Moths were doing windward heel before the foiling thing in the late 80's, and my cherub, 29er and IC all sail best upwind with windward heel.

 

Cherub sailing at intermediate speeds upwind, there was a noticeable wake ( and speed ) difference heeling one way or the other.

leeward heel made a deep V shape hole from the lee chine, and windward heel a much flatter wake and a shallow V from the windward chine ( and was faster )

 

Nice looking hull, BTW.

Share this post


Link to post
Share on other sites

What dinghy doesn't love to heel upwind, one way or the other? Does it encourage spans flow, which is good? Tom Speer, who should know, you'd think,says to sail her flat. Does it just work when your being luffed up and going slow? It's probably VMG, but NA's tell me 20 degrees is optimum on my 40er. Granted it has an 8.5' with a big ol' honking bulb keel, but a dinghy? With all my moveable weight? Or even worse, railing up ( or vanging the lift ofvthe foil) of a D2 hull, while in lining the rig to windward? And the tell tails still point straight back. And the hull squirts to windward. I've looked at all the schematic drawings in design texts, but deep down, I don't get it.

What dinghy doesn't love to heel upwind, one way or the other? Does it encourage spans flow, which is good? Tom Speer, who should know, you'd think,says to sail her flat. Does it just work when your being luffed up and going slow? It's probably VMG, but NA's (like all the guys in Bob Perry's design office that drew her) tell me 20 degrees is optimum on my 40er. Granted it has an 8.5' draft and the fin has a forward sweep with a big ol' honking bulb keel, but a dinghy? With all my moveable weight? Or even worse, railing up (or 'vanging' the lift of the foil) of a D2 hull, while Inclining the rig to windward? And the tell tails still point straight back. And the hull squirts to windward. I've looked at all the schematic drawings in design texts, but deep down, I don't get it. I mean look at the cat at the top of this page, it's not heeling. Much.

 

Rant over.

 

 

I meant span wise flow. Why does an edit turn into a reply to your own post? Aaarrrgh. Add to that out of control spellcheck...

Share this post


Link to post
Share on other sites

Then there's Matt Layden's chine runner concept. Not legal on an IC.

Share this post


Link to post
Share on other sites
Instead you have all your weight on the weather rail essentially canting the weather chine into the water to generate increasd lateral resistance.

I find it extremely hard to believe lateral resistance is anything to do with it without serious evidence due to a couple of empirical oberservations

1) Sailing at any speed we are vastly over centreboarded anyway, because the board has to be big enough to get out of a tack. So how an earth is the horrendously inefficient hull going to take up serious work from the foil which is a good number of orders of magnitude more efficient and lightly loaded.

2) The RS300 in the UK, a sort of developed 70s moth shape without signficant chines just loves being sailed heeled to windward upwind.

3) and come to think of it so does my Nethercott, which doesn't have a lot to offer in the chine department either.

Thanks IC for the thoughts on the flow separation. I'm still trying to sort out what "planing" really is beyond the classic "climb up the bow wave"... and "dynamic lift" ... precisely because of my experiences on boats like the I-14 and 49er. Both of which are exceeding "hull speed" upwind without major changes in attack angle - but yes with some trim changes (ones matching the earlier plot almost exactly.

 

Now one of the curious things I noticed was that sailing an older I-14 hull (A Bieker-3 I think, might have been a B2) there was a point at about 1.5 strings (roughly 10 knots) where both boats are still looking for more power, but in which the 49er just seems to keep powering up and going faster proportionaly and smoothly with the wind. but the I-14 hull (with T-foil and we played with the foil and the sail trim and crew positions) seemed to "get molassased" IE We'd add power but the boat didn't smoothly increase speed. I figured this was the "hump" but couldn't figure out how to get over it.

 

The only time I've experienced that on the 49er was at the Melbourne worlds where we had desert heated air mixing with over-water breeze. We didn't solve it, but one of the other team boats addressed it by pulling up the CB about 3" as though they were overpowered. WS was around 12 knots, but the boat just felt stuck and then overpowered. Thinking back on it perhaps it was the mixing of the air so that the steady breeze literally had less pressure in it (and hence less driving power) and the puffs would have the cooler denser air. And by pulling up the board you allowed for a tighter trim that gave enough power to plane in the steady breeze but with decreased board lift, when the denser puff hit, you didn't dip the leeward wing.

 

But that's the only time I've ever felt the 49er hull "stick". (choppy 2 directional wave pattern as well).

 

But in both of those cases, we had full solid separation from the hull and were exceeding hull speed by a non-trivial amount. (80%-120%)...

 

 

And I know those aren't canoes but they are both easily driven and light hulls.

 

 

As to the lateral resistance, remember one of the arguements about what jibing CB's do is that they increase the lateral resistance of the hull by increasing its angle of attack trough the flow relative to the CB. So I'm not convinced its "trivial". Remember that "short boards" don't use any daggerboard whatsoever and on my old Windsurfer One Design, we wanted to be able to reduce the size of the Daggerboard at any speeds of wind over 12 knots because it was drag and caused the "rail up " described...(though the D2 was WAY scaryier at that).

Share this post


Link to post
Share on other sites

 

 

 

 

As to the lateral resistance, remember one of the arguements about what jibing CB's do is that they increase the lateral resistance of the hull by increasing its angle of attack trough the flow relative to the CB. So I'm not convinced its "trivial". Remember that "short boards" don't use any daggerboard whatsoever and on my old Windsurfer One Design, we wanted to be able to reduce the size of the Daggerboard at any speeds of wind over 12 knots because it was drag and caused the "rail up " described...(though the D2 was WAY scaryier at that).

 

 

Oh no, gybing boards again! You'd better mean gybing boards DECREASE the hulls lateral resistance or you've got some explaining to do.

 

I could see a couple of reasons why heeling to weather could be faster on some boats. Like maybe it reduces tip loss on the sails. Or, if you picture your (non gyber board!) hull making 2 - 4 degrees of leeway maybe it presents the hull at a better angle to the flow. For me it just makes the seat hit the wave tops.

Share this post


Link to post
Share on other sites

Kite race boards are 'boat-shaped' and use a big fin vertically, kite fun boards heel well to windward but just use the rails and a tiny fin ( each end ) and push water sideways and up.

Kite speed boards are more like the fun boards, - but they only do it in force 8 +

Share this post


Link to post
Share on other sites

 

 

 

 

As to the lateral resistance, remember one of the arguements about what jibing CB's do is that they increase the lateral resistance of the hull by increasing its angle of attack trough the flow relative to the CB. So I'm not convinced its "trivial". Remember that "short boards" don't use any daggerboard whatsoever and on my old Windsurfer One Design, we wanted to be able to reduce the size of the Daggerboard at any speeds of wind over 12 knots because it was drag and caused the "rail up " described...(though the D2 was WAY scaryier at that).

 

 

Oh no, gybing boards again! You'd better mean gybing boards DECREASE the hulls lateral resistance or you've got some explaining to do.

 

I could see a couple of reasons why heeling to weather could be faster on some boats. Like maybe it reduces tip loss on the sails. Or, if you picture your (non gyber board!) hull making 2 - 4 degrees of leeway maybe it presents the hull at a better angle to the flow. For me it just makes the seat hit the wave tops.

Sorry I stated that backwards on the gybing boards... careless of me.

 

As for "heeling to weather" - In windsurfers it very much is about using the lift to reduce wetted surface (think waterstarts) so that's going to not apply in a boat that's only heeling 5 or so degrees to weather. Nor do I see how 5deg heel would help with tip loss (Moths also don't count for obvious reasons).

 

I DO KNOW that 5 degrees of weather heel on a boat like a J-24 is SLOW... but it also is tacitcally useful off the line, because it lets you project the redirected flow of the tip into the boat to weather of you in turn sucking them down into your leebow. but otherwise it is slow - mainly I suspect because the weight of the sailcloth starts to fall inwards at the low speeds you can do this at.

 

But I've never heard of 5ohs trying to heel to weather for better speed, Only boats with hard weather chines that have an "upward" curvature to the chine.

Share this post


Link to post
Share on other sites

Most CAD programs can create a flat pattern if the surfaces aren't distorted by other means than conventional engineering practices e.g folding and rolling beyond that they tend to fall over...

 

Ah the reverse engineer - nice! I've templated core before by taking a quick splash from a mould, cutting it up and moving it around flat sheet until the entire boundary has been captured. Similar thing I guess.

I know that you can "unroll" developable surfaces in rhino, but there must be a way of doing it with compound surfaces. Will have a think...

 

Pretty sure that most of the 14s that bieker built himself were largely from flatstock - the topsides anyhow, the lower hull was stripped in foam over a male frame. Pretty low cost approach!

Share this post


Link to post
Share on other sites

I think it's time to throw in the comment that I (obviously) don't know what's planing but I know what i like :P (IC's?)

 

The fact that we can produce almost a page a day for over two weeks now (with stuff that blows my mind) and still nearly be on track with the original post (nearly 2300 ago) says that we're an eclectic lot and I love it!

Share this post


Link to post
Share on other sites

As to the lateral resistance, remember one of the arguements about what jibing CB's do is that they increase the lateral resistance of the hull by increasing its angle of attack trough the flow relative to the CB. So I'm not convinced its "trivial". Remember that "short boards" don't use any daggerboard whatsoever and on my old Windsurfer One Design, we wanted to be able to reduce the size of the Daggerboard at any speeds of wind over 12 knots because it was drag and caused the "rail up " described...(though the D2 was WAY scaryier at that).

 

 

Oh no, gybing boards again! You'd better mean gybing boards DECREASE the hulls lateral resistance or you've got some explaining to do.

 

I could see a couple of reasons why heeling to weather could be faster on some boats. Like maybe it reduces tip loss on the sails. Or, if you picture your (non gyber board!) hull making 2 - 4 degrees of leeway maybe it presents the hull at a better angle to the flow. For me it just makes the seat hit the wave tops.

Sorry I stated that backwards on the gybing boards... careless of me.

 

As for "heeling to weather" - In windsurfers it very much is about using the lift to reduce wetted surface (think waterstarts) so that's going to not apply in a boat that's only heeling 5 or so degrees to weather. Nor do I see how 5deg heel would help with tip loss (Moths also don't count for obvious reasons).

 

I DO KNOW that 5 degrees of weather heel on a boat like a J-24 is SLOW... but it also is tacitcally useful off the line, because it lets you project the redirected flow of the tip into the boat to weather of you in turn sucking them down into your leebow. but otherwise it is slow - mainly I suspect because the weight of the sailcloth starts to fall inwards at the low speeds you can do this at.

 

But I've never heard of 5ohs trying to heel to weather for better speed, Only boats with hard weather chines that have an "upward" curvature to the chine.

 

Heeling any leadmine to weather (to windward) will be slow for obvious RM generation reasons.

I think that rather than the chined-ness, heeling to weather on the whole is good on boats that don't develop nasty assymetry of the waterplane at heel (accentuating the effect of dragging the hull crab style through the water) so in real terms this means narrow boats (canoes, lowrider moths, RS300) boats with pintail or canoe sterns (canoe, lowrider moth). Obviously another prerequisite is that the RM needs to be generated in such a way that the crew arent hittiong the water.

Another aspect is that it'll work better in a boat where the windward heel will ameliorate a slight excess of weather helm at level heel. Having no weather helm is pretty slow - means the dagger is being worked too hard, and the rudder is just wasted wetted surface.

Share this post


Link to post
Share on other sites

So many definitions of planing. All different and all trying to capture some aspect of what we see the boat doing. If we go back to basics what we are trying to do is understand why the speed of some designs doesn't just keep increasing with increases in wind speed ( power), or increases more slowly than other. One point about some of the discussions so far. Some have mentioned a peak in the resistance- speed curve. This is incorrect there is no such peak. The peak is in the slope of the curve. I guess most know this but it may be misleading to others. I find the talk of energy recovery and pressure recovery confuses the issue. A streamlined stern reduces the expenditure of energy, i don't think it recovers energy from the flow (convince me if I'm wrong). It is much easier if we talk about forces. For any hull which has a waterline which narrows toward the stern, the hull effectively accelerates the water inwards and forward in the direction of motion. This creates a reaction force on the hull which is a part of the resistance.

 

Share this post


Link to post
Share on other sites

So many definitions of planing. All different and all trying to capture some aspect of what we see the boat doing. If we go back to basics what we are trying to do is understand why the speed of some designs doesn't just keep increasing with increases in wind speed ( power), or increases more slowly than other. One point about some of the discussions so far. Some have mentioned a peak in the resistance- speed curve. This is incorrect there is no such peak. The peak is in the slope of the curve. I guess most know this but it may be misleading to others. I find the talk of energy recovery and pressure recovery confuses the issue. A streamlined stern reduces the expenditure of energy, i don't think it recovers energy from the flow (convince me if I'm wrong). It is much easier if we talk about forces. For any hull which has a waterline which narrows toward the stern, the hull effectively accelerates the water inwards and forward in the direction of motion. This creates a reaction force on the hull which is a part of the resistance.

 

Think of energy recovery this way... Imagine if you were towing a slug through a pipe. ahead of the slug is just air. as the slug passes any point in the pipe, a microvalve opens up and the pipe fills up laterally with water. So in essence the water has horizontal accel towards the center of the pipe.

 

When that water hits the back half of the slug, what is the net force effect on that slug?

Share this post


Link to post
Share on other sites

BalticBandit- "Think of energy recovery this way... Imagine if you were towing a slug through a pipe. ahead of the slug is just air. as the slug passes any point in the pipe, a microvalve opens up and the pipe fills up laterally with water. So in essence the water has horizontal accel towards the center of the pipe.

When that water hits the back half of the slug, what is the net force effect on that slug? "

Again not sure that the situation is the same. Why does the pipe fill with water? There seem to be two possibilities1). The water is pumped in, in which case energy is supplied from outside the system, and this is not the same as water flowing in behind a moving boat.Or2) the slug pulls the water in by suction, in which case the slug will have a force in the opposite direction to it's motion, trying to slow it down. This is more like the case of a boat moving through a fluid.

Share this post


Link to post
Share on other sites

But water will fill the hole created by the the hull. Conversely, the hull will dig a hole in the water. And if the hull is going faster than the speed water can move away from it, or come back to it, what happens?

 

It's a gravity interface. Water is incompressible. It's sticky at the interface. Pressure is modified by waves at the interface. D'Amberlot's paradox only exists in a perfect fluid of zero viscosity. The hull has friction and shape.

 

And then there's design opinion.....

 

Knock knock knocking at heaven's door?

 

:)

Share this post


Link to post
Share on other sites

But water will fill the hole created by the the hull. Conversely, the hull will dig a hole in the water. And if the hull is going faster than the speed water can move away from it, or come back to it, what happens?

 

Well isn't this why high speed hull shapes tend to have squared off sterns? You can't get pressure recovery past a certain point, so why drag the weight around? Which is why a pintail ceases to have advantages once you are "planing" (whatever that means at this point). You have pintail shapes on wind surfers but for different reasons - having more to do with steering using the hull than because they are particularly faster. And as has been pointed out for the newer boards like the kite RACE boards and the "course boards" - which spend most of their time in a straight line optimizing speed, they've largely gone back to squared off sterns.

Share this post


Link to post
Share on other sites

Low-rider moths evolved to pintails with T-foil rudders.

The moth is very short, so immersed transoms were a problem as the boats got narrower and low rocker, but by using a pintail the immersed part could be minimised at low speeds - less wetted surface and also a smaller 'hole' in the water, and at high speeds the T-foil provided the same sort of support as a wide transom - but actually better because the T-foil controlled the trim angle and reduced pitching.

 

Didn't work quite so well on my IC, being 50% longer :unsure:

Share this post


Link to post
Share on other sites

A fat tail is physically bigger than pintail. Its easier to make a small hull lighter than a big hull. The smaller hull can have a flat bottom. The bigger hulls have been tending to more shape athwarthships. So less lift and more skin friction for the big guys and more lift and less skin friction for the little guys? Per square inch of bottom.

 

Hey Andy, i'm restating your point somewhat. Sorry I was posting when you were posting.

Share this post


Link to post
Share on other sites

Low-rider moths evolved to pintails with T-foil rudders.

The moth is very short, so immersed transoms were a problem as the boats got narrower and low rocker, but by using a pintail the immersed part could be minimised at low speeds - less wetted surface and also a smaller 'hole' in the water, and at high speeds the T-foil provided the same sort of support as a wide transom - but actually better because the T-foil controlled the trim angle and reduced pitching.

 

Didn't work quite so well on my IC, being 50% longer :unsure:

Well there are kiteboards out there that have a TFoil on a 1 meter skeg, http://www.bing.com/images/search?q=foiler+kiteboard&view=detail&id=317D7BEF9C94321C5BE28ADB83991A41532E0EDE&first=0&FORM=IDFRIR But I don't think they are as fast at the top end as just essentially a waterski IE just enough surface area to float via dynamic lift when at speed and then a skosh extra to let you get launched... but at that point you are into drag minimization.

Share this post


Link to post
Share on other sites

oh, the pontificating !

get to work boys, experiment !

 

 

something I remember, transitioning from OK's to Lasers, was how stable the OK was when fast planing, while the more rounded Laser got more and more unstable the faster I went, as the hull lifted further out of the water. combined with the teeny rudder, the Laser hull remains the most difficult to keep under the rig I have sailed in survival conditions.

the Tasar I had later was fantastically stable at warp speed, winning my personal championship for the best high speed sailing hull. the spaghetti rig sucked ass though.

my Maas hull is also very stable at warp speed, unless I am dumb enough to leave the gyber active off the wind. going fast with 2 rudders is tricky.

back to snowboarding in excellent pow.

 

cheers, Kenny

 

Share this post


Link to post
Share on other sites

Amati. - "But water will fill the hole created by the the hull. Conversely, the hull will dig a hole in the water. And if the hull is going faster than the speed water can move away from it, or come back to it, what happens?"

The two situations, at the bow and at the stern, are subtly different. At the bow the boat is pushing the water away, the water is under positive pressure. At the stern the boat is pulling the water into the hole, the water is under negative pressure. For our purposes the water can be pushed with any acceleration limited only by the force applied, there is no 'faster than speed water can move away'. At the stern things are different, if you try and 'pull' the water into the hole too quickly you will reduce the pressure to the point when the water will cavitate. This is equivalent to reducing the boiling temperature of the water to the ambient temperature, so there does become a 'faster than the speed water can come back in'. Indeed i understand that this is one of the reasons that for planing powerboats the immersed volume of the hull doesn't decrease towards the stern.

Share this post


Link to post
Share on other sites

Do canoes reach these speeds?

 

Plus a finite sail area, and apparently a wind limit for racing.

 

As I stare at things out in the shop, right now I'm pondering WS vs planing area. With a pintail it seems reducing WS is accomplished best by heeling, while with the big diamond tail it is accomplished best by trimming down at the bow?

Share this post


Link to post
Share on other sites

At 2 bar of pressure, there can be cavitation from sharp hull geometry changes at around 60 knots ( http://proceedings.f...cuments/514.pdf ). This is with respect to submerged craft. This was not a sharp value where cavitation suddenly appeared, but is an example speed (actually 30 m/s) where it was a substantial effect.

 

At 1 bar (atmospheric), I don't know what the relation would be... perhaps 70.7% the speed as a guess? As that would be half the kinetic energy.

 

In that case, somewhere around 40 knots?

 

I had never seen cavitation raised as a concern with regard to the hulls of sailing vessels. I would attribute bubbles to entrained air.

Share this post


Link to post
Share on other sites

I don't think (but i don't know) that sailing boats suffer from cavitation around the hull, but has been discussed in papers on planing theory applied to power boats. I was giving the extreme case to make a point as to how different the bow and stern are. More likely would be the effect of vortex formation at the stern if the hull has a lot of curvature. These would also extract energy from the boat and require power for their maintenance.

Share this post


Link to post
Share on other sites

Re Chris's sucky time, and me wondering how far back max beam can be before sudden curvature leads to same, if you haven't seen it before,

scroll down to plans. Mr. Storer has sailed skiffs, so he has an interesting perspective, and some of the linkys have nice pics. Perspective at any rate. Got me crazed about an aero battened dipping lug for weeks. :blink: Some things about the planform work. Details though, details. His goat skiff has an interesting rig...

 

http://www.storerboatplans.com/Beth/beth.html

Share this post


Link to post
Share on other sites

I don't think (but i don't know) that sailing boats suffer from cavitation around the hull, but has been discussed in papers on planing theory applied to power boats. I was giving the extreme case to make a point as to how different the bow and stern are. More likely would be the effect of vortex formation at the stern if the hull has a lot of curvature. These would also extract energy from the boat and require power for their maintenance.

That's true that there are different extreme boundary parameters, but if you've got a DC or skiff hull that is hitting those speeds, you need to contact Mr. Ellison as he would make you a rich man.

 

OTOH the only dinghy's I've heard of dealing with actual trailing edge cavitation are the foilers sailing in 5oC and below conditions (there's a thread about that around here somewhere). Now for a 6KSB, the "pin tail" shape makes sense for a couple of reasons: heeled over it lengthens waterline and when "flat" it does get energy recovery.

 

In this case the acceleration vector in the aft section is provided by gravity accelerating the water laterally. We cannot fully extrapolate the "slug in a pipe" because the slug is fully immersed whereas the hull is operating in boundary conditions where absent gravity water would not need to flow to level in the hull's wake, IE you could very well leave a trough behind the hull. you also have the bernoulli effect of the hull shape as well providing acceleration towards the center. But remember that F is a SQUARE function in time of distance so the shorter the distance (ie the longer the chord from max thickness to TE) accelerated the shallower the slope of that power function and thus the net force from parasitic drag is reduced.

[speculation]

So it seems to me that a pintail works fine for slower boats precisely because you minimize parasitic drag and mazimize energy "recovery". And this of course is modulated by the viscosity of water, and hence will have an optimal ratio. of length to width.

 

But once you get to higher speeds the gravitational accel is not fast enough so parasitic drag starts to dominate. And hence squared off sterns have lower CDs... We see this in cars as well. The (then) funny looking Honda CRX had an intentionally chopped off tail because wind tunnel testing showed that to have a lower CD at speed than a more teardrop shape. Whereas low-speed vehicles like the Fuel Efficiency champs and solar vehicles and human powered airplanes are all operating in that much lower realm and tapered sterns work.

[/speculation]

 

And I might be full of it since I'm kinda thinking out loud here.

Share this post


Link to post
Share on other sites

So how would you differentiate theoretically between a pin tail and a fat 45 degree diamond tail, which seem to be the two extremes allowed by the IC rule?

 

 

How did Steve (?) put it, canoes have pointy ends?

Share this post


Link to post
Share on other sites

So how would you differentiate theoretically between a pin tail and a fat 45 degree diamond tail, which seem to be the two extremes allowed by the IC rule?

 

 

How did Steve (?) put it, canoes have pointy ends?

 

Well it would seem to me that the diamond tail would have speed advantages at "forced" condition point and above, and the pintail at speeds below that, but I don't know enough about the actual hull dynamics to really know. I've hard Paul Bieker explain that the reason he likes designing I-14s is that they are challenging because unlike something like the 49er or the 18, they are too short to have adequate floatation (I think I'm getting this right, this is from memory so I'm likely wrong) to without generating too much drag in light conditions, but getting that floatation into the design without compromising planing abilities is challenging.... - at least that's the gist of what I understood him to be explaining.

 

And earlier someone was talking about how tacking was problematic with certain canoe designs that I would not have expected to have problems, so clearly there is a helluva lot more going on than i have a clue about.

Share this post


Link to post
Share on other sites

BalticBandit - "In this case the acceleration vector in the aft section is provided by gravity accelerating the water laterally. We cannot fully extrapolate the "slug in a pipe" because the slug is fully immersed whereas the hull is operating in boundary conditions where absent gravity water would not need to flow to level in the hull's wake, IE you could very well leave a trough behind the hull. you also have the bernoulli effect of the hull shape as well providing acceleration towards the center. But remember that F is a SQUARE function in time of distance so the shorter the distance (ie the longer the chord from max thickness to TE) accelerated the shallower the slope of that power function and thus the net force from parasitic drag is reduced."

I'm still not convinced that energy can be 'recovered' from the water flowing around the hull. I do agree that the slug analogy doesn't fully model the boat sailing at a water/air interface. I didn't follow your argument, but didn't think that gravity has much to do with it except astern of the boat. Gravity produces forces vertically downwards only. If you mean that it produces a higher pressure away from the hull, then this is possible. Perhaps we agree that the pressure at the surface of the hull is less than that away from the hull. Even in your scenario this is the only way to get horizontal water movement. However it is the boats movement reducing the pressure adjacent to the hull which produces the pressure difference and so the flow of water.

Perhaps we could do a thought experiment, and move the boat very slowly such that the water surface remained essentially flat. Remember that i can specify this speed as near zero as i like and however close i make the speed to zero the water would still have to flow in behind the boat, but there would be no way gravity could act to produce a force. It is the fact that a void cannot occur between the hull and the water that ensures that the water flows in behind the boat. This is ensured by the forces between the molecules of the hull and those of the water, the water molecules pull the hull back and out and the hull pulls the water in and forward. Your point about what would happen in the absence of gravity is well taken, and I entirely agree that it is gravity which is affecting the water astern of the boat and filling any wake effects.

Share this post


Link to post
Share on other sites

I think I can see, ARE, how on your slow moving boat the energy it takes to push the water aside would be (completely?) "recovered" by the water flowing back together over the after part of the hull. So at low speeds form drag is near nil. But we're going faster than that one would hope which makes me think that IC Nutter is right - that there is a speed, I'm going to guess about 5kts, at which you transition from energy recovery to "suck". I'm going to say it again: On ICs you need to design to minimize suck in the aft section and the drag from wind waves. Ignore wave making.

Share this post


Link to post
Share on other sites

I think I can see, ARE, how on your slow moving boat the energy it takes to push the water aside would be (completely?) "recovered" by the water flowing back together over the after part of the hull.

 

I wonder if that's a a tad simplistic Chris. For all that water is theoretically virtually incompressible, It seems to me that for our purposes it compresses just fine - upwards into the air. And as soon as its pushed up into the air and into waves the energy in those waves must surely be scattering and so at least some of it is unavailable for pressure recovery. There are a lot of complicated things going on and, rightly or wrongly, I can't get my head into ignoring those waves, losing energy through internal friction and scattering it round the environment...

 

You know if you watch a boat in a canal you see a big mound of water pushed up in front, and you see a rapid accelleration of water alongside the boat in the reverse direction of travel as that mound is pushed back by gravity to fill the hole behind the boat. And then it's not just a random heap, it all goes into circulation systems, standing waves relative to the boat.

 

What I can't do is to get straight in my head what I really want that water to do... I don't like the idea of pushing a lump of water in front of me, but if there isn't a lump of water in front of me there must be an increased flow round the boat in the reverse direction as the water rushed to the stern. So the skin friction isn't that of the 4 knots to boat is travelling, its a lot more than that...

 

Too hard for me at this time of the night/morning!

 

tacking was problematic with certain canoe designs that I would not have expected to have problems, so clearly there is a helluva lot more going on than i have a clue about.

Well, there are various different tacking problems and it depends on technique as well. I'm not flexible and fast like some of these guys, so I run round behind the boom rather than duck under it. I'm heavy too so that means the finer the stern the deeper it goes under the water and the more the brakes go on, but on the other hand if you're light then the water slips past the fine stern better, maybe less brakes...

Then there's just the plain problem of stability, its suprisingly easy to fall off mid tack if you lose your balance. Then coming out of the tack, well those of us that aren't quick enough to just leap put on the plank the right amount may have to pause for a second or two standing on the gunwhale, and beam means righting moment and gets you going faster, although the weight of the plank helps.

Share this post


Link to post
Share on other sites

Well, as an utterly amateur and quick look at it, if we mostly push the water forward, then there will tend to be a heap of it (or some amount anyway) in front of us, which can't be good.

 

If on the other hand we tend to push it more to the sides (narrower half angle of entry), then that would seem better in that regard.

 

With regard to filling in the moving hole, that's going to happen in any case and it's not obvious we have any control over it or whether it matters, except that the more waves that are created, the more energy must be lost to do that, as you point out.

 

Whether for a given half angle of entry (or length to beam ratio; I don't know which is the better way to look at it) and DLR and considering hulls that are good designs, one can really do anything much about further reducing wave-making drag, I don't know and I'm happy to stipulate Chris's position that this is not the place to look.

Share this post


Link to post
Share on other sites

I'm certainly not saying that waves aren't created or that the creation of those waves doesn't slow the boat. I'm just saying that there are more important things to consider when designing a narrow, light, fast hull that spends it's life transitioning through a wide range of speeds. It may well be that the hull that shows the least resistance to wind waves upwind or that accelerates easily on each wave or with each gust downwind also makes the smallest waves but I wouldn't count on it, or let wave making influence a hull design.

Share this post


Link to post
Share on other sites

OK Lets try this thought experiment on gravity as the accelerating vector for water flowing in behind the hull:

 

Consider a cylinder with a disconnectable cover on the bottom. We insert this cylinder into the water vertically. Now - because this is a thought experiment we can do this - we instantaneously remove the cylinder, leaving the bottom in place so that no water can come up from below. What FORCE causes the water surrounding the column of air to move laterally?

 

It is gravitational force pushing/pulling down on the surface and that translates into lateral deflection just as if you take a steel post, put it in a press and push on its ends. Its just much more dynamic since you are talking about a fluid.

 

Parasitic drag - which is part of bernoulli's effect - only occurs when the lateral acceleration necessary to fill the void is GREATER than that which gravity can affect.

 

 

 

The idea that we 'compress water upwards' isn't accurate either. Because there is no increase in pressure. We simply displace it upwards, and in turn displace the air above it.

 

 

[speculation]

Perhaps this is it though - again thinking out loud. Hulls that create a significant bow wave - ones with lots of "spring" are displacing ever increasing amounts of water vertically against gravity until they "pop up onto" a plane (ie the dynamic lift is enough to effectively eliminate a significant amount of this vertical displacement.

 

Whereas hulls that are finer in their entry, and thereby accelerate the water laterally rather than laterally AND vertically, lack this 9.8mS2 component of force required

 

[/speculation]

Share this post


Link to post
Share on other sites

Baltic Bandit -Consider a cylinder with a disconnectable cover on the bottom. We insert this cylinder into the water vertically. Now - because this is a thought experiment we can do this - we instantaneously remove the cylinder, leaving the bottom in place so that no water can come up from below. What FORCE causes the water surrounding the column of air to move laterally?

For this thought experiment you are perfectly correct. It is the water pressure which generates the water movement and this is due to gravity. Hence you could say that gravity is the driving force. However this is very unlike a boat moving thro' the water. In your experiment you insert the cylinder vertically, where does the energy for this process come from? When a boat moves thro the water we never see a 'hole' adjacent to the boat. Consider a thought experiment which, I think, is identical to yours. Consider two hull shaped surfaces, initially in the same place and let one of them move forward, leaving the other in it's original place, so there is a gap between them.( this is essentially the same as your cylinder immersed in the water, except for the shape). Next we remove the surface that didn't move. The water flows in to fill the gap under the action of gravity. This is exactly right in the thought experiment, and would be exactly right if we could do the experiment in the real world. I have absolutely no problem with it. Again in this thought experiment we have neglected the actions needed to produce the void below the water surface. To move the surface which we have associated with the boat and create a void we would need to apply a very large force to the surface. this is equivalent to saying that a lot of energy is needed to move the hull forward. To create such a void is the same as cavitation and is very energy expensive.Even if you stipulate that your cylinder is neutrally buoyant you stil have to do work ( supply energy) for the experiment. The cylinder will be neutrally buoyant at only one depth. To move it to this depth we have to apply a force and move it though a distance.

Share this post


Link to post
Share on other sites

 

I wonder if that's a a tad simplistic Chris. For all that water is theoretically virtually incompressible, It seems to me that for our purposes it compresses just fine - upwards into the air. And as soon as its pushed up into the air and into waves the energy in those waves must surely be scattering and so at least some of it is unavailable for pressure recovery. There are a lot of complicated things going on and, rightly or wrongly, I can't get my head into ignoring those waves, losing energy through internal friction and scattering it round the environment...

 

The way I like to think about this is that although water is an incompressible fluid, the free surface allows free vertical movement. This make the fluid, near the free surface, quasi-compressible in two dimensions.

 

Viscous effects, the internal friction in the fluid, is certainly a component of residual resistance, albeit a small one.

Share this post


Link to post
Share on other sites

CMaas - I think I can see, ARE, how on your slow moving boat the energy it takes to push the water aside would be (completely?) "recovered" by the water flowing back together over the after part of the hull. So at low speeds form drag is near nil. But we're going faster than that one would hope which makes me think that IC Nutter is right - that there is a speed, I'm going to guess about 5kts, at which you transition from energy recovery to "suck". I'm going to say it again: On ICs you need to design to minimize suck in the aft section and the drag from wind waves. Ignore wave making.

I think I must have worded me post very badly, as you took exactly the opposite meaning to that intended. In the example I was trying to avoid the complications of the wave pattern set up by the boat, and so avoid adding gravity as a possible source of energy. The boat pulls the water in around the stern, and this takes energy. So that energy is lost thro pushing the water apart at the bow and pulling it in at the stern. No recovery.

Share this post


Link to post
Share on other sites
The boat pulls the water in around the stern

!! I tend to think of pulling water to be akin to pushing rope. Semantics I guess.

Trouble is its all too easy for these sorts of discussions to be about the use and understanding of technical english amongst the partipants rather than enhanced understanding of what's actually happening. Especially of course, if we invoke the pl***ng word:-)

Share this post


Link to post
Share on other sites

Not wanting to go round in circles, but... Planing is simply water hitting the underside of the hull presented at an angle (typically 2-3 degrees). That water is deflected down and so there is a force pushing the boat up. Simultaneously as water flows over the convex curvature of a hull bottom (more curvature = more rocker) there is a suction force pulling the boat down. With little enough rocker, a nice wide and flat hull and low weight, the planing forces will be greater than the suction forces an the boat will rise. There's nothing magic or especially complicated about it.

 

On the subject of transom designs. I would have thought that pintails only really worked on deeply submerged, low rocker, transoms. Or on short boats where you are trying to keep the bow from burying. Not sure they would be so good on a canoe.

Share this post


Link to post
Share on other sites

Not wanting to go round in circles, but... Planing is simply water hitting the underside of the hull presented at an angle (typically 2-3 degrees). That water is deflected down and so there is a force pushing the boat up. Simultaneously as water flows over the convex curvature of a hull bottom (more curvature = more rocker) there is a suction force pulling the boat down. With little enough rocker, a nice wide and flat hull and low weight, the planing forces will be greater than the suction forces an the boat will rise. There's nothing magic or especially complicated about it.

 

On the subject of transom designs. I would have thought that pintails only really worked on deeply submerged, low rocker, transoms. Or on short boats where you are trying to keep the bow from burying. Not sure they would be so good on a canoe.

 

Seems like we already know what is fast. Or Chris does anyway.

 

But the pintail shape evolved when the boats were heavier. And were paddled.

 

Which reminds me that one pretty direct way to figure out how much resistance a hull has (at least at low sucky speeds) is to paddle it. Maybe we should reinvoke paddling races to learn a little more about the subject?

 

It's a lot more work than typing, but then most things are.

Share this post


Link to post
Share on other sites

 

Seems like we already know what is fast. Or Chris does anyway.

 

But the pintail shape evolved when the boats were heavier. And were paddled.

 

Which reminds me that one pretty direct way to figure out how much resistance a hull has (at least at low sucky speeds) is to paddle it. Maybe we should reinvoke paddling races to learn a little more about the subject?

 

It's a lot more work than typing, but then most things are.

 

Paddling a canoe?! That's how it all started...

Share this post


Link to post
Share on other sites

Don't forget heel affects the weather helm. Heeling to windward reduces helm because the turning moments become more balance around the centreboard (generally). This means there is no need to use the rudder, hence no rudder drag.

 

Just another item to consider in the grand scheme of making dinghies go faster upwind.

Share this post


Link to post
Share on other sites

OK Lets try this thought experiment on gravity as the accelerating vector for water flowing in behind the hull:

 

Consider a cylinder with a disconnectable cover on the bottom. We insert this cylinder into the water vertically. Now - because this is a thought experiment we can do this - we instantaneously remove the cylinder, leaving the bottom in place so that no water can come up from below. What FORCE causes the water surrounding the column of air to move laterally?

 

It is gravitational force pushing/pulling down on the surface and that translates into lateral deflection just as if you take a steel post, put it in a press and push on its ends. Its just much more dynamic since you are talking about a fluid.

 

Parasitic drag - which is part of bernoulli's effect - only occurs when the lateral acceleration necessary to fill the void is GREATER than that which gravity can affect.

 

 

This thought experiment is very similar to the one I used when creating my hull resistance spreadsheet. The difference is that I imagined the cylinder (of infinite length) lying horizontally with the waterline passing through the centreline of the cylinder. The cylinder represents a hull of infinite length with a semi-circular cross section shape. I imagined that when the cylinder vanishes, the boundary of the void would accelerate towards what was the centreline of the cylinder.

 

My question was, how long does it take to fill the void? After looking for similar fluid dynamics solutions and after much trial and error comparing my calculations with tank test data, I came up with the following answer:

 

t = 2.718282*SQRT(2*r/g)

 

where: t = time to fill the void

r = the raduis of the cylinder

9 = acceleration due to gravity (9.81 m/s^2)

 

The constant 2.178282 in the transcendental number 'e'. What this suggests is that the boundary starts off accelerating inwards with an acceleration of 'g' however the rate of acceleration decays so that when to void is filled the acceleration is zero.

 

Now of course hull cross sectional shapes are not all semi-circular and I spent a lot of time trying to work out what effect different cross sectional shapes would have on the value of 't'. In the end I had to conclude that (for the range of hulls I hada data for) the section shape has little effect. All you need to have to work ot 't' is the maximum cross sectional area of the hull and then work out the radius for equivalent semi-circular section.

 

Once you have a value for 't' it is then easy to work out what I call the reasonant velocity (Vr)for the hull thus:

 

Vr = L/2t

 

where L is the virtual waterline length i.e. the length that the waterline would be if it were not truncated by an immersed transom. L/2 is also the distance from the bow to the point of maximum cross sectional area of the hull.

 

To cut a long story short, if we assume that the same varying acceleration mecahanism is at play when generating the bow wave (but in reverse), it becomes possible to calculate the work done by the bow half of the hull in displacing its own volume of water as it moves. It is also possible to calculate the work done at the stern half of the hull by the water flowing back in against it as it tries to fill the void. This is the prerssure recovery part of the algorithm. The ability to recover pressure falls as boat speed increases and reaches zero at 'Vr'.

 

Another useful analogy is that the hull launches the water into a low orbit around itself. The bow wave is a high pressure region corresponding to the blast required to launch a projectile. The stern wave corresponds to the pressure wave from the impact when the projectile lands. For any projectile (ignoring losses) the the launch energy is equal to the impact energy. However the hull is moving towards the launch zone and away from the impact zone so the pressure is increased at the bow and reduced at the stern. Thus there is a net resistance to movement.

 

 

The idea that we 'compress water upwards' isn't accurate either. Because there is no increase in pressure. We simply displace it upwards, and in turn displace the air above it.

 

 

I have to disagree with this. When you displace the water upwards you are raising the height of the water surface and therefore increasing the pressure head.

Share this post


Link to post
Share on other sites
The boat pulls the water in around the stern

!! I tend to think of pulling water to be akin to pushing rope. Semantics I guess.

Trouble is its all too easy for these sorts of discussions to be about the use and understanding of technical english amongst the partipants rather than enhanced understanding of what's actually happening. Especially of course, if we invoke the pl***ng word:-)

What magic magnetism exits in the same amount in wood, steel and fiberglass (as well as carbon) that "pulls" water?

 

Its an important question. And the "means of creating the hole" does not matter - unless you believe that water has consciousness and "understands" that it should behave differently based on 'how' the hole was created.

 

This is sloppy thinking folks.

Share this post


Link to post
Share on other sites

OK Lets try this thought experiment on gravity as the accelerating vector for water flowing in behind the hull:

 

Consider a cylinder with a disconnectable cover on the bottom. We insert this cylinder into the water vertically. Now - because this is a thought experiment we can do this - we instantaneously remove the cylinder, leaving the bottom in place so that no water can come up from below. What FORCE causes the water surrounding the column of air to move laterally?

 

It is gravitational force pushing/pulling down on the surface and that translates into lateral deflection just as if you take a steel post, put it in a press and push on its ends. Its just much more dynamic since you are talking about a fluid.

 

Parasitic drag - which is part of bernoulli's effect - only occurs when the lateral acceleration necessary to fill the void is GREATER than that which gravity can affect.

 

 

This thought experiment is very similar to the one I used when creating my hull resistance spreadsheet. The difference is that I imagined the cylinder (of infinite length) lying horizontally with the waterline passing through the centreline of the cylinder. The cylinder represents a hull of infinite length with a semi-circular cross section shape. I imagined that when the cylinder vanishes, the boundary of the void would accelerate towards what was the centreline of the cylinder.

 

My question was, how long does it take to fill the void? After looking for similar fluid dynamics solutions and after much trial and error comparing my calculations with tank test data, I came up with the following answer:

 

t = 2.718282*SQRT(2*r/g)

 

where: t = time to fill the void

r = the raduis of the cylinder

9 = acceleration due to gravity (9.81 m/s^2)

 

The constant 2.178282 in the transcendental number 'e'. What this suggests is that the boundary starts off accelerating inwards with an acceleration of 'g' however the rate of acceleration decays so that when to void is filled the acceleration is zero.

 

Now of course hull cross sectional shapes are not all semi-circular and I spent a lot of time trying to work out what effect different cross sectional shapes would have on the value of 't'. In the end I had to conclude that (for the range of hulls I hada data for) the section shape has little effect. All you need to have to work ot 't' is the maximum cross sectional area of the hull and then work out the radius for equivalent semi-circular section.

 

Once you have a value for 't' it is then easy to work out what I call the reasonant velocity (Vr)for the hull thus:

 

Vr = L/2t

 

where L is the virtual waterline length i.e. the length that the waterline would be if it were not truncated by an immersed transom. L/2 is also the distance from the bow to the point of maximum cross sectional area of the hull.

 

To cut a long story short, if we assume that the same varying acceleration mecahanism is at play when generating the bow wave (but in reverse), it becomes possible to calculate the work done by the bow half of the hull in displacing its own volume of water as it moves. It is also possible to calculate the work done at the stern half of the hull by the water flowing back in against it as it tries to fill the void. This is the prerssure recovery part of the algorithm. The ability to recover pressure falls as boat speed increases and reaches zero at 'Vr'.

 

Another useful analogy is that the hull launches the water into a low orbit around itself. The bow wave is a high pressure region corresponding to the blast required to launch a projectile. The stern wave corresponds to the pressure wave from the impact when the projectile lands. For any projectile (ignoring losses) the the launch energy is equal to the impact energy. However the hull is moving towards the launch zone and away from the impact zone so the pressure is increased at the bow and reduced at the stern. Thus there is a net resistance to movement.

Cool on the numbers Nutter!!!

 

 

The idea that we 'compress water upwards' isn't accurate either. Because there is no increase in pressure. We simply displace it upwards, and in turn displace the air above it.

 

 

I have to disagree with this. When you displace the water upwards you are raising the height of the water surface and therefore increasing the pressure head.

"Pressure Head" is not compression. "Pressure head" is the Potential Energy due to gravity. consider a 1KG bucket of water with a hose that hase a pressure gauge at the end of it. Start with the bucket on the floor and the gauge at head height. You should have "negative" pressure on the gauge even though the water's boiling point is not affected

 

Now raise the bucket so the top of the waterlevel is equal height to the gauge. It now reads zero even though you engaged in a fair amount of "Work" (as in Force x Distance) to achieve this level

 

Now drop the gauge end to the floor but don't move the bucket, "pressure head" has gone up.

 

Now raise the bucket another meter higher above the gauge, you've engaged in "work" (Force x Distance) and the "pressure" goes up DIRECTLY PROPORTIONALLY TO THE "WORK" done.

 

 

The "pressure" at the bottom of the bucket has not changed (as you can validate if you have a gauge there) What has changed is that the bucket of water has moved to greater Potential Energy IN THE GRAVITY FIELD.

 

 

So a bow wave is simply water DISPLACED VERTICALLY IN THE GRAVITY FIELD, there is no change in "pressure" as per "compressibility" of water.

Share this post


Link to post
Share on other sites

Don't forget heel affects the weather helm. Heeling to windward reduces helm because the turning moments become more balance around the centreboard (generally). This means there is no need to use the rudder, hence no rudder drag.

 

Just another item to consider in the grand scheme of making dinghies go faster upwind.

 

Well there is an effect of reduced weather helm, but I'm not convinced that is the effect (though I could be wrong) because lift generated by the rudder foil is VMG LIFT.

Share this post


Link to post
Share on other sites

"Pressure Head" is not compression. "Pressure head" is the Potential Energy due to gravity. consider a 1KG bucket of water with a hose that hase a pressure gauge at the end of it. Start with the bucket on the floor and the gauge at head height. You should have "negative" pressure on the gauge even though the water's boiling point is not affected

 

Now raise the bucket so the top of the waterlevel is equal height to the gauge. It now reads zero even though you engaged in a fair amount of "Work" (as in Force x Distance) to achieve this level

 

Now drop the gauge end to the floor but don't move the bucket, "pressure head" has gone up.

 

Now raise the bucket another meter higher above the gauge, you've engaged in "work" (Force x Distance) and the "pressure" goes up DIRECTLY PROPORTIONALLY TO THE "WORK" done.

 

 

The "pressure" at the bottom of the bucket has not changed (as you can validate if you have a gauge there) What has changed is that the bucket of water has moved to greater Potential Energy IN THE GRAVITY FIELD.

 

 

So a bow wave is simply water DISPLACED VERTICALLY IN THE GRAVITY FIELD, there is no change in "pressure" as per "compressibility" of water.

 

You are right in that the pressure equidistant below the surface, following the wave profile, will be the same. If however you take a horizontal slice through at some level below the surface, you will have a varying pressure field due to the varying water height above that level. The pressure will be higher under the wave peaks and lower under the wave troughs.

 

An analogy for the two dimensional compressibility of the water at the free surface is to take a sheet of paper, lie it on a table and push two of the edges together. The paper will bend up in the middle (in a wave shape). We are able to move the two edges together (compress the incompressible sheet of paper) by allowing the centre of the sheet to move upwards.

Share this post


Link to post
Share on other sites

All this theory is messing with my head.

Mal, does the cylinder theory only apply to (Hollow) Log shaped boats or does it cover all those derived form wood?

Share this post


Link to post
Share on other sites

All this theory is messing with my head.

Mal, does the cylinder theory only apply to (Hollow) Log shaped boats or does it cover all those derived form wood?

 

Wood, plastic and even cement, at least, as far as I know, but wooden boats are always slightly faster (something to do with wood elves). All unproven. No guarantees.

Share this post


Link to post
Share on other sites

Hey, does anybody have any ideas about how to make an IC go faster?

 

 

I keep finding ways, apparently, to make them go slower. :lol:

 

But I do love that moment when a bad idea seems good. So simple. So obvious......

Share this post


Link to post
Share on other sites

Hey, does anybody have any ideas about how to make an IC go faster?

 

 

Chris

for me personally the best idea is to loose a few kilos of weight and doing some excersises during the winter to avoid being exhausted after the first round of the course when we get back on the water.

I think i`m far away to realize the full potential of your green beauty ...

 

Roger

Share this post


Link to post
Share on other sites

All this theory is messing with my head.

Mal, does the cylinder theory only apply to (Hollow) Log shaped boats or does it cover all those derived form wood?

 

yes I agree. All this Theory makes it seem like you need to be a rocket scientists before getting into the canoe.

I look forward to the DC settling down a bit. For me I would have to wait. I couldn't afford to build a boat, only to find it is second or third string, no matter who sails it.

Share this post


Link to post
Share on other sites

My version of back to basics...;)

 

post-26260-060084200 1327301393_thumb.jpg

 

sweet! I can have a IC and a SUP at the same time!

Share this post


Link to post
Share on other sites