Winglets on sails?

[oldsailor7]

All this theory is very nice ---but in practical use on full size sailboats it is probably quite impractical

The nearest you can get to it is a horizontal endplate on the top of the mast
and that would have to be large enough to cover the top of a square topped sail. The extra windage would have to be taken into account and also the extra weight high up.

Increased aspect ratio is probably the most practical way to go.

A flat plate on the top of the boom would help with the foot vortex. but would be inconvenient with modern rigs.

 

[peterraymond]

The nearest you can get to it is a horizontal endplate on the top of the mast and that would have to be large enough to cover the top of a square topped sail. The extra windage would have to be taken into account and also the extra weight high up.

The top is where people first think of doing something, but in addition to adding weight, it also moves the center of effort up which is not so good.

Increased aspect ratio is probably the most practical way to go.

Increased aspect ratio should be taken as far as available righting moment allows. The question is, what do you do next?

A flat plate on the top of the boom would help with the foot vortex. but would be inconvenient with modern rigs.

There is more theory at: http://aero.stanford.edu/Reports/VKI_nonplanar_Kroo.pdf They don't mention endplates and I have to think that is because a winglet is more efficient. When you are controlling and redirecting the flow of air, a flat plate is less than ideal. On the other hand, a winglet has the disadvantage that it's sticking out there where you could run into it. I'm thinking though that if your head doesn't hit the boom, you can learn to duck under the winglet too. It also looks like there is some flexibility in where you put it. You might be able to have a design that sits stationary on either side of the mast. Or, you might mount it further back, but some distance up from the base on a wing sail. Winglets near the base move the center of effort down and add power, so they promise some combination of more power and less drag, without increasing heeling moment.

You don't have to do a perfect job. I see the task as finding the best most practical design. Maybe you have two winglets mounted at 90 derees to each other, viewed from above, so that on each tack one is under the boom and the other sticks straight out to leeward. the one under the boom would not do much, but the one to leeward would be optimally cambered and have the correct angle of attack.

I do however think that large heel angles might complicate things, so I'd do the first experiment on an A-cat. An A-cat would also let you take two similar boats and do side by side testing and the result would be class legal too.

 

[Perm Stress]

There is very simple, cheap and effective way to reduce losses on sail: just seal the gap between the boom and hull, as it is done on racing boats with headsails. At least the foremost part of it. If this sealing is made from transparent material and do not intrude in the cockpit space -here you are: extra drive, lower center of effort, no extra weight aloft.
The drawback is most your marina neighbors will laugh at it...

 

[Doug Lord]

There is very simple, cheap and effective way to reduce losses on sail: just seal the gap between the boom and hull, as it is done on racing boats with headsails. At least the foremost part of it. If this sealing is made from transparent material and do not intrude in the cockpit space -here you are: extra drive, lower center of effort, no extra weight aloft.
The drawback is most your marina neighbors will laugh at it...

--------------
Steve Dashew, for one, tried what you suggest and said there was a dramatic improvement on his boat-I'll try to find the article....

 

[peterraymond]

There is very simple, cheap and effective way to reduce losses on sail: just seal the gap between the boom and hull, as it is done on racing boats with headsails. At least the foremost part of it. If this sealing is made from transparent material and do not intrude in the cockpit space -here you are: extra drive, lower center of effort, no extra weight aloft.
The drawback is most your marina neighbors will laugh at it...

Wait, that's too easy! On many boats the boom position is hard to lower, but some skiff boats have that extension from the below the boom to the mast. On a cruising boat you might benefit from a wish-boom type boom and a sail that brushes the cabin roof for part of the way back. Still even in this case, a lot of times part of the crew goes over the roof on a tack. You also have the problem of the boom vang. The jack from above may make sense.

Boats hulls aren't typically designed for smooth airflow to the sail, but the flow around the boom you are interacting with is probably more from span-wise flow anyway. I don't know if you are better off or not on a cat. there might be some potential with the tramp, but with a cat you have to cross under the boom too. A cat hull with minimum freeboard ahead of the main beam will cause less disturbance of the flow to sail area near the base.

 

[Doug Lord]

Interesting discourse found by Brian Eiland, post# 50 "Sail Aerodynamics here:

Hi Steve and Linda, Thanks for all of the excellent books and tapes on you adventures. They have been a great help. I have noticed the winglets on airplane wings over the last few years. Has any one tried making a "plate" at the top of the mast, maybe using carbon fiber as a frame covered with sail cloth, to form a device which would reduce the vortexes created by a headsail & main combination? If if would work with a plate on each side of the mast, to tending would be needed during tacking or gybing. Asked my sailmaker about it but he deals with racers more than cruisers, so he is not too interested in the idea. Since you seem to be interested in making cruisers go faster with less effort, thought this idea might be for you. Thanks for thinking about it. Crawford
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Hi Crawford: Interesting concept, and as a glider pilot, with some very long and exotically shaped "winglets" I can relate to what you are suggesting. However, in a sailboat situation there are a whole series of variable which make this idea impractical.

On the other hand, there is another approach which we've used over the years which does work in some cases. This is to "endplate" or seal off the bottom of the boomed sails. If you can achieve this for even half of the foot length, the increase in efficiency is dramatic.

On our 67' ketch, Sundeer, we were able to pick up five degrees in weatherliness--without losing boat speed, when we sealed the main and mizzen. We've just had seals made for Beowulf which we'll be testing in the near future, and will write up for SetSail.

The area added is down low, where it is in turbulent air flow and where the breeze is much lighter. However, the seal effect is very powerful, and if you can make it work with your rig and deck structure, will generate a huge improvement. Note--the less efficient your keel, the more this will help as it reduces induced drag--which hit cruising keels harder than those found on racing boats. Regards--Steve__________________

 

[Petros]

I have thought about this issue for a long time, but I think the advanage would be limited. Any effect of end plating, either a winglet at the top or a gap seal at the boom, has the same effect as increasing the aspect ratio. For the same sail area it would be far better to just increase the aspect ratio in terms of efficiency (less losses). But this has the undesirable effect of raising the aerodynamic center on the sail. If you can deal with that there are real and practical benefits (not just theoretical) for sail performance.

The extra weight up top, and extra complexity of a winglet at the top of the mast would have only one area of advantage (over a larger sail) that I can think of. the current way the rules are written the sail area is calculated as the projected area of the mast and sail. That means presumably they would not count the area of the winglet (nor the cabin roof either if you seal the sail below the boom). That would be your advantage since the effective sail area would than be larger than the projected sail area. This would at first be an advantage, until all the new records are set with sail with winglets, and they change the rules.

So it is simply a way to "cheat" the intent of the rules, and that likely will only be a temporary condition.

 

[peterraymond]

I added a couple more lines to the graph I stole from Tom Speer to compare the effect of sealing the bottom to adding aspect ratio. If you seal the bottom - perfectly, the optimum plan form changes and that lowers the heeling moment. You can then increase the aspect ratio by about 30% and the result will be the same lift and heeling moment from the sail, but less than 1/2 the induced drag. This is not particularly new and just repeats what I said in my first post in this thread.

If you wanted the same reduction in induced drag, while maintaining a 5% gap, you would have to increase the aspect ratio around 60% and the heeling moment would go up around 29%, for the same lift. This shows the cost of using aspect ratio, while the first example shows the big benefit of doing something at the bottom.

(The 60% and 29% numbers, depending on how Tom set up the graph, may not be quite right, because they assume the gap will stay 5%, but if you did increase the aspect ratio 60%, the same physical gap at the bottom would now be less: 5/1.6 = 3.13% of the new hoist.)

Whether winglets or sealing the bottom would work better must depend on a bunch of stuff, not the least being class rules. The response of rule makers will vary from class to class too. Certainly in some classes, whatever improvement you came up with would be gone if the boat did well at a World Championship. I suspect that in, for instance A-cats, the response would be to congratulate you. Here I think that sealing the base might be tough. Winglets? To go a good job would probably take a 3 CFD analysis, or some informed design and testing. I also don't think it would buy you as much as sealing the bottom.

 

[ScottyBooth]

Thanks for all the good discussion guys. In windsurfing it's called closing the gap when you put the foot of the sail right down against the board. It definitely makes you go faster.

 

[jimburden]

The most efficient modern aircraft have forward swept wings or very high aspect ratio wings with semutiar tips to reduce losses. No modern aircraft of any kind has had the equivalent of a wing seprated from the hull like we have rigs that are not airtight with the cabin or deck top to reduce the possibly greater lower losses. Some diagrams but no wind tunnel studies that I have ever seen show the wind strength losses off the under boom and sail top. returning to whale back hulls might also improve conditions for the sailand flairing the hull into the wing mast or sail with fillets. You can add extra wing length to a planes wings but unless the basic structure is improved this can weaken the wing under shock or cyclic loads. Long wing fenses are no longer used on inner wings to prevent stalls and winglets are added to existing wings to reduce stresses for a slight improvement in lift to drag ratio. Boeing is claiming top glider lift to drag ratios on some future designs that have not been tested yet. Sailboats could benefit from this accept they have to operate under many times the airflow direction, huge macro and micro surface turbulence and speed variability planes see only partly on landing and noe as slow motion. Sailboats have to handle all situations as well as possible ideally doing it all as efficiently as possible. Most hulls run out of upper speed potential before little differences are much noticed accept in a competition where often a few boat lengths over miles is visible design proof. Why do we not experiment before we discouragingly comment and at least give it a chance? If future hulls were designed to be lower drag and faster then maybe winglets would at some point be useful.

 

[Doug Lord]

In the last few years the positive effect of closing the gap partially or fully under a wing or sail has been proven beyond a shadow of a doubt.
In the this years Little America's Cup and in the previous version of that race the winner, Groupama, sealed their wing to the tramp. In AC 34 Oracle did the same with their wing on the AC 72. This year the A Class featured two of the top guys using a rig that sealed the main to the tramp-both are convinced that it helps a lot.

A Class rig by Mischa Heemskerk:
(Henk de Grad picture)

 

[champ0815]

I think although it is easier to close the bottom gap of the sail, from the dilettante viewpoint on sailing aerodynamics it would be more beneficial to add a winglet on the top of the sail since there I would expect more losses of efficiency due to higher air speeds at the height of the upper mast end.
However, no reason to omit the easy gain in efficiency on the bottom... .

 

[gggGuest]

In the last few years the positive effect of closing the gap partially or fully under a wing or sail has been proven beyond a shadow of a doubt.

I thought it was proved comprehensively about 50 years ago when within the space of two or three years pretty much every dinghy class started rigging their jibs so that they touched the foredeck...

 

[Doug Lord]

seal

I'd say you were right.

 

[CT249]

I think although it is easier to close the bottom gap of the sail, from the dilettante viewpoint on sailing aerodynamics it would be more beneficial to add a winglet on the top of the sail since there I would expect more losses of efficiency due to higher air speeds at the height of the upper mast end.
However, no reason to omit the easy gain in efficiency on the bottom... .

The Boeing aerodynamics experts on this forum have repeatedly explained that winglets are not such an easy gain. If you have the capacity to fit them it's better to just extend the rig height.