Fat-headed mainsail

[rom]

Originally Posted by Stumble

I have miv d from a pinhead to a square top both made by North, both from 3DL, both sailed by the same crew.

The square headed main was faster and pointed higher upwind, and faster and deeper down wind. The difference was such that we had to reprogram the boats polar program because it was no longer a reasonable approximation of boat speed.


I have been involved in converting at least 3 boats from high quality pin head racing mains to square top mains, and I need every case the effect has been the same. You point higher and go faster upwind, and go faster and deeper downwind.

Kim's grasp of theory is far beyond mine, so I can't engage, but I think his theory is right (and I don't believe it is) then the theory is wrong. Because direct observation across multiple boats, multiple, classes, and thousands of trials has proven onthe water that flat heads are faster sails.

Thank you Stumble for sharing that real life experience. And I gather that you would strongly advice me to buy a square top when the time comes to replace my main sail. But I am not sure I understand correctly, your experience is based on a rather fast cat right ? Do you yourself have comparison experience on condomarans like mine ?

[Stumble]

Originally Posted by rom

Thank you Stumble for sharing that real life experience. And I gather that you would strongly advice me to buy a square top when the time comes to replace my main sail. But I am not sure I understand correctly, your experience is based on a rather fast cat right ? Do you yourself have comparison experience on condomarans like mine ?

My direct experience is on a TP-52, Olson 30, and a Corsair Trimaran. These are the boats I was directly involved in the conversion of, and sailed a lot both before and after the conversion.

I have seen the same results on a Tripp 40, Melges 30, Andrews 26, Colgate 26, Olson 29, J-22, and probably a few more I am forgetting. Again in every case the boats point higher and go faster with the square top compared to the pin head.

Finally you can look at every single development class that constrains sail area or mast height. In your case because you are not going to replace your mast the best example are classes with fixed mast height like the Aussie 18 skiffs that have no limit on sail area, but a hard limit on mast height. They basically sail with rectangular sails, because it works the best.

The A-Class Catamaran only constrains sail area, so what did we do? 30' tall masts and square tops. We could have gone to taller masts, and pin head sails, but they are slower so we didn't (though people have tried it).

I hesitate to make unconditional statements, but I can not think of a single class where mast height or sail area is restricted, but sail shape is not, that hasn't gone to a square top main. And it isn't because they are slower. It's as close to a universal rule as I can't think of. And it comes with other benefits as well...

1) When reefing the CP of the sail just moves down, it doesn't move forward (well as much to be complete), making the boat easier to keep balanced.

2) Fully battened sails (pretty much a requirement for a square top) flog less in light air and chop making for a much quieter sail that lasts longer.

3) Automatic gust responce. In effect the first reef I saw the head of the sail opening up. Meaning you can carry more sail longer for the lulls and no time be overpowered in the gusts.

The two major downsides however need to be addressed.

1) main sheet loads go thru the roof. It takes a lot of sheet tension to keep the top batten at the right twist. Substantially more than a pin head. So it's may be necessary to beef up the traveler to cope.

2) they can be a pain to flake in the bag correctly. The top batten is not parallel to the rest (it's somewhere around 45 degrees) so you have to deal with it somehow.

[Stumble]

As always I am not an engineer, but the primary reason that square heads are faster (for the same sail area) is that they reduce induced drag. Specifically tip vortex strength at the top of the sail.

Think of it this way, as the air is passing over the sail it will make a strait line for the shortest distance across the sail. The molecules don't care I find that is up, or laterally, or thru the sail, they just want to make the shortest trip to the low preassure side of the sail. So take out a piece of string and attach it to the front o the mast, and measure the shortest distance the air can travel. On a pin head sail that I sail going to be upward at an angle defined by the leech angle (the angle between the boom and the leech of the sail). That's the shortest distance the air can travel, so that's what it does.

On a pin head sail the wind then may be moving upward as much as it is moving backward, possibly more. When the air runs out of sail, the upward motion of the wind hits the lateral movement of air from the back of the sail and creates a vortex. This vortex is pure drag, and the faster the wind is moving upwards the stronger the vortex will be.

On a square head the upward flow angle is minimized because of the higher leech angle, so much so that only at the very top of the sail does flow pass over the top instead of out the back. This reduces the vortex strength, and thus the drag.

Now ideally you would use an elliptically sail, but it's near impossible to build, and there are more complexities like twist we haven't dealt with yet that come into play... A pin head is most efficient when the leech can be bent to windward, and elliptical when it is directly in line with the flow of the air, and a square head when it is bend to leeward.

So which is easiest to achieve on a sail? Why allowing the top to spill off a bit to leeward, in fact it is almost impossible to get a sail to do anything else (except rigid wings). So not only does a square head reduce induced drag just by dint of its shape, it also is most efficient when the tip I sail bend off to leeward, which is what's happening.

Describing all of this mathematically is beyond me, but there are plenty of good NA articles that deal with it.

Note: That all of this is exactly the same for keels, daggerboard, and rudders. Except that with them it is possible to build rigid structures so elliptical shapes are more common.

[Cavalier MK2]

I'm convinced. I'll keep my conventional main but get to work on my crabclaw topsail for zephyrs. That will give me a stylish square top and keep the center of effort low for normal winds.

[thinwater]

I think a better question is "which is better for the cruiser--high roach or square top?"

I'm tired of the aerodynamic arguments. I'm an engineer and 30-year sailor, I understand them, but mostly it is wasted wind. Race results prove square tops are faster period. The professional opinion that started the whole thread was obviously myopic in the extreme.

I'm tired of the strawman that the big main/small jib formula is wrong. For many boats it is, and many are under powered. It sells because people like self tacking and they don't want to sell boats into the charter market that require skilled crew. I have high roach and a big genoa (not in the Avatar--that's not actually my boat) and it works great. It is also too high powered to hand over to a charter crew in a breeze. I've also modified the keels to match it all up. But non of that addresses the high roach/square top puzzle.

Some designers can't imagine how you tension the forestay without a backstay. The answer, as any competitive beach cat sailor knows, is mainsheet tension.

----

The difference is sail area is not great. The demands on the sail cloth up high are greater. Hoisting is problematical (form what I understand) because of the typical diagonal batten. Cost is higher. When I ordered my last main, I asked the sailmaker's to be open ended in their quotes, and still, all were for high roach, full batten conventional sails. Although the top is not square, there is a lot of area up high and foot is basically constant length until at least 1/3 hoist.

Let's discuss this^^.

[Just Another Sa]

Originally Posted by Kim Klaka

Hello, I am back. After spending far too much of my remaining life trying to reconfigure my argument, I now have what I hope is a clearer explanation of my statement that started all this:
There is no advantage in a high lift-drag ratio rig on a low lift-drag ratio hull of a multihull with stub keels.

This post is a bit long, it is divided into 3 parts: some background notes, rig efficiency and sail efficiency.

Notes:
1. This is no longer about all the pros and cons of fat-head mains, it is about their windward performance.
2. This is not a definitive explanation, there are generalisations which mean that the argument does not always apply; but it does work for most boats and sailing conditions.
3. I have not considered the impact of sail-carrying power on the argument. It will doubtless influence it, but I don’t think it negates it.
4. The book I refer to as “Garrett” is Symmetry of Sailing by Ross Garrett, published 1987 by Adlard Coles; and “Marchaj” is Sailing Theory and Practice by Tony Marchaj, published by Dodd Mead & Co 1982 i.e. the revised edition, not the original 1964 one. I have also looked at other sources but I have not referenced them here in the interests of brevity (already failing!)

Hull efficiency:
By “hull” I mean canoe body, keels and rudders. I will not examine hull efficiency in detail, rather I will make use of the on-water observation that high efficiency hulls point higher and have less leeway than low efficiency hulls. This appears to be supported by VPPs, as evidenced by the design approach taken by many/most designers. All I need assume is that a multihull with stub keels will sail at around 40 deg to Apparent Wind Angle (AWA) and 5-10 deg leeway, making for a total sailing angle (path relative to apparent wind) of 45-50 deg. (open to refinement, but doesn’t make much difference to the overall argument). This compares with a high efficiency hull which sails at, say, 30 deg AWA and 3-5 deg leeway (total sailing angle 33-35deg).

Rig efficiency:
Consider the lift v. drag (L/D) curve of a typical mainsail. L/D max occurs at the point tangent to the origin, but a boat will almost always sail at a point further along the curve (i.e. away from the origin), at a higher lift value. This is because it will operate at the point of maximum thrust, which is the point tangent to a line perpendicular to the sailing path, which is further along the curve than the L/D max point. Therefore increasing L/D max does not of itself increase thrust (hence boat speed, hence VMG for a given sailing angle). What matters is how a change in the rig alters the shape of the L/D curve beyond the point of L/D max.
Consider then a change in rig efficiency. The nearest data I have for flat-top v triangular mainsail is the Lionheart 1980 elliptic rig tests shown in Marchaj p83 fig 52. However, I suspect this data is probably flawed (it shows the elliptic rig reducing profile drag but not induced drag; opposite of theory). So I will have to generalize from flat-top v triangular, to high aspect ratio (AR) v. low AR…..
Look at the L/D curves in Garrett p110 fig 4.21 (replicated in Marchaj p150 fig 100), showing the L/D curves for 3 different AR. Ignore the curve for the highest AR (it is an aberration for the purposes of this argument – a mast interference effect – but if you choose not to ignore it, it strengthens my argument).
The data shows that the high AR rig will generate more thrust than the low AR rig at low sailing angles (no surprises there), but less thrust at high sailing angles (the “gaff rig phenomenon”). The crossover point between the two rigs is at a sailing angle of about 45 deg. I made the observation earlier that the sailing angle of the low hull efficiency stub-keel cat is about 45-50 deg. It should now become clear that the high AR rig does not improve the performance if put on the low efficiency hull. Depending on the exact numbers for a particular boat, the low AR rig might even generate the greater thrust, and certainly not less thrust. Conversely, if you put the high AR rig on a high efficiency hull, you will indeed reap the benefits of the higher rig efficiency, because you are operating lower down the curve. There is an assumption here that increased thrust equates to increased VMG. This is only true for unchanged sailing angle so there is a second order correction to be made. There is also the heeling force to consider, but as I said near the beginning, I don’t think that will negate the conclusion.
So this argument supports my original statement, that there is no advantage in putting a high efficiency rig on a low efficiency hull. It is not a rigid proof – there are far too many generalizations and approximations – but I hope it explains the point rather better than my previous attempt.

"I have not considered the impact of sail-carrying power on the argument."
Good, because with stub keels (assuming here you mean shallow draft), the increased drag from those keels when leeway is increased due to maximizing aerodynamic thrust reduces upwind vmg a lot sooner than limited righting moment. For optimum vmg less than maximum thrust from the sails is needed, the difference being much greater with stub keels than with deep daggerboards.
Do you realize a squaretop main with the same mast and boom length and thus same P and E measurement, is lower AR than a pinhead? More area and the same span.
Thrust of the rig can not be determined by AR and sail angle, even with same conditions. Sail area is very relevant and can not be ignored.
"Look at the L/D curves in Garrett p110 fig 4.21 (replicated in Marchaj p150 fig 100), showing the L/D curves for 3 different AR"
I don't have those books available, but have read the latter decades ago. You or the author is leaving something very relevant information out. Was that pic perhaps only valid for the case with identical sail area and identical planform, and therefore not supporting your generalized argument?

[Just Another Sa]

Originally Posted by Stumble

As always I am not an engineer, but the primary reason that square heads are faster (for the same sail area) is that they reduce induced drag. Specifically tip vortex strength at the top of the sail.

On a square head the upward flow angle is minimized because of the higher leech angle, so much so that only at the very top of the sail does flow pass over the top instead of out the back. This reduces the vortex strength, and thus the drag.

For the same lift and area, tip vortex strength at the top of the sail is higher for the squaretop. The reason is in bold type font in the above quote.
Greater change in lift while moving up leads to higher vortex at the tip.
Because it is so only at the tip, a squaretop has less vortex and drag overall under the given assumptions.

And please, please forget the the shortest trip idea. The air molecules have no way of knowing where the shortest path might be, neither have they brains to act accordingly. The pressure gradient gives the direction of force acting on the air particles and thus direction of acceleration, definitely not the path.

[Stumble]

Originally Posted by Just Another Sa

For the same lift and area, tip vortex strength at the top of the sail is higher for the squaretop. The reason is in bold type font in the above quote.
Greater change in lift while moving up leads to higher vortex at the tip.
Because it is so only at the tip, a squaretop has less vortex and drag overall under the given assumptions.

And please, please forget the the shortest trip idea. The air molecules have no way of knowing where the shortest path might be, neither have they brains to act accordingly. The pressure gradient gives the direction of force acting on the air particles and thus direction of acceleration, definitely not the path.

Wait you mean air molecules aren't intelligent actors that behave according to their whim? I had no idea. The fact that they follow the shortest distance to equalize preassure however is good science though based on physics not free will.

I won't debate well settled science, but feel free to raise the issue with Eric Sponberg https://www.ericwsponberg.com/wp-con...ding-masts.pdf who has more rig designers under his belt than anyone I know of.

Or C.A. Marchaj Who literly wrote the book on sailing aerodynamics. 'Aero-hydrodynamics of Sailing'

[Just Another Sa]

Originally Posted by Stumble

Wait you mean air molecules aren't intelligent actors that behave according to their whim? I had no idea. The fact that they follow the shortest distance to equalize preassure however is good science though based on physics not free will.

I won't debate well settled science, but feel free to raise the issue with Eric Sponberg https://www.ericwsponberg.com/wp-con...ding-masts.pdf who has more rig designers under his belt than anyone I know of.

Or C.A. Marchaj Who literly wrote the book on sailing aerodynamics. 'Aero-hydrodynamics of Sailing'

You won't debate well settled science?
Perhaps you won't debate, but you disagree with it. No need to raise the issue with Eric Sponberg or C.A. Marchaj, who already agree with me.
You link was about rig design, not aerodynamics, and it did not support you false claim at all. There were no path related nonsense in it.

[Stumble]

Originally Posted by Just Another Sa

You won't debate well settled science?
Perhaps you won't debate, but you disagree with it. No need to raise the issue with Eric Sponberg or C.A. Marchaj, who already agree with me.
You link was about rig design, not aerodynamics, and it did not support you false claim at all. There were no path related nonsense in it.

Fine, find an example of a highly efficient wing that uses a trianglular end, well anything other than supersonic aircraft wings. Because supersonic travel changes all the rules. From long distance gliders, boeings new winglets (wing end plates), to AC foiling boats every foil designed for maximum efficiency uses either an elliptical, or rounded tip, unless they find a way to add an end plate (Boeing winglets first act as a new endplate for the wing, then use a square head to reduce the winglets drag).

So far as I can tell the only application of wings in the world that have stuck with pointed ended wings are sailboats. But hey, maybe the entire Aero space, high performance sailing, and glider industry is wrong and you have it right.

Hey, maybe I have the explanation wrong, I can certainly imagine that, but the result? No the end result I said correct. Below are four exceptionally well designed foils, all of which share the same basic form, long, thin, and flat (ish) at the tip to reduce induced drag.

[Just Another Sa]

Originally Posted by Stumble

Fine, find an example of a highly efficient wing that uses a trianglular end, well anything other than supersonic aircraft wings. Because supersonic travel changes all the rules. From long distance gliders, boeings new winglets (wing end plates), to AC foiling boats every foil designed for maximum efficiency uses either an elliptical, or rounded tip, unless they find a way to add an end plate (Boeing winglets first act as a new endplate for the wing, then use a square head to reduce the winglets drag).

So far as I can tell the only application of wings in the world that have stuck with pointed ended wings are sailboats. But hey, maybe the entire Aero space, high performance sailing, and glider industry is wrong and you have it right.

Hey, maybe I have the explanation wrong, I can certainly imagine that, but the result? No the end result I said correct. Below are four exceptionally well designed foils, all of which share the same basic form, long, thin, and flat (ish) at the tip to reduce induced drag.

It seems you are asking me to provide evidence that would prove me wrong. I can't do that. Flat heads are more efficient than pinheads if amount of lift, span, and flow speed and fluid density remain the same, and that is not going to change no matter how much evidence I try to find according to your request. Distance along the path has no relevance to the matter. Entire Aero space has it right and so do I.
Like I already said in post #67: "a squaretop has less vortex and drag overall under the given assumptions."

[Kim Klaka]

Originally Posted by Just Another Sa

"I have not considered the impact of sail-carrying power on the argument."
Good, because with stub keels (assuming here you mean shallow draft), the increased drag from those keels when leeway is increased due to maximizing aerodynamic thrust reduces upwind vmg a lot sooner than limited righting moment. For optimum vmg less than maximum thrust from the sails is needed, the difference being much greater with stub keels than with deep daggerboards.
Do you realize a squaretop main with the same mast and boom length and thus same P and E measurement, is lower AR than a pinhead? More area and the same span.
Thrust of the rig can not be determined by AR and sail angle, even with same conditions. Sail area is very relevant and can not be ignored.
"Look at the L/D curves in Garrett p110 fig 4.21 (replicated in Marchaj p150 fig 100), showing the L/D curves for 3 different AR"
I don't have those books available, but have read the latter decades ago. You or the author is leaving something very relevant information out. Was that pic perhaps only valid for the case with identical sail area and identical planform, and therefore not supporting your generalized argument?

You raise a difficulty that always exists when measuring or calculating the effect of changing one parameter at a time - other parameters also change. The data I refer to above is for rigs of the same sail area but different aspect ratio. That is a standard scientific procedure, and none the worse for it. However, it does inadvertently change other parameters e.g. Reynolds number. So you never know definitively whether the change in performance is due to the change in the parameter you chose, or some other parameter that also changed. nobody has found a solution to this, other than to accept that there is more than one change happening at a time, then using knowledge of physics or whatever to deduce/intuit/infer which of the changes is the more likely to cause the effect found. I think that is what Okkam's Razor is all about, though not certain.

[Kim Klaka]

Originally Posted by SVNeko

Does it make any sense to combine AWA and sailing angle (leeway)? Seems apples and oranges to me, or at least tangerines and oranges.

It is just one of the many standard ways of defining your angles. Adding leeway to AWA gives you the apparent wind angle to the boat's path. If you leave the leeway out, you are omitting a part of the efficiency jigsaw puzzle. It is the way Marchaj, Garrett and others analyse the problem in order to explain the matter. I found it got a lot more difficult to understand if you don't include the leeway angle. It's not wrong to do that though, just not as clear for most of us.

[Stumble]

Originally Posted by Just Another Sa

[/B]For the same lift and area, tip vortex strength at the top of the sail is higher for the squaretop. [/B]The reason is in bold type font in the above quote.
Greater change in lift while moving up leads to higher vortex at the tip.
Because it is so only at the tip, a squaretop has less vortex and drag overall under the given assumptions.

And please, please forget the the shortest trip idea. The air molecules have no way of knowing where the shortest path might be, neither have they brains to act accordingly. The pressure gradient gives the direction of force acting on the air particles and thus direction of acceleration, definitely not the path.

I am not sure how you square the above two bolded statements? Is this an autocorrect issue? Because I think we may be arguing the same side at cross purposes and my admittedly weak aerodynamic knowledge is hampering things.

[jdazey]

How many bolded statements?