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DDW

Deck scrapping vs high cut jib - difference in pointing?

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Now, I know this is SA, so I will be getting plenty of opinion and conjecture, probably flames, hyperbole, etc., as well.

 

But the question is, does anyone have some data on pointing ability with a deck scrapping jib vs one that is cut a few feet off the deck? Theory and wind tunnel data suggest that sealing the gap between deck and sailplan will significantly reduce induced drag by increasing effective aspect ratio.That should improve the L/D of the rig and therefore relative wind angle and VMG. I think the consensus is that this works, since that is what everybody uses. I am trying to quantify this effect from real experience. Do you gain 3 degrees, 4 degrees, less, more?

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I have very high cut furling sail, old style sheets outside lifelines at toerail, borrowed bigger low clew sail difference in my application was at least 8-10degrees better, weather that day was 6-9knots wind, flat water.

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I had a sailmaker tell me once that the air 3-4 feet off the water is disturbed anyway by waves, objects etc. and that a foil's lowest section is least efficient because of it....

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I had a sailmaker tell me once that the air 3-4 feet off the water is disturbed anyway by waves, objects etc. and that a foil's lowest section is least efficient because of it....

 

Hate to say it but the sailmaker is/was a bonehead.

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I have very high cut furling sail, old style sheets outside lifelines at toerail, borrowed bigger low clew sail difference in my application was at least 8-10degrees better, weather that day was 6-9knots wind, flat water.

 

That is a huge difference, especially in light air, how much different was the jib otherwise (how much bigger, was it sheeted tighter inside shrouds etc.)?

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I had a sailmaker tell me once that the air 3-4 feet off the water is disturbed anyway by waves, objects etc. and that a foil's lowest section is least efficient because of it....

 

Hate to say it but the sailmaker is/was a bonehead.

There is definitely a wind gradient effect, also the hull and deck of the boat disturbs the airflow near the foot. However if you look at the published data on measured wind gradients, it has a small effect on a larger boat. A dinghy operates low in the gradient and suffers the most. The main boom on a 50 footer is already 10 feet off the water, so the gradient may only amount to a couple of knots, foot to head.

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Until recently, the models used for air flow at different heights were all land based measurements. A few years ago, the actual measurements made on water were found to be significantly different from the land based measurements. The primary difference was in the increaced flow of air directly on the water's surface. It was much greater than the land based flows would have indicated. In particular, when the water (current) and the air flowed the same direction, there was a significant increase in air volocity close to the water's surface.

 

The direct visible result of this new data can be seen in the new formula windsurfing sails that all literally sweep the deck. And that deck is a hell of a lot closer to the water than your boat's foredeck.

 

The thinking is that the land, being static, creates more "resistence" to the air passage. thus there is a substantial difference in the air flow at .5', 5', 25' and 50'. The water, being fluid, tends to go with the wind and can actually augment the flow of the wind down low. Ths diifferentials are much less than in the land models with the primary diffeence being the increaced volocity at the lower levels.

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Until recently, the models used for air flow at different heights were all land based measurements. A few years ago, the actual measurements made on water were found to be significantly different from the land based measurements. The primary difference was in the increaced flow of air directly on the water's surface. It was much greater than the land based flows would have indicated. In particular, when the water (current) and the air flowed the same direction, there was a significant increase in air volocity close to the water's surface.

 

That is quite interesting. The only data I have is old and land based. Do you have a reference/URL to look at?

The direct visible result of this new data can be seen in the new formula windsurfing sails that all literally sweep the deck. And that deck is a hell of a lot closer to the water than your boat's foredeck.

Any quantitative/qualitative estimate of the difference in efficiency? Is it a marginal difference or substantial enough that the non-deck sweeper is no longer competitive (particularly on windward legs)? The windsurfer case is actually more applicable to what I am interested in (being a cat rig) but I thought I would have to extrapolate from sloops.

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Anyone who has sailed a windsurfer in high winds knows that putting the sail down on the deck is like kicking on the turbos! End plate effect.

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I’m curious about this topic too. I race a Columbia Sabre which is roughly 33' long with a 6' 4" beam.

She point like shi# at the moment because she's got a C.D.I furler, thus no way to further tension the Luff of the sail (no runners as of yet) the end result is the head stay sags off when pointing and places us in the 45 deg off apparent wind category (shitty for W/L). If I ditch the furler will I regain the 6 sec? Or should I go with a furler that allows me to, further, tension the luff by using a Halyard?(The C.D.I is an internal halyard and does not attach to the mast; it relies on the head stay for luff tension.)

Two part question: Ditch the furler altogether for a deck sweeper? Or keep the 6 sec and get more tension on the luff by switching to a Harken or a similar furler?

Also how close to the deck should I go? Sweeper or standard furler?

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all else being equal the one with the most area (power) will be higher

Once fully powered up, the question is which is closer pointing? Not necessarily the largest area. but rather the most efficient. Suppose I have the same jib so everything is exactly the same, but attach the tack with a lanyard to open a two foot gap at the deck. The sail will be in more wind. Will I point higher or lower? And by how much? That's the information I am looking for.

 

The windsurfer guys are saying put the sail on the deck and you will get more power. Is that more lift (faster on a reach, or on a beat less than fully powered) or is it more efficient (closer pointing angle)? Or, as theory predicts, both?

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Read Marcaj....

Been there. Done that. He quotes some theory and did some wind tunnel tests on scale models. And shows some pictures. (And some words hedging the theory and tunnel tests). But on the theory that theory is theory and data is data, I am looking for data. For some reason in sailing this is the scarcest of commodities. Everyone has conjecture. Most have theories. Very few have real data.

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Don't remember which book but there was one chapter that dealt with deck scraping or high cut foot. My understanding is that it's best low. High cut to stop the foot catching water. Air passing under the foot creates turbulence on the back of the sail and reduces flow. Hard reading Marcaj..

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The deck actually acts like a winglet preventing high pressure air from spilling under the foot of the jib to the low pressure (inboard) side of the jib . Projected sail area is projected sail area(the more the better) the key is having as much clean attached flow as you can.

If you want data just look at any modern airplane wing.

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all else being equal the one with the most area (power) will be higher

Once fully powered up, the question is which is closer pointing? Not necessarily the largest area. but rather the most efficient. Suppose I have the same jib so everything is exactly the same, but attach the tack with a lanyard to open a two foot gap at the deck. The sail will be in more wind. Will I point higher or lower? And by how much? That's the information I am looking for.

 

The windsurfer guys are saying put the sail on the deck and you will get more power. Is that more lift (faster on a reach, or on a beat less than fully powered) or is it more efficient (closer pointing angle)? Or, as theory predicts, both?

 

thats the point yas are not getting , and why you'll never point

 

all else being equal, the hull, fins moving faster thru the water will be the boat that points the highest, thus the overriding factor point is sail area, not the profile of the jib

 

if you are saying you have ample area with a strop or high-cut clew/foot then maybe kidding oneself, if hypothetically this was the case then the end-plate effect of the deck may be more efficient

 

however THIS config (deck-sweeping or close) has more area which brings you FULL CIRCLE back around my original point.

 

On hi-po light dinghies where effects are amplified or more obvious 'tacked down' jib is always quicker once powered up, this may be the end-plate OR the freer more open slot to the mainsail

 

 

also the helm has the option of trading speed for height in a periodic sequence.

 

if you are racing go to a racing furling system like the lumbo 30 or synergy or whatever it was they talked about in the FT10m forum, with a low cut jib and tensionable forestay

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thats the point yas are not getting , and why you'll never point

 

all else being equal, the hull, fins moving faster thru the water will be the boat that points the highest, thus the overriding factor point is sail area, not the profile of the jib

 

if you are saying you have ample area with a strop or high-cut clew/foot then maybe kidding oneself, if hypothetically this was the case then the end-plate effect of the deck may be more efficient

Actually, point angle is numerically equal to the sum of the hydrodynamic drag angle of the hull and the aerodynamic drag angle of the rig (and hull). More power from the rig may, or may not improve either of these angles and therefore may, or may not improve pointing angle. But an increase of efficiency of the rig (i.e. an improvement in aerodynamic drag angle) WILL improve pointing ability by a like amount. Actually a little more because the keel load is less (and the hydrodynamic drag angle also slightly improved).

 

You are confusing power with efficiency, or lift with drag angle. They are related, but not the same.

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what the fuck , you asked a dumbass question, now you are answering it YOURSELF ? (from the book of theoretical crapola)

 

mate the bigger headsail on the fuckin deck ( more power) will outpoint the other due to the boat going quicker thru the water, book of practice not theory, ......... in less than ideal water conditions it will be even more marked.

 

'ability to point' WAS THAT NOT THE QUESTION

 

what was the point of posting the question ?? so you can come back and regurgitate some theoretical babbledeegook

--------------------------------------------------------------------------------------------------------------------------------

 

aaahh , got it - YOU are the theorist, earlier ; that thru calculation to & from App.angle , rejected that a boat could be tacking thru an angle of 88^ LOL reckoned it was too closewinded to be true !

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what the fuck , you asked a dumbass question, now you are answering it from the book of theoretical shyte

 

mate the bigger headsail on the fuckin deck ( which is larger ) will outpoint the other due to the boat going quicker thru the water, book of practice not theory, in less than ideal water conditions it will be even more marked.

 

'ability to point' WAS THAT NOT THE QUESTION

 

what was the point of posting the question ?? so you can come back and regurgitate some theoretical shit.

Sorry I offended you.

 

You are simply wrong about the larger jib always outpointing a smaller one as any dumbass (such as myself) knows (yes, from practical experience). Pointing ability is close hauled apparent wind angle, at max VMG, not speed. What I quoted was mathematical fact, not theory, there is a difference. And I did ask for data. You have offered none. But, can I suggest a couple of good anger management self help books?

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WHY did you ask the dumbass question , did your ( btw theoretical ) mathematical fact not exist before you posted.

 

mate mathematical FACT does not take into account water condition, velocity fluctuations, helms ability to stay in the groove

 

mate your books and mathematical FACTS are THEORY, that do not take into account real-world variables.

 

WHY did you ask the question if your armoury of crapola held the secrets?

 

I am trying to quantify this effect from real experience. Do you gain 3 degrees, 4 degrees, less, more?

the question you posed was what was the 'real world' effect (i.e. IN PRACTICE) , I answered that, and you countered with theoretical ignorance ??

 

give it a break ( btw I 'feel' it would be in the order of at least 5^)

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Anyone who has sailed a windsurfer in high winds knows that putting the sail down on the deck is like kicking on the turbos! End plate effect.

 

Yes; you can feel the acceleration.

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Testing of the sort described takes time and costs money.

 

"One boat" testing is going to have low repeatability and high error (OK, we've got data for *this* jib on *this* wind range; swap jibs -- oops the wind just picked up two more knots!)

 

"Two boat" testing takes more time, more sailors, more money.

 

The people usually willing to spend this time, effort, and money in an attempt to build a big repeatable data set:

 

1) Are usually trying to get a competitive edge, and therefore unlikely to share the data gained

 

2) Usually won't bother running expensive tests that confirm what they already know (Hey, it turns out there are *no* wind ranges that will allow us to make better VMG to windward with a high cut jib vs. a sweeper!)

 

Further, the data table sought would be fairly specific to boat type:

 

dinghy

 

fin keel ultra-light

 

fin keel light

 

fin keel medium

 

full keel heavy

 

-- or at least, it is non-obvious that results on one class would extend meaningfully to the others, unless it were proved by experiment. (Again, excepting the obvious platitudes)

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all else being equal, the hull, fins moving faster thru the water will be the boat that points the highest, thus the overriding factor point is sail area, not the profile of the jib

I think you're greatly over simplifying there. The faster you go through the water (all else being equal which of course it isn't remotely) then the lower you point because the apparent wind comes back. Turn on an engine when you'r boat is close hauled and the sails will flap until you point lower. On the other hand The boats that really point at the sky aren't necessarily the ones with the best vmg, as you rightly say faster boats often have better vmg, because up to a point speed beats height. For optimum vmg its necessary to achieve the right balance between power and direction. Pointing too high you have low power and good direction. Footing off for speed too much and you have poor direction and good power. Writing this has given me some thoughts about the effectof drag which are counter intuitive, I'll talk about these below.

 

Bethwaite has some interesting stuff of main/jib interaction on the Tasar Yahoo mailing list, post 3360, which gives some theory behind why recent Bethwaite boats have the luff extension of the main which he calls a cuff. This would seem to have some relationshp to our discussion.

 

a. The first is that most races are sailed in winds of less than 12 knots.

 

b. The second is that what we want from our sails when sailing to windward in winds of less than 12 knots is higher force, not best lift/drag. (for the purist, optimum power factor.)

 

c. The third is that a sail which is set to exhaust into the area of lowest pressure of the sail behind it acts like a turbine blade. The pressure at the leading edge of the jib is greater than the pressure at the trailing edge (the jib leech) which is just to leeward of the mast where flow speed is highest and pressure lowest. So the flow around the lee side of the jib is accelerated and the sail pulls harder and develops greater force [snip] What was happening was that the cuff was extending the area of lower pressure just to leeward of the mast down to the deck level of the lower jib, and this lower jib was suddenly beginning to pull much harder than it can when there is no cuff. And this low level is exactly where it serves best.

 

Interestingly Bethwaite is claiming that in lighter condition power is king, you can ignore L/D. I can't argue either way. Now you could take his theory about the cuff to suggest that it would be better to move the jib up so that it was all overlapping the main, rather than have the jib deck sweeping and lower than the main. However he's talking about exploiting the pressure differences round the sail, and those pressure differences are greatly reduced by circulation round the foot which is what deck sweeping aims to reduce.

 

But real measured data on this stuff is really rare, its all people talking like this with sometimes less actual backup to the words than you might think.

 

Now to drag, from the first paragraph. We all think about Lift to drag as being cirtical to upwind performance, and for sure no boat goes upwind well towing a bucket. But how does drag actually change when an individual boat points higher or foots faster? As you point higher and go slower hull drag mustreduce and wind drag on standing rigging and the like must also reduce. On the other hand the induced drag from foils and sails must increase as they work harder I guess. Is that the bigger number? I seem to remeber reaidng somewhere it is. Any good data?

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Now to drag, from the first paragraph. We all think about Lift to drag as being cirtical to upwind performance, and for sure no boat goes upwind well towing a bucket. But how does drag actually change when an individual boat points higher or foots faster? As you point higher and go slower hull drag mustreduce and wind drag on standing rigging and the like must also reduce. On the other hand the induced drag from foils and sails must increase as they work harder I guess. Is that the bigger number? I seem to remeber reaidng somewhere it is. Any good data?

 

Given that water is around 780 times denser than air, I suspect drag from hull and foils is much more significant than drag from sails. I've never seen a detailed analyis that confirms that. However here is a little comparison between air and water which illustrates what I mean. A modern open class sailplane weighs around 500kg and achieves 60:1 L/D at around 60 knots. A little arithmetic shows that is equivalent to around 4HP in energy consumption. Now what would it take to drive a 500kg RIB at 60 knots? 150HP? I don't actually know but it is certainly upwards of an order of magnitude more.

 

The question on whether drag from foils would increase or decrease as you point higher is actually a complex one, because as you point higher you go slower. Induced drag decreases with velocity whereas parasitic drag increases. Induced drag also increases with angle of attack (leeway) which will increase as you point higher. I don't think you can work out what would happen without more measurement data than I have to hand.

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jim , dogwatch BJ we are assumming all else equal here, say 2 OD's, because the question is contained to the differnce in heady profile only, also as not mentioned we have to assume mid-range windspeed.

 

one OD with a high clew jib off the deck, one with a sweeper

 

this scenario is a given example of a boat , jib profile the difference

 

It would be extremely unlikely that the high clew heady would have the same area as the deck sweeper in the said scenario.

 

This is not as theoretical as is assummed, he is asking for a practical view, of course the off-deck example will be lower and slower

 

its not a pure theory question

 

Jim I know you have some practical exp as well, bethwaite doesn't have to 'claim' , it's obvious, even a good skiff helm blindfolded could feel the diff.

 

A boat lacking grunt will be hobby-horsing, with grunt it can be bow down, with helm , and charging by comparison. This is so obvious it's not funny. put away the slide-rules.

 

Even two control boats with the same jib profile can demonstrate this if one has a heavier cloth and less powerful cut, it's that BIG a difference.

(i.e. an improvement in aerodynamic drag angle) WILL improve pointing ability by a like amount.
YES that is the point by filling in that area with sail - and an endplate, power is increased out of proportion with the slightly increased drag, thus the above quoted excerpt is TRUE. slot effect is also gained, it is not a question that could go each way as yr opening post suggested. Why rebutt down-thread ??

 

DDW you need not worry I will never reply to yr ( marchaj answer in hand) questions again

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Just threw it to "Ask the experts"

What are we paying the big bucks to Ed and the experts for anyhow?

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Interestingly Bethwaite is claiming that in lighter condition power is king, you can ignore L/D. I can't argue either way. Now you could take his theory about the cuff to suggest that it would be better to move the jib up so that it was all overlapping the main, rather than have the jib deck sweeping and lower than the main. However he's talking about exploiting the pressure differences round the sail, and those pressure differences are greatly reduced by circulation round the foot which is what deck sweeping aims to reduce.

From practical experience, I believe Bethwaite is correct, power is king until you have enough. He seems to be concerned with slot effect and increasing Cl more than reducing induced drag. I am wondering specifically about reduced induced drag due to the "end plate" or "mirror image" effect.

But real measured data on this stuff is really rare, its all people talking like this with sometimes less actual backup to the words than you might think.

You certainly have that right! While two boat testing is expensive, simple measurements can often be made, but most don't bother. It is tricky business, but I hoped someone on the board would have done it.

Now to drag, from the first paragraph. We all think about Lift to drag as being cirtical to upwind performance, and for sure no boat goes upwind well towing a bucket. But how does drag actually change when an individual boat points higher or foots faster? As you point higher and go slower hull drag mustreduce and wind drag on standing rigging and the like must also reduce. On the other hand the induced drag from foils and sails must increase as they work harder I guess. Is that the bigger number? I seem to remeber reaidng somewhere it is. Any good data?

Everything you change on a boat affects everything else. But if we simplify to the case of a boat in the upwind condition sailing at max vmg, then magically reduce the induced drag of the rig by half what happens? The resultant vector from the rig tilts forward, the load on the keel is still the same (same lift from rig) so the boat either goes faster (changing everything) or heads up at the same speed (changing very littel except heading). This is all just geometry. The unknowns are, how much of the "end plate effect" can be realized in practice? and How much of total rig drag is induced drag? The latter is better studied than the former.

 

My actual problem has to do with una rigs and adding a cuff similar to what Bethwaite is doing (but obviously no slot effects). I asked about jibs because I believe it is analogous, and the body of knowledge would be greater. I had not expected the windsurfers to have run the experiment. Still their statements so far indicate a power increase. Is there also a pointing angle advantage?

 

The question on whether drag from foils would increase or decrease as you point higher is actually a complex one, because as you point higher you go slower. Induced drag decreases with velocity whereas parasitic drag increases. Induced drag also increases with angle of attack (leeway) which will increase as you point higher. I don't think you can work out what would happen without more measurement data than I have to hand.

But, if the speed is the same, the induced drag of the rig lower, then the hydrodynamics should be identical, but the apparent wind angle less. I don't doubt this, just want to know what the practical difference is.

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It's not really a fair question DDW. The reason some sails are high cut doesn't

relate to point.

 

If you have both planforms sheeted to the same angle, and they both have

the same camber, both will stall and luff at the same angle of attacks.

The deck sweeper will be developing more power due to the end plate effect.

 

 

So you may ask "Why two types? Why would anyone build anything

but low clewed deck sweepers?"

 

The reason has to do with the apparent wind angles that the high clewed

sail can handle without flogging that the low clewed sail can not.

What happens when you ease sheet on a low clewed sail? The top twists

off more than on a high clewed one. That means the top is flogging to a

greater degree than it would on a high clewed jib. Offshore, or in

high winds and big water, sometimes you want a jib that you can ease

out without excessive flog being the result. The part about letting

water through is a nice, but unintended, side effect.

 

So you see high cut clews in heavy weather sails, or reaching-biased

sails for offshore work, but never in sails used in moderate conditions

on course racing boats.

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It's not really a fair question DDW. The reason some sails are high cut doesn't

relate to point.

 

If you have both planforms sheeted to the same angle, and they both have

the same camber, both will stall and luff at the same angle of attacks.

The deck sweeper will be developing more power due to the end plate effect.

 

 

So you may ask "Why two types? Why would anyone build anything

but low clewed deck sweepers?"

 

The reason has to do with the apparent wind angles that the high clewed

sail can handle without flogging that the low clewed sail can not.

What happens when you ease sheet on a low clewed sail? The top twists

off more than on a high clewed one. That means the top is flogging to a

greater degree than it would on a high clewed jib. Offshore, or in

high winds and big water, sometimes you want a jib that you can ease

out without excessive flog being the result. The part about letting

water through is a nice, but unintended, side effect.

So you see high cut clews in heavy weather sails, or reaching-biased

sails for offshore work, but never in sails used in moderate conditions

on course racing boats.

 

Agree about sheeting angles off the wind but for offshore reaching - keeping the sail out of the waves is even more important - if you have the sail close to the water it will be torn to pieces very quickly by the waves - which in turn makes sheeting the sail pretty meaningless

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If you have both planforms sheeted to the same angle, and they both have

the same camber, both will stall and luff at the same angle of attacks.

The deck sweeper will be developing more power due to the end plate effect.

 

 

So you may ask "Why two types? Why would anyone build anything

but low clewed deck sweepers?"

I understand why one might use one or the other. I just want to know how much pointing angle is gained, if any, from the deck sweeper. Theory says you double the effective aspect ratio. Tunnel testing says there is some gain, but not double. Practice has lined up on the side of some gain, I was hoping someone had done some testing and quantified it. But perhaps not.

 

I gather we have drained the pool of knowledge on the subject, so I will let it die.

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Anyone who has sailed a windsurfer in high winds knows that putting the sail down on the deck is like kicking on the turbos! End plate effect.

 

Yes; you can feel the acceleration.

well to be the devils advocate i agree BUT alot of other things happen. When you close the gap , that means you rake the sail back and thus the boom and yourself. Result, your stance changes as well, the centre of effort of the sail goes back over the fin . At that point you riding the fin big time, wetted surface area goes way down.

BUT if one was to do that with a race sail versus a wavesail, i do believe most windsufers would agree the race sail is faster. But to be devils advocate, the race sail has camber inducers most likely and has a different cut.

Ok hmmm case and poit : cut to the quick (which i have not done) speed sailors have a flat bottom panels and strive to close the gap.

Jef E

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To answer your question, you need to understand that the angle of attack

that a given sail stalls and luffs at is determined by shape, not planform.

 

That being said, there is some pointing advantage that the deck sweeper

gains due to the greater apparent wing angle the leading edge of

such a sail "sees". The apparent wind angle of a boat is bent

by the high pressure air created by the sails on the windward

side of the boat. The decksweeper will create a greater amount of

high pressure air down low. Therefore, the lower sections are seeing

a "lifted" apparent wind opposed to what a high clewed sail would see.

 

By exactly how much? Depends. Nobody really

cares to quantify it. Too much depends on just how high a clew

we are talking.

 

Hope that helps.

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I understand why one might use one or the other. I just want to know how much pointing angle is gained, if any, from the deck sweeper. Theory says you double the effective aspect ratio. Tunnel testing says there is some gain, but not double. Practice has lined up on the side of some gain, I was hoping someone had done some testing and quantified it. But perhaps not.

 

I gather we have drained the pool of knowledge on the subject, so I will let it die.

 

I think we can conclude:

1. No one has done testing on this specific question you ask.

2. Practice, insofar as we can tell, agrees with theory that there is some gain, but not much.

3. The amount gained is enough to be noticeable, in theory and practice, but the amount varies by boat type and the specific conditions (both wind & water) under which the testing might occur.

 

In short, there is no reason to test to find the answer to a question we already know the answer to, when the answer will be less reliable than practical experience because of the pure mass of testing and data required to get a test result that is accurate above any error factors, for even a single test condition, not to mention all test conditions.

Why, out of curiousity, do you want to know the 'answer' to such an open-ended question in the first place?

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In short, there is no reason to test to find the answer to a question we already know the answer to, when the answer will be less reliable than practical experience because of the pure mass of testing and data required to get a test result that is accurate above any error factors, for even a single test condition, not to mention all test conditions.

Why, out of curiousity, do you want to know the 'answer' to such an open-ended question in the first place?

I am afraid your summary is correct. As for the reason to ask, I am involved in a una rig design which would require significant complication to close the deck gap, but it could be done. The question is: is the gain worth the trouble? I can estimate the trouble, but I am having a hard time estimating the gain. As I alluded to earlier, I asked about jibs because I believe a sloop acts as a single slotted foil with the deck gap closed forward, and there is very little knowledge in the world about large Una rigs (and I had forgotten about sailboards). It didn't seem like such an open question to me - if you own a sloop with both jibs, how do you know when to use which one? Just guess? If you shift to the high cut jib in a seaway to avoid damage, how to you know how much risk to take, if you don't know how much performance you are giving up?

 

I suppose I am unusual in that I test these things. I just don't have convenient access to a sloop with appropriate sail choices at this moment. I thought someone might have the answer at hand.

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The answer is at hand-- If high clewed genoa's pointed higher you would see them on the AC boats----- the highest pointing sailing vessels on the planet.

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Not sure that I agree.

 

If the proposition were true then why don't the ACC boats have deck sweeping booms? Such a setup would ensure full closure of the foil ends.

 

I suspect that, as usual the answer is in the rulebook. Main is specified size therefore you have it as high as possible.

 

Jib also a specified size, luff is limited length which means the rest of the area has to be placed somewhere.

 

I would have thought that if the luff length and the LP length was specified (LP being the perpendicular of the luff) then it would be best to have the area therefore the clew further up (provided you can sheet it from an appropriate place in the boat) where it has most effect on the slot (since the boom is usually much higher).

 

I would have thought that the ACC's are not necessarily optimum since a large amount of sail area of the jib is not part of the slotted airfoil which seems to be an important part of the power generating capability of the rig.

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Check out this Star sail plan. It shows that the maximum jib luff allowed to be considerably longer than any Star that I've even seen sailing with - look how high the clew is! I can think of only one reason - the effectiveness of the "deck sweeper" or "end plate" found with the jib laying on the deck.

 

I'm pretty sure that the Lightning class as well as the 5.5/6 metre classes will also show the same evolutionary results.

 

http://www.starclass.org/rules_items/sailplan.pdf

 

BTW, it's Marchaj with a "h". Here's a link to his book - the updated version. I remember reading his book (the original version) back in the late 60s or early 70s, complete with small scale models and people holding smoke wands in front of the sails. I seem to remember a scale model Dragon that was used for some tests.

 

http://www.sailgb.com/p/sail_performance_by_c_a_marchaj/

 

Another book well worth reading on this subject (and written not so long ago) is that by Tom Whidden (he of North Sails fame).

 

http://www.amazon.com/gp/product/031204417...glance&n=283155

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The answer is at hand-- If high clewed genoa's pointed higher you would see them on the AC boats----- the highest pointing sailing vessels on the planet.

As I have said several times, the consensus is that the deck sweeper points higher. The answer would consist of data indicating how much. Is the gain worth the pain? I know the pain, I want to know the gain. On an AC boat you would say that even an infinitesimal pointing gain is worth any amount of pain, winning is all that counts. Not true in a less extreme cases, now there is a cost/benefit tradeoff to be made.

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Check out this Star sail plan. It shows that the maximum jib luff allowed to be considerably longer than any Star that I've even seen sailing with - look how high the clew is! I can think of only one reason - the effectiveness of the "deck sweeper" or "end plate" found with the jib laying on the deck.

 

I'm pretty sure that the Lightning class as well as the 5.5/6 metre classes will also show the same evolutionary results.

Interesting, that again supports the consensus of improvement, but still without quantifying it.

BTW, it's Marchaj with a "h". Here's a link to his book - the updated version. I remember reading his book (the original version) back in the late 60s or early 70s, complete with small scale models and people holding smoke wands in front of the sails. I seem to remember a scale model Dragon that was used for some tests.

I have read Marchaj's work. Unfortunately he is a bit schizo on the subject - on the Dragon test the hull helped, but on later tests quoted by him, the gain was not nearly as high as theoretically supposed or previously tested (though still about a 15% reduction in overall rig drag). A wind tunnel test published in his earlier work showed an increase in L/D of 10% in a Finn rig just by lowering the boom a few percent of luff length. However these are all scale tests, I am trying to find real world confirmation (and quantification).

 

Wash,

 

Subject is really about the "end plate effect" of the deck, more than headsails...just thought everyone would have tried it with headsails.

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Understand--- But as I see it----- why discount the absolute best test subject of AC boats built to get to the weather mark first?

 

Is not that the best real world confirmation?

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In short, there is no reason to test to find the answer to a question we already know the answer to, when the answer will be less reliable than practical experience because of the pure mass of testing and data required to get a test result that is accurate above any error factors, for even a single test condition, not to mention all test conditions.

Why, out of curiousity, do you want to know the 'answer' to such an open-ended question in the first place?

I am afraid your summary is correct. As for the reason to ask, I am involved in a una rig design which would require significant complication to close the deck gap, but it could be done. The question is: is the gain worth the trouble? I can estimate the trouble, but I am having a hard time estimating the gain. As I alluded to earlier, I asked about jibs because I believe a sloop acts as a single slotted foil with the deck gap closed forward, and there is very little knowledge in the world about large Una rigs (and I had forgotten about sailboards). It didn't seem like such an open question to me - if you own a sloop with both jibs, how do you know when to use which one? Just guess? If you shift to the high cut jib in a seaway to avoid damage, how to you know how much risk to take, if you don't know how much performance you are giving up?

 

I suppose I am unusual in that I test these things. I just don't have convenient access to a sloop with appropriate sail choices at this moment. I thought someone might have the answer at hand.

 

Don't suppose any longer than necessary, as with many budding rocket scientists you would use a sloop headsail test bed , with all it's other variables (slot / area etc) , as a test pig for a una-rig ? , and basically like most of us you know the answer already

 

If you asked the question here about the una-rig, SHC or a multi developer, or a Fbethwaite type would have answered your question, anyone on 'soft-sail C's or all the 500mtr world speed record machines, or the M-20, or the french 60' twin UNA hydroplane cat.

 

In your test plan you assume "" and there is very little knowledge in the world about large Una rigs "" what a load of rubbish "" ! Is Team Phillips big enough for you ??

 

I thought only for a sec this was Doug Lord ? but he would never use a jib example for a una-rig riddle, nor would he assume little knowledge/data out there on single element rigs.

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test pig for a una-rig,

 

Una-rig: deck sweeper, or higher clew was the apparent question. Not sure I can resolve your pointing issue/question, Induced Velocity Distribution

might give you an answer. Coef. of Drag is reasonably known.

 

Empirical results on "pointing" from Star, Lightining, Soling, ... one design XX, must give one a reasonably good "feel", if not quantitatively with tables in html or Excel. The challenge is what testing was done, thru the decades, probably didn't have access to instrument readouts as you apparently expect. The question I ask myself this evening, given the best instruments I might be able to purchase, do I trust instruments to measure 2-4 degrees difference accurately in a seaway (SF Bay) with the boat causing the mast to pitch, roll, accelerate, decelerate, ... and the wingmast rotates anyway.

 

2-3 Degrees = Rounding error.

 

 

Tom Speer's site, specifically at the link to Optimal planforms may be of interest. Summary wording, "Design charts showing optimal span loadings, planform shapes, induced drags, and centers of effort are derived for isolated surfaces, single sails in ground effect, ..."

 

Mr. Speer's work has some nice references at the end. There may be easier reading somewhere else, can't remember anything right off. Tom Speer does work with landyachts, rigid wings. Tom is a sailor, think an F-24 (Mk I or II?) ; he posts on the yahoo F-boat list. Nice guy, his email is listed on the site. He ~might answer a direct question.

 

End-Plate Effect = Good ?:

An equation on drag, lift, gap ( E ) follows, along w/some of Mr. Speer's words:

 

Image351.gif

 

"E is the Oswald efficiency factor, and is a function of the planform geometry and the surface effects. The value of e for the minimum drag planforms is shown in Figure 5. For zero gap, e is two, which is in agreement with classic wing theory. With as little as a 1% gap, however, e drops by two-thirds to 1.36. "

 

Happy New Year, Champagne is Chilling

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Cool project.

 

I would think that you (and we all) would agree that to have a deck-sweeper sail is more efficient than not. Taking that for granted, we can modify the question to:

'When should I use/design for/plan for ausing my deck-sweeping sail/design/rig?'

The answer is so dependant on your boat, and wind and sea conditions that the onlw real answer one could give is:

'You have to rely on your sailing experience to properly asses the situation.'

For example, I suspect that the only reason we have not ever seen a deck-sweeping main on an AC boat is that it would hinder the normal operation of the boat and crew. Certainly 12m boats try to get the boom as low as possible, to the point that grinders work in seriously recessed well on some of them. I always thought it might be neat to try hanging some of those heavy plastic heavy mylar sheet things that you have as doors in warehouses/walk-in freezers/etc from the boom of an AC boat. People could walk through them (not fast), but the efficiency gain should be worth the slightly slower crewwork in tacks/gybes.

So, for your project, I would think that if you can make it work within the strictures of the rest of the design, you should do it. Don't ask 'how much will I gain in increments?'. Ask instead 'can I make this work and still let people move about the deck?

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DDW you need not worry I will never reply to yr ( marchaj answer in hand) questions again

GybeSet, why didn't you keep this promise?

 

Neither Bethwaite nor Goss has researched this aspect in any depth, talk to them if you don't believe me. The number of large Una rigs is outnumbered by sloops perhaps one to million, it should not surprise you that the body of experience in them is far smaller. Team Phillips was a singularity, although Goss has told me he felt the rig worked well. But in it they were attempting to solve entirely different problems. Please point to all the other Una rigs in 40-60 foot boats: I know of only a handful.

So, for your project, I would think that if you can make it work within the strictures of the rest of the design, you should do it. Don't ask 'how much will I gain in increments?'. Ask instead 'can I make this work and still let people move about the deck?

I think I may go ahead and do it - just to see if it does work - since I cannot find a definitive anwer. Operationally, the deck sweeping part would not hinder any crew work. The cost is mainly in construction and engineering.

 

Slackwater said:

Tom Speer's site, specifically at the link to Optimal planforms may be of interest. Summary wording, "Design charts showing optimal span loadings, planform shapes, induced drags, and centers of effort are derived for isolated surfaces, single sails in ground effect, ..."

I am familiar with Speer's work and have conversed with him about the rig, though not this specific aspect. However even he will tell you that those are computer simulations, left to be verified in practice. And there are several wind tunnel tests indicating that practice departs from theory significantly in this area. That is why I was asking for anyone's practical experience.

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Anyone who has sailed a windsurfer in high winds knows that putting the sail down on the deck is like kicking on the turbos! End plate effect.

 

Yes; you can feel the acceleration.

well to be the devils advocate i agree BUT alot of other things happen. When you close the gap , that means you rake the sail back and thus the boom and yourself. Result, your stance changes as well, the centre of effort of the sail goes back over the fin . At that point you riding the fin big time, wetted surface area goes way down.

 

But if you just move weight back without also closing the slot, will you also accelerate? Probably not, unless your stance was wrong to begin with. Therefore, it is closing the slot that makes the difference.

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A wind tunnel test published in his earlier work showed an increase in L/D of 10% in a Finn rig just by lowering the boom a few percent of luff length.

 

If you look at how Europe sailors tune their boats, they carefully adjust rake for the conditions so that the boom is just sweeping the deck when sheeted in. They think it is worth the trouble of tuning it to be so before each race, and ducking the very low boom that results on every tack and gybe. That, I suggest, is further "full-scale" confirmation that closing the slot works. I suspect Finn helms do the same - anyone know?

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DDW said:

I am familiar with Speer's work and have conversed with him about the rig, though not this specific aspect. However even he will tell you that those are computer simulations, left to be verified in practice. And there are several wind tunnel tests indicating that practice departs from theory significantly in this area. That is why I was asking for anyone's practical experience.

 

You spoke with Mr. Speer, so you are ahead of me. Suggest Steve Clark

has more practical experience on the water with una-rigs than most people on the planet.

Unsure what level of instrumentation was on the C-class cat. You might inquire with

Mr. Clark for real-world.

 

Suggest focus on L/D ratio as the success factor & consider pointing by 2-3 degrees the

rounding error that is not completely testable w/o full-scale models in controlled conditions.

 

Please encourage Mr. Clark to write-up his latest boat-building effort(s). I'm a fan of his

boat-repair / build advice on Sailing Anarchy. It gives encouragement while doing epoxy

work looking up at the surface. :ph34r:

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Suggest Steve Clark has more practical experience on the water with una-rigs than most people on the planet.

 

Arguably the C class are not una rig boats downwind with the slot(s) open...

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DDW you need not worry I will never reply to yr ( marchaj answer in hand) questions again

GybeSet, why didn't you keep this promise?

 

Neither Bethwaite nor Goss has researched this aspect in any depth, talk to them if you don't believe me. The number of large Una rigs is outnumbered by sloops perhaps one to million, it should not surprise you that the body of experience in them is far smaller. Team Phillips was a singularity, although Goss has told me he felt the rig worked well. But in it they were attempting to solve entirely different problems. Please point to all the other Una rigs in 40-60 foot boats: I know of only a handful. That is why I was asking for anyone's practical experience.

Your geekdom inspires me is the short answer, Ask Bethwaite does he know anything about mainsail dynamics in isolation of jibs ?, he'd could write you 500 pages , I could follow this thru but we know the answer

 

and there is very little knowledge in the world about large Una rigs (and I had forgotten about sailboards).

why do I have to find a 60'er example when yr 'light bulb' lit up about a sailboard being large enough ?

 

This post is a little reminiscent of the famous Doug Lord " oh , I had forgotten about the Aussie 18' skiffs " sheez

rocket scientists !! lol ; how on earth can you fathom out something apparently problematic when the real world OBVIOUS examples slip your mind ?

 

I hold my line on plenty of data available on una-rigs , C class , board, and the 'speed week' platforms just scratching the surface. Also I doubt your project is 60' btw.

 

Any designer of multis could comment on una-rigs I feel, e.g. Morr. & Mel went a tiny jib option on Connor's Stars and Stripes, yet it would have won as a Una I would hazard. The Cunninghams, YPEndeavour MacqInnovatoin people, hundreds have worked Una technical aspects, absolute hundreds, if not thousands.

C's, ns14's and Moths could go sloop or Una, decades of refining either way, SHC has discussed his ICanoe Una experiment here on anarchy

 

Jim C, the comment Cclass wing "not una downwind" , Clarky has plenty of experience in C when soft sails and single element wings were de rigeur

 

DDW why not get two OD rc boats and cut a sweeper main exactly the same area and side-by-side test. jibs on or off? (off would be a better test sample if the boats balance is ok)

 

" but I thought I would have to extrapolate from sloops. " man that is the most flawed research that one could come up with. This thread HAS produced some food for thought , but only since you fessed up that you were asking a 'mainsail' question', more would come if you explained your platform.

btw seen a pic of the 'extreme' end-plated Sailrocket skimmer Tri ?wind_rocket_wingsail_boat.jpg

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why do I have to find a 60'er example when yr 'light bulb' lit up about a sailboard being large enough ?

 

This post is a little reminiscent of the famous Doug Lord " oh , I had forgotten about the Aussie 18' skiffs " sheez

rocket scientists !! lol ; how on earth can you fathom out something apparently problematic when the real world OBVIOUS examples slip your mind ?

 

I hold my line on plenty of data available on una-rigs , C class , board, and the 'speed week' platforms just scratching the surface. Also I doubt your project is 60' btw.

 

DDW why not get two OD rc boats and cut a sweeper main exactly the same area and side-by-side test. jibs on or off? (off would be a better test sample if the boats balance is ok)

You really are a special case.

 

C-class cats are hard sails, sailboard and ODs and RCs are much smaller. Not directly applicable due to scale effects, etc., but data points none the less. I was hoping someone would have already done the experiment, even if it wasn't directly applicable but none have stepped forward. Several have suggested some good leads. most of which I have already chased. I would be happy to get more.

 

I wish you good luck in your life of ridicule (3183 posts in a little over a year?). I have to get back to building my boat.

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thanks for the compliment cobber

 

I am sure if you define the 'task at hand' you will flush out the REAL expert comment .i.e lightweight, heavy, inshore, offshore, multi, speed weeker, room to tack behind clew etc

 

this is the 'revenge of the geeks' nightmare I did NOT have after spikkin wid ya wingsail_outrigger_diagram.jpg

the one where FOYD and DL and the 'association of sailing geeks' are chasing me (in my trusty Topper sailfish) with revenge in mind, everything I had scoffed at was brought to fruition ( only cos they pooled their resources ! not fair) and a laser cannon added

It was like a bad combo of Star Trek and Waterworld.

NO, No, no oh, nnnnnnnnnnnnnnnnnnnnnnoooooooooooooooooooooohhhhhhhhh

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Some observations on this discussion.

 

1) Unlike the aviation industry, the sailing industry is to small to pay for quantifiable wind tunnel [wind tunnel time is not cheap] data sets of a given design, besides, it could constrain the marketing department....bad engineer...bad engineer. Also, if the data existed why would you assume it applied to your configuration?

 

2) If the clew is too low, it is difficult shape the sail. Sail shape will always trump pressure losses at the margin

 

3) End plate effect, I think that analogy would be more apt if one was to compare to the wing root? If that is the case then the overriding concern is interference drag. Take a look at wing root fairings and see if your deck and sail match any of the wing root configurations you can find?

 

4) Fluid Velocity will play a big role here, at low velocities, the pressure differential between the foil surfaces is also small, the high to low pressure is likewise small, a small shelf is more than adequate for these velocities. As wind speed increases so does angle of heel and what that does to the hull/deck wing root interference is anybody's guess.

 

5) As mentioned above, the wind surfer is doing more than bringing the wing root to the water, he is also changing sweep angle, lowering the CP and bring the CP aft. Bear in mind a 2-3% efficiency increase is not going to produce a large acceleration. Additionally, a sailboard deck/hull is tiny in relation to the sail area when compared to conventional sailboats so I am not sure a comparison can be made without a large amount of hand waving. Nothing against windsurfers, they have produced a lot of progress, but do those guys do a lot of upwind sailing in low wind conditions? And speaking of high clews, don't windsurfers have higher clews if scaled to sail area than sailboats? Have look at the gap beneath the sail, note the clew. The reason sailboarders go so fast [this has been related elsewhere] is because they use the sail to "lift" a portion of their bodies weight off the board and thusly reduce hydrodynamic drag. And while we on the subject of Aero-efficiency, does anybody, upon reflection, find anything aerodynamic about a sailboarders body flopping in the breeze? C'mon guys aero efficiency is not the sailboard's forte, it's the low hydro-resistance.

 

http://www.proofboard.com/news/index.html

 

BTW, these guys go fast too, their sails have to be much flatter and they are restricted in area so small losses loom big. Again, it is not aero efficientcy that produces speed, it's lack of resistance underneath.

 

http://www.iceboat.org/bladerunner/0304.htm

 

http://na.northsails.com/Racing_Sails/ISA.htm

 

So the original answer remains unanswered, but [per board rules] that won't stop me from adding my two cents worth.

 

Here is what I did when I made sails, which preceded my university education [Aero- Engineer] and still seems apt subsequently:

 

1) Size the sail to the guy's existing hardware, if he can control shape with a deck scraper...fine, but rare.

 

2) Cut the sail foot so that it sweeps the first 45 - 60% of the distance to the clew and curve upwards to raise clew so that the sail can be shaped for different wind conditions. On mainsails droop the boom as much as is permissible it's just extra sail area and that really pays off downwind. Unless the main is loose footed, the main has a shelf which is an "end plate".

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All foils make best power at an angle of attack, for the same foil shape high cut or low cut foot does not change the angle of attack (if the sheeting angle is maintained, hard to do with most track locations) so the pointing angle of the sail is not changed. However things like end plate, do change the power produced by the sail and may change the way the keel etc work to change pointing in that way and VMG will be effected as the boat should be faster with the more power.

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DB to your points see my replies below,

 

"...All foils make best power at an angle of attack, for the same foil shape"

 

That is true, but my point was that shape of sails is a variable not a constant.

 

"...high cut or low cut foot does not change the angle of attack...if the sheeting angle is maintained"

 

And my point was you can't control shape [twist and angle of attack] without mechanical advantage, hence the need to raise the clew.

 

"...things like end plate do change the power produced by the sail"

 

The end plate is really the hull/deck wing root, which I note above "the overriding concern is interference drag."

 

Have a look at this and note the horizontal stabilizer, note the "massive" "end plate losses"

 

http://www.airforce-technology.com/projects/jsf/

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Some observations on this discussion.

 

1) Unlike the aviation industry, the sailing industry is to small to pay for quantifiable wind tunnel

Sadly, all too true.

3) End plate effect, I think that analogy would be more apt if one was to compare to the wing root? If that is the case then the overriding concern is interference drag. Take a look at wing root fairings and see if your deck and sail match any of the wing root configurations you can find?

Once you close the gap, then it looks like a wing root, and an ugly one at that. With the gap open it looks a lot more like a tip. You can see this by tufting the sail near the foot. However others have speculated that the reason you don't get anything like the theoretical improvement from closing the gap has to do with junction interference drag, along with gradient effects.

5) As mentioned above, the wind surfer is doing more than bringing the wing root to the water, he is also changing sweep angle, lowering the CP and bring the CP aft. Bear in mind a 2-3% efficiency increase is not going to produce a large acceleration. BTW, these guys go fast too, their sails have to be much flatter and they are restricted in area so small losses loom big. Again, it is not aero efficientcy that produces speed, it's lack of resistance underneath.

Which is why I keep asking, "does it point better?" Sailboards are an interesting case in that they don't go to weather particularly well, so any increase in efficiency in the rig pays large dividends. If you can already point at 20 degrees, a few degrees improvement is going to help a little. If you can only point at 35 degrees, the same few degrees improvement will help a lot more.

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The reason sailboarders go so fast [this has been related elsewhere] is because they use the sail to "lift" a portion of their bodies weight off the board and thusly reduce hydrodynamic drag.

 

No, they don't; at least not for straight-line sailing. For best performance, the rig is held in a vertical plane, just like any other sailboat. This has been known for 20+ years.

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A few degrees in 20 is alot larger percentage improvemant than a few degrees 35

 

If your say a 'percentage improvement' will ring a bit truer.

 

more sail should point better, why not PM shc

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A few degrees in 20 is alot larger percentage improvemant than a few degrees 35

 

If your say a 'percentage improvement' will ring a bit truer.

Lets say we are doing 7 knots and pointing at 20 degrees apparent. That is a VMG of 6.11 knots (and a true wind angle of 29 deg). If we improve that by 4 degrees, then the VMG goes to 6.42 (and true wind angle to 23 deg), and improvement of 5%.

 

Now lets say instead we start at 7 knots pointing 35 degrees apparent. That is a VMG of 4.45 knots (Ta is 51 deg). Improve that by 4 deg and the VMG goes to 4.96 (Ta 45 deg) an improvement of over 11%,

 

So your gain is better both in both percentage (11% vs 5%) and in absolute VMG (0.51 knots vs 0.31) for the 35 deg case. Now, it is going to be a lot harder to get a 4 deg improvement on the boat that already points at 20 deg.

 

By the way, where did you get the plans for my boat, you dirty bastard! wingsail_outrigger_diagram.jpg

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Principles of Yacht Design by Larsson and Aliasson has a chart with wind tunnel testing of gaps between sail and deck (Chapter 7, fig 7.8)

 

As the gap increases, you lose both lift and increase drag. If the gap is 1% of mast height, you lose 4% of your lift and increase drag 7%. The rate you lose efficiency drops off as the gap increases (i.e. the first 2-3% of gap is the worst).

 

Here's some real world data - buy me a case of beer and I'll instrument the boat to get a real scientific study :)

We did a Vic-Maui race a few years ago and took a high clewed jib-top instead of the deck sweeping #1. We lost about 2-3 degress in point but then in that race you're faced with maybe 50 miles of light air beating and then 2300 miles of reaching and running, and I think the jib-top won us the race. I should also mention that the boat gets no end plate effect - it's got a 6' bowsprit so the headsails don't actually start getting close to the deck until about the shrouds (here's the sailplan, it's a Bob Perry boat:

http://www.perryboat.com/largeview/crhper-...r:+41-11+Cutter ).

The sail area is pretty much the same between the JT and the #1.

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"...no end plate effect - it's got a 6' bowsprit "

 

And little interference drag, a much cleaner flow.

 

to all...go back and look at the link I provided above if you think the wing root gap is a silver bullet.

 

http://www.airforce-technology.com/projects/jsf/

You need to explain this a bit more. There is nothing obvious to me on that link. In general high speed jets have almost nothing at all to do with low speed sail aerodynamics, including wing root junction interference.

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Principles of Yacht Design by Larsson and Aliasson has a chart with wind tunnel testing of gaps between sail and deck (Chapter 7, fig 7.8)

 

As the gap increases, you lose both lift and increase drag. If the gap is 1% of mast height, you lose 4% of your lift and increase drag 7%. The rate you lose efficiency drops off as the gap increases (i.e. the first 2-3% of gap is the worst).

 

Here's some real world data - buy me a case of beer and I'll instrument the boat to get a real scientific study :)

We did a Vic-Maui race a few years ago and took a high clewed jib-top instead of the deck sweeping #1.

Thanks for an actual data point! Don't know how to weight it, given the bowsprit. Bob can draw a pretty boat when he wants to, can't he?

 

Messrs Larsson and Eliasson don't provide attribution for their figure, but from the rest of the text we can assume it is based in Marchaj. His published work suggests a 10-15% increase in L/D, which probably equates to around the same improvement in VMG - certainly worth pursuing.

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"high speed jets have almost nothing at all to do with low speed sail aerodynamics" - DDW

 

DDW,

 

The F-35 aircraft operates in flight regimes from 0~1K+ knots, so this statement is based on you misunderstanding this aircraft's flight envelop as well as wing theory. Perhaps some rudimentary research should proceed extremely broad statements that comport to neither to theory or practice.

 

You get two options for fluid flow, compressible or non-compressible that's it...no magic. As a side note the losses are even higher as speed increases...up to mach 1, clearly the designers thought the losses worth the weight saving in structure...that after extensive wind tunnel studies...that should give a clue.

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"high speed jets have almost nothing at all to do with low speed sail aerodynamics" - DDW

 

DDW,

 

The F-35 aircraft operates in flight regimes from 0~1K+ knots, so this statement is based on you misunderstanding this aircraft's flight envelop as well as wing theory. Perhaps some rudimentary research should proceed extremely broad statements that comport to neither to theory or practice.

 

You get two options for fluid flow, compressible or non-compressible that's it...no magic. As a side note the losses are even higher as speed increases...up to mach 1, clearly the designers thought the losses worth the weight saving in structure...that after extensive wind tunnel studies...that should give a clue.

I asked for clarification on exactly what you were talking about, but you have provided none. One thing I can guarantee: when the F-35 is flying between 0 and 30 knots (the speed we are interested in) it is sure as hell not flying on aerodynamic lift. My boat does not have an " engine is coupled with a shaft-driven lift fan system for STOVL propulsion. The counter-rotating lift fan, developed by Rolls-Royce Defence, can generate more than 20,000lb of thrust. " Does yours? Sure would make it go, though - better than an engine powered canting keel.

 

If you are talking about losses at the wing fillets in compressible flow, that is not of interest here. If you are talking about the lifting strakes, that is also not of interest here. Take a look at a modern glider. It doesn't look the slightest like the F-35, nor do the wing fillets, nor does the wing, nor the tail. And most have winglets.

 

Please let us know, what are you talking about?

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Arguably the C class are not una rig boats downwind with the slot(s) open...

 

You are correct. Typing w/o thinking, not uncommon from my side. Cheers from drizzly SF Bay,

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"...engine is coupled with a shaft-driven lift fan system for STOVL propulsion. The counter-rotating lift fan". - DDW,

 

Okay explain this:

 

http://www.globalsecurity.org/military/sys...t/f-15-pics.htm

 

Note same basic config., but no lifting fan, again, clearly the designers thought the aero losses not worth the weight saving in structure.

 

If you want an answer to match your preconceived ideas, just say so.

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This thread is so cool---- It demonstrates to me why we still need the out of the box artists and designers who dream and actually dare to build and perform (ala Burt Rutan) ---

 

The engineers just do not seem to "get it" ------- they are so buried in the #'s that they lose sight of task at hand. But they are certainly necessary and without peer when thay have to make the dreamer's ideas work in the real world!

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The deck actually acts like a winglet preventing high pressure air from spilling under the foot of the jib to the low pressure (inboard) side of the jib . Projected sail area is projected sail area(the more the better) the key is having as much clean attached flow as you can.

If you want data just look at any modern airplane wing.

 

BINGO! Added a furler to a 42' 18,500 boat. Using the same racing headsail (if that's what you call them on our boat) with a 12" higher tack height the boat was lower and slower. The increased air flow in the narrow channel is faster than the surrounding air, and the pressure decreases in this faster flowing area. This creates a chain reaction. As new air approaches the leading edge of the sail and splits, more of it flows to the leeward side - air flow is attracted to low pressure areas and repulsed by high pressure areas. Now an even larger mass of air must travel faster to squeeze through the channel caused by the convex sail and the free air flow, causing an even lower air pressure. This continues to build until the maximum speed is achieved for the existing wind condition, and a maximum low-pressure area is created on the lee side. Without the deck there is lower pressure differential. "It's a scientific fact." - H. Simpson

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"...engine is coupled with a shaft-driven lift fan system for STOVL propulsion. The counter-rotating lift fan". - DDW,

 

Okay explain this:

 

http://www.globalsecurity.org/military/sys...t/f-15-pics.htm

 

Note same basic config., but no lifting fan, again, clearly the designers thought the aero losses not worth the weight saving in structure.

Cool link. But oh my god how far off can you be? A jet fighter has absolutely nothing to do with sail design. Nothing at all. If you cannot see this from simple observation, let alone the slightest cursory research, then certainly nothing I say will help you.

 

edit: No wait, I just got the joke. He's kidding right? Somebody tell me he's kidding!!

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You get two options for fluid flow, compressible or non-compressible that's it...no magic.

 

It's not that simple. The Reynolds number is a scaling effect related to velocity and fluid behaviour is different at different Reynolds numbers. Also, the relative importance of different sources of drag is very different at different speeds. For example, induced drag is important for sailplanes and boats but not important at high subsonic speeds.

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With respect to windsurfers and the end plate effect: it's really not very clear, because when you close the gap, you are also sheeting in that last few inches, which means you have to be driving off the fin much more as well. someone else mentioned that the rake increases, when you close the gap, and it does, but it's always been my opinion that the better sheeting angle is what gives the burst of speed.

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With respect to windsurfers and the end plate effect: it's really not very clear, because when you close the gap, you are also sheeting in that last few inches

 

Don't agree. Increasing rake and sheeting in are two different things, whether you are on a sailboard or a dinghy.

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you can also see under a high clewed sail a hell of a lot easier than you can a low cut. note that im talking genoas here. other than that, you all know my knowledge base.

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S B, you seem keen on planes, so lets use an areoplane picture:

 

tlv.jpg

 

thats what u get with no end plate effect (or foil symmetry), I think its called tip vortex, and in term of drag

 

is quantified as induced drag. Its definitly not good news so if u can avoid it u do so, hence you close the gap

 

between the deck and genoa (look at IACC pics...)

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With respect to windsurfers and the end plate effect: it's really not very clear, because when you close the gap, you are also sheeting in that last few inches

 

Don't agree. Increasing rake and sheeting in are two different things, whether you are on a sailboard or a dinghy.

 

did i say they were the same thing?

 

i'm not sure what you disagree with.

 

what i was trying to say was that it's hard to judge the effect of closing the gap on a windsurfer, because it's not a controlled experiment. the mechanics of the windsurfer rig mean that when most sailors close the gap, they are actually also changing the sheeting angle, and also changing the rake angle. it is precisely because these are "different" things, that you can't judge the effect of closing the gap.

 

in order to measure the effect of closing the gap, you'd like to be able to do so without also changing the sheeting angle, and without changing the rake angle - but you really can't do so on a windsurfer.

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Ch_rlie650,

 

Great picture!, I don't have a lot of time for this today, suffice to say, winglets or not, wing vortices will exist. Think about it, high pressure is always going to move to low pressure in the absence of a structure to prevent it, once the wing has past, the pressure will equalize, the picture would look the same with or with winglets. What would change is the position of the pressure drop off on the wing by few inches...and the drag created by the winglet itself.

 

My point has been all along, that boat itself is far from being an "end plate", the flow on the deck is not even close to being parallel to the flow across the sail which is what you would need to have an "end plate". If you look at the speed board photos on the windsurfer site I provided, you'll note that there is no "end plate" to talk about, all of them show something like 6 inches of gap or greater, which is far more than needed by the volume flow we are talking about. And when it comes to dirty aero configurations, having a body lying perpendicular to the flow is hardly a vote for aero-efficiency. Again, if you are looking for speed on the water, you are looking at the wrong thing, hydro resistance is the problem. Before I'd deal with the problems related to properly shaping a full deck scraper, I'd clean the bottom of the boat just before the race.

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Here are a couple of exerpts from the piece. It was on a windsurfing forum discussing the introduction of wide sleeve formula sails. I'll get a url and another reference when I can.

 

Hope this throws an interest in the sailing end vs the jet end.

 

About jets. I remember very clearly hanging from a crack and having a F-111 pass through the Valley below me. It was loud, but interesting to look down into the cockpit at the jock. Heard he was in a little trouble for going below the Valley walls.

 

Picture_11.pdf Picture_12.pdf Picture_9.pdf

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A few degrees in 20 is alot larger percentage improvemant than a few degrees 35

 

If your say a 'percentage improvement' will ring a bit truer.

Lets say we are doing 7 knots and pointing at 20 degrees apparent. That is a VMG of 6.11 knots (and a true wind angle of 29 deg). If we improve that by 4 degrees, then the VMG goes to 6.42 (and true wind angle to 23 deg), and improvement of 5%.

 

Now lets say instead we start at 7 knots pointing 35 degrees apparent. That is a VMG of 4.45 knots (Ta is 51 deg). Improve that by 4 deg and the VMG goes to 4.96 (Ta 45 deg) an improvement of over 11%,

 

So your gain is better both in both percentage (11% vs 5%) and in absolute VMG (0.51 knots vs 0.31) for the 35 deg case. Now, it is going to be a lot harder to get a 4 deg improvement on the boat that already points at 20 deg.

 

 

NO NO NO, the same 'degrees' improvement was assumed on BOTH models ( by you) IS MY POINT,

not how difficult it would be to achieve. YOUR WORDS here

If you can only point at 35 degrees, the same few degrees improvement will help a lot more.

 

above therefore is blah blah