Winglets on sails?

[champ0815]

Is there a reason for not using winglets on sails?
On planes they are used to increase the efficiency of the wings (http://en.wikipedia.org/wiki/Wingtip_device), why shouldn't they be used on sailboats?

[yipster]

http://www.boatdesign.net/forums/showthread.php?t=16657 from the bit harder to find boatwiki http://boatdesign.net/wiki/Main_Page
read under Sail Plan - Needs additional contributions arvelgentry on how the jib acts as LEX and search various threads (and check my gallery) for winglet
another great thread, sail airodynamics http://www.boatdesign.net/forums/showthread.php?t=457

[champ0815]

Wow, that's something to read...
Thanks a lot - must have missed it in my short survey about the topic...

[tspeer]

The use of winglets hinges on the question, "Does a winglet reduce the induced drag due to lift more than it adds in parasite drag?" If you are adding surface area the size of the winglet, the least induced drag is obtained when that area is used to extend the span of the lifting surface itself.

If the span is limited for some reason, say, in the case of a Standard Class sailplane or the need to fit airplanes next to each other at terminal gates, then a winglet makes sense. Winglets make sense on keels because the depth is limited by grounding considerations.

But for a sail rig, there are rarely any a priori limits on the height, so it's better to just make the rig taller. An exception might be if you are designing to get under the bridges on the Intracoastal Waterway, etc.

Other factors include not being able to reef a winglet, getting a design that works for both tacks and at a number of angles of heel, and weight aloft.

[champ0815]

Other factors include not being able to reef a winglet, getting a design that works for both tacks and at a number of angles of heel, and weight aloft.
Well, greater height of a sail is a problem since the heeling momentum is increased, or am i wrong?
As for the reefing, i thought about a square top sail with the whatsoever winglet attached to the top batten, so it would reef with the sail...

[yipster]

thanks again for the points Tom, very interesting subject nevertheles http://aero.stanford.edu/Reports/VKI_nonplanar_Kroo.pdf
more reading from under the search button on winglets http://www.boatdesign.net/forums/search.php?searchid=1251173

[champ0815]

Nevertheless, most of the posts about winglets are about the underwater side of the boat. There is no thread dedicated to the aerodynamic effects on the upper (and lower) end of the sail. (at least, I haven't found one)
I just wanted to start a discussion about the potential increase in efficiency of a sail (same area, same aspect ratio) when the flow around the upper and lower edge of the sail are somehow diminished.

[quicksail]

I have to agree with Tspeer on this. As we all know design is all about compromise. Yes you can make the "sail" more effiecient by adding winglets but at what cost. Will the suface drag be increase? What about the added weight aloft and the reduction in righting moment? This consequences might hinder the overall performance of the boat.

I to have been interested to see if winglets can be used. Especially now with large square topped mainsails. But how big would they need to be? It is hard enough to support the square tope mainsails now. How much added weight would winglets be? Lots of questions that would need to be answered. Sounds like a good project for a student. Could be fun.

[champ0815]

Will the suface drag be increase?
Sure it would, but it would depend on the material and the size... What about the added weight aloft and the reduction in righting moment?
Can you explain the reduction in righting moment? I thought it would result in a reduction of heeling moment... .:confused: Or do you mean reduction in righting following additional weight in the mast top?
But how big would they need to be?
Judging from the size of winglets on planes, the area would not be to so large. But maybe it's a question of velocity...:?:
It is hard enough to support the square tope mainsails now. How much added weight would winglets be? Lots of questions that would need to be answered. Sounds like a good project for a student. Could be fun. What about endplates on the lower edge of the sail? Maybe some structure on the boom or only a tiny gap between boom and the roof of the cabin on larger boats (each tack and each jibe cleans your roof top window or your solar panels:D )? There, the added weight (if any) would be closer to the COG of the boat.

[TTS]

Does anyone have a design for sail winglets. It might be interesting to try on an A-Cat.

[Doug Lord]

Tom, contact Magnus("High on Carbon"); he and Fredo tried them on a C CLass cat. Find his contact info here(click on "High on Carbon") :
http://boatdesign.net/forums/showthread.php?t=18998&page=2

[miloman]

The first thing to do in order to increase the efficiency of a sail is to change its shape. Triangles are poor foils, rectangles, trapezoids, and elipses all generate much more lift for their drag than triangles. You've probably already noticed that there are no triangular keels. You can see on many modern rigs that mainsails are growing more eliptical or rectangular. Fat headed rigs, and rigs with fully battened mains and huge roaches are examples of this trend.

The reasons that modern sails tend to be triangular is that they are a natural shape to fit on our tall masts with standing backstays. The gaff main is at least in theory not a terrible shape for a sail. Don't let anyone tell you that the gaff disappeared because it was less efficient than the marconi sail. The gaff rig disappeared because the International and Universal rules only measured the amount of sail area a boat could set, and this doesn't take account for the shorter gaff rigs ability to carry more sail. Steve Dashew's Beowulf shows the potential of shorter more efficiently shaped sails. The masts on Beowulf are stubby and short but they carry full-battened mains with a practically eliptical, she is quite fast. Eric Sponberg also has some information about sail shape on his website.

Basically changing the shape is a much quicker way to increase the efficiency of a sail than adding winglets.

[champ0815]

Well, I completely agree with you! Winglets on triangular sails wouldn't make sense, since there is no top edge around which the air can stream from high to low pressure side (although there should be some tip vortice?).
Anyhow, I have to repeat my question:
For a given efficient sail shape, preferably rectangular at the top, and a given area, would there be any advantage to add winglets in order to increase the efficiency?
Or the other way round, can winglets increase the efficiency in a way to allow for the reduction of mast height with equal performance to a non-winglet sail? I think, this is an interesting question, since a lower rig results in lower heeling momentums and therefore in faster and safer sailing for both mono- and multihulls.

[oldsailor7]

IMHO an attempt to put winglets on the top of a sail is impractical for the reasons already stated. There is also the fact that a pitching motion of the boat is going to be continually altering the AOA of the winglet,thus reducing its efficiency.

However any sail configuration has to be related to the hull of the boat it is going to be driving.

It would be an incongruity to put a high aspect ratio square top fully battened sail on a Drascombe Lugger for instance. A short masted lug rig is more effective, despite its poorer L/D ratio, because it is more suited to the limited hull speed of the Lugger.

On the other hand the tall higher efficiency sails are eminently suitable on boats such as an A Class Catamaran, or any boat that has a fine and slippery hull shape and low windage.

[tspeer]

Well, I completely agree with you! Winglets on triangular sails wouldn't make sense, since there is no top edge around which the air can stream from high to low pressure side (although there should be some tip vortice?).

This is a widely repeated explanation for the induced drag of a lifting surface, but it is wildly inaccurate and misleading. Induced drag is due to the fact that the lifting surface has a finite span, but air escaping around the tip has very little to do with it.

Induced drag is the result of deflecting a finite amount of air through a finite angle, imparting a sideways velocity to it. It's not just the air at the tip that counts, it's the entire wake that matters. In effect, a boat sails in a header of its own making. This self-generated header results in the lift vector being tilted backwards compared to the free-stream velocity upstream, and this looks like a drag component.

The trailing vortices are the result of the rest of the air getting out of the way of the slice of deflected air, and flowing in behind to replace it. They are exactly the same as the vortices you see at the edge of a canoe paddle as you draw it through the water. With the paddle, it's clear that the water is flowing outboard from the center of the paddle, circulating around and coming in behind the paddle.

The same thing happens to the air in the wake behind a sail. The deflected air on the windward side of the wake is shoved to windward and towards the top and bottom of the wake. The deflected air has to be replaced by air to leeward, and at the edges of the wake the air is flowing around and coming back in.

The planform shape, along with the camber and twist, determine how the lift is distributed along the span. The lift at a given spanwise station is proportional to how much the air is deflected at that station.

The amount of vorticity (swirling) shed into the wake is proportional to how much the lift is varying along the span. At the tip, the lift has to go to zero. So there's a lot of vorticity shed there as the lift goes from being substantial to being nothing. A triangular planform ramps the lift down over a longer distance, so the vorticity is more spread out. But it's still there.

For a given efficient sail shape, preferably rectangular at the top, and a given area, would there be any advantage to add winglets in order to increase the efficiency?

The minimum induced drag is obtained when the induced velocity (the self-generated header) is uniform along the span. This is true, even if the freestream is not uniform, or what the planform shape is, or if the surface is experiencing interference from some source such as the water's surface.

If you have a non-planar lifting surface, like a sail rig with winglets, the minimum induced drag is obtained when the induced velocity is uniform along each panel and proportional to the cosine of the dihedral angle of that panel. In other words, if the sail is perpendicular to the apparent wind and the winglet is perpendicular to the sail, then because the sail has a zero dihedral angle and the winglet has a 90 degree dihedral angle, the induced velocity (called the "downwash" in aeronautical contexts) will be uniform along the sail and zero along the span of the winglet - the winglet will be providing just enough lift to cancel out the induced velocity that would be swirling inboard to leeward of the head (or foot) of the sail.

When you calculate the optimum planform shape with and without the winglet, you'll find the lift distribution does not go to zero at the ends of the sail with a winglet, as it would have to do without the winglet, and the planform shape will be fuller toward the junction between sail and winglet.

Or the other way round, can winglets increase the efficiency in a way to allow for the reduction of mast height with equal performance to a non-winglet sail? I think, this is an interesting question, since a lower rig results in lower heeling momentums and therefore in faster and safer sailing for both mono- and multihulls.

There is, indeed, and interesting tradeoff here. If you design for the minimum induced drag for a given heeling moment, then instead of a uniform induced velocity distribution, the optimum is a linearly varying spanwise induced velocity distribution. It is stronger at the foot and weaker at the head. Here is what the comparison looks like for single planar surfaces:

http://www.tspeer.com/Planforms/Fig20.gif

The solid dot corresponds to a half-ellipse planform sealed to the water's surface. The span, gap, and induced drag are taken as a ratio to this case to show the sensitivity of different design variations. Every point on this graph represents a different design, optimized for the particular combination of span, gap, and induced velocity distribution, while producing the same lift.

The dashed lines correspond to the uniform induced velocity distribution, with variations in span and the gap between the foot and the water. As the span is made longer, the induced drag drops significantly - inversely proportional to the square of the span. The gap also has a significant effect, increasing the induced drag. As you'd expect, the center of effort is higher if you design for a greater span or larger gap.

The solid lines correspond to the case where the induced velocity varies from a maximum at the foot to zero at the head. This shifts the lift downward, reducing the height of the center of effort. For a given span, the induced drag is greater for the tapered induced velocity distribution than for the uniform velocity distribution. However, for the same height of the center of effort - same heeling moment - you can make the mast taller with the tapered velocity distribution and get less induced drag. So it really depends on what your basis for comparison is - fixed span, or fixed height of the center of effort.

The dash-dot line shows limiting case where the chord goes to zero at the head and the induced velocity actually reverses sign near the head. The upper trailing vortex is effectively being shed some distance below the head.

A winglet changes this tradeoff in detail, but not in degree. A winglet changes the effective span because you can always get the same induced drag of a rig with a winglet by making the rig taller. So the effectiveness of a winglet on the drag can be expressed as a multiplier to the span. The winglet will have a lower center of effort but the same drag as a single surface whose physical span is the same as the winglet's effective span.

You can add the winglet span and winglet dihedral angle to the trades shown by the figure above. If the sail span is kept the same, increasing the winglet span (for, say, a 90 degree winglet) will decrease the induced drag but increase the height of the center of effort somewhat because of the fuller lift distribution of the sail and the fact that the downward-directed lift of the winglet displaced to leeward adds to the heeling moment. So the trend lines will have a similar shape, but different slopes.

However, there's another factor for the winglet. If you keep the sail area the same and add winglet area, then the total wetted surface is increasing. This brings with it a parasite drag penalty. There may also be a rating penalty if the winglet area is judged to count as sail area. On this basis, you might want to ask the question as to what produces the minimum induced drag, constrained by both total area and heeling moment. That would keep the parasite drag the same.

Even if the winglet area doesn't count, so the basic sail area can be held constant, the increase in effective span you get with a winglet requires more winglet area than the increase in sail area required to extend the tip for the matching physical span. In other words, when you optimize a winglet's dihedral angle to minimize the parasite drag penalty, it turns out that the optimum winglet dihedral angle is zero - the winglet is a straight extension of the sail.

It would be an interesting tradeoff to constrain both the height of the center of effort (heeling moment) and induced drag, and see how the parasite drag changes with winglet dihedral angle. The span and induced velocity distribution would be changing to meet the constraints.

[champ0815]

Wow, thank you very much, Tom! This is an answer I can think about!
If I get you right, it's about the same effect if you add the area of the winglet perpendicular to a given sail or add it vertical? Very interesting! So the fuss with winglets on planes is all about reducing the length? Well, at least on a sail boat this would result in a lower heeling moment from rig weight - center of gravity would be lower.
The effect of the center of effort of the sail remains unclear to me. Stays it on the same height with winglets as with a sail where the area of the winglet is added to the top as an extension?

[oldsailor7]

The effect of the center of effort of the sail remains unclear to me. Stays it on the same height with winglets as with a sail where the area of the winglet is added to the top as an extension?

I guess it depends on HOW you get the extra area on the top of the sail.
If you have to extend the top of the mast to achieve it, the extra weight of the mast extension and its parasitic drag may well nullify the benifits.

The modern fathead sail with an elliptical planform would appear to solve this problem and also keep the CE of the sail at the same position. :D

[tspeer]

...
If I get you right, it's about the same effect if you add the area of the winglet perpendicular to a given sail or add it vertical? Very interesting! So the fuss with winglets on planes is all about reducing the length?

And style. Standard Class sailplanes use winglets because their span is limited by the class rules. Airliners are sometimes constrained by the width of the area at terminal gates. But if you look at Boeing's 787, for example, it doesn't have winglets. Winglets do provide the same drag reduction with less bending moment (equivalent to heeling moment), so they can be more attractive as retrofits than simple span extensions.

Here's another way to look at it. The lift is often fixed by the stability of the boat. So there are really only two things the winglet can do. It can provide a reduction in induced drag, which is equivalent to an increase in span, or it can provide the same drag with a reduction in heeling moment. But there are other means to achieve the same end, and it becomes a matter of tradeoffs of other drag sources and practical considerations as to which route to go.

Well, at least on a sail boat this would result in a lower heeling moment from rig weight - center of gravity would be lower.
The effect of the center of effort of the sail remains unclear to me. Stays it on the same height with winglets as with a sail where the area of the winglet is added to the top as an extension?

The winglet would raise the center of effort, if the winglet and sail have the optimum loading. However, it doesn't raise the center of effort as much as a physical increase in span equal to the winglet's equivalent increase in span.

With regard to the center of effort, there are really two centers to consider. The instantaneous center of effort is the heeling moment divided by the lift. Twisting off the head of the sail will lower the instantaneous center of effort.

Then there's the aerodynamic center. This is the location where the change in moment divided by a change in lift is zero. It's where the additional heeling moment from a gust will appear to act if the twist is kept constant.

Here's the exact same comparison as before, but this time the vertical axis is the height of the aerodynamic center instead of the instantaneous center of effort:
http://www.tspeer.com/Planforms/Fig21.gif

It's interesting that designing for a different induced velocity distribution, with a corresponding change in both twist and planform shape, doesn't change the tradeoff between the drag and aerodynamic center. The taller rig may have the same center of effort because it's twisted off, but in a gust, it will still feel like a taller rig.

The square-head sails are effective in two ways. They have a planform shape that is closer to the optimal planform for their span, and they twist off to reduce the height of the center of effort when hit by a gust. So the height of the aerodynamic center is effectively reduced by the feedback of the loads on the sail.

[champ0815]

First of all, thank you again for your elaborate answers! Since I am no engineer, it is very likely that some of the implications of your answers get lost on me. So excuse me if I ask again even when to you the answer has been already given somewhere above...
How would a square-head sail with a winglet perform? I imagine a lightweight winglet attached to the top batten of the sail, so that the flexibility of the upper part of the sail is maintained and gust response is not affected.
And how about the lower edge of the sail? In some book the theory of keels was explained, with the statement that the hull acts like a endplate making the keel so efficient. Would a cabin roof swiping boom improve the efficiency of the sail likewise?

[tspeer]

...How would a square-head sail with a winglet perform? I imagine a lightweight winglet attached to the top batten of the sail, so that the flexibility of the upper part of the sail is maintained and gust response is not affected.

You would have to do an engineering analysis to answer this question. It would depend on the planform of the sail, the twist, the camber (draft). And the jib and its interaction with the mainsail. The results I've shown above were calculated with a spreadsheet (http://www.tspeer.com/DesignTools/vortex95.xls) (which apparently doesn't run any more in the latest Windows versions) that could be modified to handle more than one surface. I've meant to do that for some time, but never got around to it.

And how about the lower edge of the sail? In some book the theory of keels was explained, with the statement that the hull acts like a endplate making the keel so efficient. Would a cabin roof swiping boom improve the efficiency of the sail likewise?

There is a huge vortex generated at the foot of the sail, and its effect is shown on the graphs above. The simple theory shows a big increase in drag for the smallest gaps, but some experimental data indicates the effect of small gaps may not be so bad:
http://www.tspeer.com/Planforms/Fig29.gif
However, for gaps bigger than about 5% of the span, the theory is not too bad.

Technically, what really matters is how the lift is distributed along the span, and that depends on how much lift is carried by the hull as well as the rig. That requires a really sophisticated computational fluid dynamics program to capture the relevant physics.

I think the practical message is the optimum planform looks a lot like a sailboard rig. We can get more out of the same sail area by raising the clew to maybe a third of the luff length, instead of having the maximum chord at the foot.

[oldsailor7]

Back in the day--(Early 1960s) when the only way to get a multihull was to build it yourself, we had wooden booms.

In the interest of stiffness with lightness we built them of 1" x ?" stock. (usually Douglas Fir for stiffness).

We made them in the form of a "T" with typically the vertical 1"x 6", and the horizontal top 1"x 10".

With the sail track screwed down the centreline of the flat top we had a Virtual "End Plate" which we hoped would help to reduce the foot vortex.

Of course we had no evidence of how successful it was. But it made us FEEL better about it. :D

BTW having been an enthusiastic Windsurfer for 10 years, I heartily agree withthe idea of raising the clew--particularly on a Catamaran with a loose footed main. Efficiency + more headroom when jibing.

[PI Design]

According to Fluid Dynamic Drag by Hoerner, the junction of two foils (such as a horizontal winglet atop a squre head sail) creates an interference drag that is typically equal in magnitude to the drag created from a foil 10 times longer than the length of the connection. In other words, if the square head was 1m in chord and an endplate was placed on top to prevent tip loss, the interferece drag between the sail head and the endplate would be equal to the drag one would expect from a 10m wing. So whilst you will reduce tip loss drag, you will gain an interefence drag. It will depend on the particular geometry of the design as to whether the gains outweigh the losses.

[Chris Ostlind]

A couple of years ago, I fiddled with this concept, but came at it from a totally different conceptual angle. I was working on the designs for a family of smallish (14-22') trimarans for speedy recreational sailing.

With capsizing and righting at the forefront of objections to multihulls, I wanted to develop a masthead flotation system that would keep the boats from turtling, allowing them to be righted quickly by the average guy with little hassle.

There have been several strategies over the years for this very issue. The Hobie "Bob" is one and also a system of installing an inflatbale bag in the masthead itself should the boat get tossed. Both of them have their own particular issues associated, so I looked for another solution.

I came up with a concept for building square topped sails that had foam pieces installed at the top of the sail providing the necessary flotation for the intended work. That the design allowed the foam to be shaped to act as an end plate device, also came into play, perhaps allowing one design concept to comfortably work for two beneficial solutions to multihull sailing issues.

There have been no further developments in the concept to date and I'm not even sure that it would work well in the endplate application when one applies the previous comments from Tom and PI. Still, it's worth a look by a savvy sailmaker who would be willing to experiment with the concept to conclusion, or discard it all together.

If anyone here has any suggestions, one way or another, it would be fun to hear what you have to say about the potential.

[yipster]

studying all things concerned i still say that interesting and thanks for showing
than there are inflatable wings (http://www.olypen.com/dkaseler/Jpegs%20and%20Gifs/Inflator-Gallery/thumbsinflator/pages/r&dsail5_jpg.htm) and came acros this '66 future rig (http://www.boatdesign.net/forums/attachment.php?attachmentid=14073&d=1181472233) again

[oldsailor7]

According to Fluid Dynamic Drag by Hoerner, the junction of two foils (such as a horizontal winglet atop a squre head sail) creates an interference drag that is typically equal in magnitude to the drag created from a foil 10 times longer than the length of the connection. In other words, if the square head was 1m in chord and an endplate was placed on top to prevent tip loss, the interferece drag between the sail head and the endplate would be equal to the drag one would expect from a 10m wing. So whilst you will reduce tip loss drag, you will gain an interefence drag. It will depend on the particular geometry of the design as to whether the gains outweigh the losses.

I feel this must be an exaggeration, otherwise you wouldn't see the normal tailplane and fin arrangements on aircraft.

[markdrela]

According to Fluid Dynamic Drag by Hoerner, the junction of two foils (such as a horizontal winglet atop a squre head sail) creates an interference drag that is typically equal in magnitude to the drag created from a foil 10 times longer than the length of the connection. The interference drag that Hoerner shows is indeed huge, but it's for 30% thick struts. Not surprisingly, there's massive separation at the intersection. So it certainly does not follow that you'll get similar drag penalties from the wing/winglet junction.

Much more relevant is Hoerner's figure 26. This shows the drag added by the strut intersection as a function of t/c. For t/c<10%, the added drag is practically nil. So the drag penalty of a typical airplane T-tail intersection is expected to be nil.

For a winglet/wing intersection, a more relevant parameter would be the local Cl rather than t/c, since the Cl is in the lifting case a better indicator of adverse pressure gradients. It's pretty safe to say that the wing and winglet away from the intersection must be reasonably far from local Clmax to prevent any separation problems at the intersection.

[Doug Lord]

I used the "wingtips" illustrated here to prevent these rc tri's from turtling. Used the same thing on a 16 footer in 1975. Both worked well for that purpose and their endplates may have worked-I'm not sure. To get an idea of the size of these rc boats click on the left picture and notice the man standing in the upper left corner!(Dr. Sam Bradfield)

[Petros]

If designed properly they may help, because the principles are the same for both wing, keel and sail. But as pointed out it is way more efficenct to just increase the span or aspect ratio. However in racing classes where the size of the sail is rule limited, it would be a way of "cheating" the rules. It has the effect of making the sail act bigger with a higher AR. This might work for a season or two, until they out law winglets too (or include their area in the sail area).

I have been thinking of try it out myself, just for fun.

[PI Design]

The interference drag that Hoerner shows is indeed huge, but it's for 30% thick struts. Not surprisingly, there's massive separation at the intersection. So it certainly does not follow that you'll get similar drag penalties from the wing/winglet junction.

Much more relevant is Hoerner's figure 26. This shows the drag added by the strut intersection as a function of t/c. For t/c<10%, the added drag is practically nil. So the drag penalty of a typical airplane T-tail intersection is expected to be nil.

For a winglet/wing intersection, a more relevant parameter would be the local Cl rather than t/c, since the Cl is in the lifting case a better indicator of adverse pressure gradients. It's pretty safe to say that the wing and winglet away from the intersection must be reasonably far from local Clmax to prevent any separation problems at the intersection.

That'll teach me to to read properly! You're quite right, the equation related to Fig 26 is what I should have used.
Please ignore my previous comments.

[peterraymond]

This thread is almost 2 years old, but I couldn't find any newer ones that were as close to what I've been thinking about. I've drawn some lines and guessed some numbers from Tom Speer's post 15 and you can see the result attached below.

My red lines compare two designs with optimum planform, one with a 5% gap at the bottom and one sealed. Of course there are all the complications of flow around hulls and variation of wind strength with height above the water, but bear with me on this.

The design that is sealed at the base has around 0.68 normalized induced drag, while the second design, with the same lift and same heeling moment, has around 1.82 normalized induced drag. The design with the gap at the bottom has more than twice the induced drag. There are several reasons for this, including the fact that with a gap at the bottom you want to taper the sail at the base similar to a windsurfing sail and the fact that to maintain the same overturning moment the sail can't be as tall.

With this potential for improvement, I think that in classes where ISAF sail measurements are used something can be done. I say this because they do not count the area of nominally horizontal elements so long as they are no larger than 10% of the normal measured sail area. It seems like the only questions are how much can be done and how do you do it.

In a very simplistic example, there is a vortex trailing off the lower edge of the sail. If you put a propeller into this vortex it would slow the vortex down and also create forward thrust. This is similar to what you get from a winglet. A winglet will reduce induced drag at the base and that will let you increase the width, or cord, of the sail there. Both lower the center of effort of the sail. This lowered center of effort lets you make the mast taller, for a further reduction in induced drag. essentially you are moving to the left along the horizontal red line I added to Tom's graph.

In a source I saw recently and can no longer find, it stated that the position of a winglet front-to-back doesn't change it's effectiveness, although I assume it does change the optimum design. I had thought that the best place for a winglet would be at the trailing edge, since that is where you have the most span-wise flow. I think that there is obviously some room for design exploration and optimization.

There is another advantage too, which is that the windward winglet would produce a force downward and the leeward a force upward, so the winglets directly provide a little righting moment. Although, I may really be counting the same benefit twice here.

I picture two tapered winglets with symmetric profiles, one on each side of the sail, that counter-rotate so that one is pitched up while the other down. More complicated would of course be if each had to be designed as a two element wing and I suppose symmetric profiles would be less effective, but I think we are looking at low hanging fruit here.

At the limits, you might be able to put foils around the main beam at the mast, or mount foils on struts above the rear cross beam, but attached somewhere to the sail, or wind-sail, seems cleaner/better.

[oldsailor7]

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

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...

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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.