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In an earlier topic, three aspects of Wings (Winglets on Keels and Wings) were missed or understated, which might simplify understanding some of the questions raised.  Span constraints, inherent disadvantage, and design subtlety and difficulty.

Span Contraints.  Winglets on airliners might seem to be powerful evidence of their performance enhancement.  No; initially airports did not have enough large enough docking spaces for a wave of new aircraft on the way.  In boating, along with the multiplicity of old-age constraints on draft, the 12-metre AC rule had a 9' draft limitation.  The winglets on the AC boats fit the boats to the rule, deeper keels would have been superior (for performance) in all respects.  The winglets on airliners, initially, fit them into their parking spaces.

The 787, freed by larger spaces, exploits the advantage of not having winglets.  Its outer span has subtle shaping and shape changing flexibility which add to that advantage in a way that tacking on even clever winglets to unconstrained span do not - the A380 has little winglets both for marketing and to simplify adding virtual span long after the initial design.  Winglets are being marketed as well to all sorts of smaller General Aviation aircraft - both for span constraints in existing hangars, and for marketing, and, again, and ultralight tack-on is simpler in every way than redesign and renovation of an existing wing.

Difficulty & Subtlety of Increasing Effective Span.  Draft constraints on keels can be sharp, or just desirable, so the search for lower CG and greater effective span is quite legitimate.  But very difficult to get right.  Scheel headed in the right direction, but understandably, missed.  If someone other than Burt Rutan designs a Canard airplane, watch out.  If a team tacks winglets on a 12 metre without the insight and analysis of the early Australian boat, hold that bet.  The twin keels are a great alternative either side of the English Channel for sitting down gracefully when the tides out without resorting to stilts, but they are not a match for performance for the same effective deeper single span (and sitting down can be a lot worse than graceful.)

If Scheel had gone further: a gentler, truly progressive increase in foil thickness toward the base of the keel, and a near-sharp edge at the base as virtual fence (with zero surface area) for flow (pressure) escaping below, before reaching the trailing edge.  Perhaps progressing to a foil at the base which is net more effective at higher foil thickness.  It might have worked quite well.  Searching for lower VCG, "Marry it or Divorce it": a keel should lift, a bulb should be as invisible as possible (not easy), not lifting or dragging more than inevitable.

Inherent Disadvantage.  Winglets will only get you a virtual span increase of 1/2 or a little more of their own span.  And they'll generally have twice the surface area (or more), of simple span increase.  They'll introduce interference drags the simple span lacks. 

Advances in other keel shaping to reduce induced drag (the "bad" wake vortices) might change those deficits.  You won't have to wait for an example: Centerboards.

At first sight, a long post wouldn't seem to simplify anything, but sometimes seeing that a situation is simpler than it appears gets complicated.  

 

regards,

 

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A few years ago, my boat spent a winter on the hard surrounded by about 15 other wing keel boats. It was interesting to see how a more or less standard approach was stretched this way and that until no two were alike, even near sisters from the same designer-builder. Also interesting was how crude the shaping and construction was on a few.

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without some pretty sophisticated tank/wind tunnel/CFD most winged keels are pretty draggy and useless. At best they keep the VCG low with shallow draft.

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5 hours ago, pschwenn said:


In an earlier topic, three aspects of Wings (Winglets on Keels and Wings) were missed or understated, which might simplify understanding some of the questions raised.  Span constraints, inherent disadvantage, and design subtlety and difficulty.

Span Contraints.  Winglets on airliners might seem to be powerful evidence of their performance enhancement.  No; initially airports did not have enough large enough docking spaces for a wave of new aircraft on the way.  In boating, along with the multiplicity of old-age constraints on draft, the 12-metre AC rule had a 9' draft limitation.  The winglets on the AC boats fit the boats to the rule, deeper keels would have been superior (for performance) in all respects.  The winglets on airliners, initially, fit them into their parking spaces.

The 787, freed by larger spaces, exploits the advantage of not having winglets.  Its outer span has subtle shaping and shape changing flexibility which add to that advantage in a way that tacking on even clever winglets to unconstrained span do not - the A380 has little winglets both for marketing and to simplify adding virtual span long after the initial design.  Winglets are being marketed as well to all sorts of smaller General Aviation aircraft - both for span constraints in existing hangars, and for marketing, and, again, and ultralight tack-on is simpler in every way than redesign and renovation of an existing wing.

Difficulty & Subtlety of Increasing Effective Span.  Draft constraints on keels can be sharp, or just desirable, so the search for lower CG and greater effective span is quite legitimate.  But very difficult to get right.  Scheel headed in the right direction, but understandably, missed.  If someone other than Burt Rutan designs a Canard airplane, watch out.  If a team tacks winglets on a 12 metre without the insight and analysis of the early Australian boat, hold that bet.  The twin keels are a great alternative either side of the English Channel for sitting down gracefully when the tides out without resorting to stilts, but they are not a match for performance for the same effective deeper single span (and sitting down can be a lot worse than graceful.)

If Scheel had gone further: a gentler, truly progressive increase in foil thickness toward the base of the keel, and a near-sharp edge at the base as virtual fence (with zero surface area) for flow (pressure) escaping below, before reaching the trailing edge.  Perhaps progressing to a foil at the base which is net more effective at higher foil thickness.  It might have worked quite well.  Searching for lower VCG, "Marry it or Divorce it": a keel should lift, a bulb should be as invisible as possible (not easy), not lifting or dragging more than inevitable.

Inherent Disadvantage.  Winglets will only get you a virtual span increase of 1/2 or a little more of their own span.  And they'll generally have twice the surface area (or more), of simple span increase.  They'll introduce interference drags the simple span lacks. 

Advances in other keel shaping to reduce induced drag (the "bad" wake vortices) might change those deficits.  You won't have to wait for an example: Centerboards.

At first sight, a long post wouldn't seem to simplify anything, but sometimes seeing that a situation is simpler than it appears gets complicated.  

 

regards,

 

Aircraft ……Aero and Hydro are considerably different fields and don’t make for good observations crossing between fields.

 Winged keels are really draggy outside of a narrow range of conditions. On boats they are more about getting ballast low than any real robust advantage. The real lift advantage from a winged keel is limited to smooth water  and a narrow range of headings and heel angle.

 

Scheel keel wasn’t a great design idea it was just well marketed but the shape didn’t come out of any rigorous testing regime, low aspect keels get their lift from vortice generation not from aerofoil effects. Rounding the bottom edge actually has an adverse effect on lift as it delays the cross flow vortice separation which isn’t what you want at all.   

 

Bulbs are really draggy too, but less so than trying to incorporate a NACA 0030 foil to get enough internal volume to get the ballast low and or keep the WSA low and or keep attached laminar flow on the foil..

 

Nothing ideal once you get away from an elliptically loaded aerfoil it’s all compromises and cycling through fads and fashions.
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9 hours ago, MikeJohns said:

 Winged keels are really draggy outside of a narrow range of conditions. On boats they are more about getting ballast low than any real robust advantage. The real lift advantage from a winged keel is limited to smooth water  and a narrow range of headings and heel angle.

What's the difference in the real world. Here are some comparisons of base ratings for some Catalina models (YRA/LIS data):

Catalina 270 - 198

Catalina 270W - 204

Catalina 34 - 156

Catalina 34 Wing - 165

Catalina 400 DK - 108

Catalina 400 W - 111

 

So the wing is assumed to have a penalty of 6-10 sec/mile around the whole race course. The upwind penalty is higher, but the offwind penalty is negligible, and the wing might actually be an advantage there. Cruisers sail upwind a smaller percentage of the time than racers. It's no wonder that cruisers find the wing keel boats satisfactory.  

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38 minutes ago, SemiSalt said:

 The upwind penalty is higher, but the offwind penalty is negligible, and the wing might actually be an advantage there. Cruisers sail upwind a smaller percentage of the time than racers. It's no wonder that cruisers find the wing keel boats satisfactory.  

It's fairly common for shoal draft versions of boats to have more ballast than their deep keeled sisters to make up the loss in righting moment.    As an example, my boat has about 10% more ballast than the deep keel version.   And the shoal draft keel often has more wetted surface and drag than the deep keel.   So the shoal draft version can be slower on all points of sail.

My impression is that the wing keel fad has died down somewhat and most current production shallow draft keels just use a bulb like or flaired shape to get the ballast low enough for the required righting moment. 

Of course a shoal draft boat is far faster than a deep keel boat if the water is shallow enough.

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And, to add on against a lot of what the OP said - the 'winglets' at the end of the wings on planes are there to increase overall efficiency and lift by reducing tip vortices caused by lack of an end plate.  Part of the same reason that the foiling AC boats started sealing the bottom of the mainsail up against their hull.

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21 hours ago, pschwenn said:


In an earlier topic, three aspects of Wings (Winglets on Keels and Wings) were missed or understated, which might simplify understanding some of the questions raised.  Span constraints, inherent disadvantage, and design subtlety and difficulty.

Span Contraints.  Winglets on airliners might seem to be powerful evidence of their performance enhancement.  No; initially airports did not have enough large enough docking spaces for a wave of new aircraft on the way.  In boating, along with the multiplicity of old-age constraints on draft, the 12-metre AC rule had a 9' draft limitation.  The winglets on the AC boats fit the boats to the rule, deeper keels would have been superior (for performance) in all respects.  The winglets on airliners, initially, fit them into their parking spaces.

The 787, freed by larger spaces, exploits the advantage of not having winglets.  Its outer span has subtle shaping and shape changing flexibility which add to that advantage in a way that tacking on even clever winglets to unconstrained span do not - the A380 has little winglets both for marketing and to simplify adding virtual span long after the initial design.  Winglets are being marketed as well to all sorts of smaller General Aviation aircraft - both for span constraints in existing hangars, and for marketing, and, again, and ultralight tack-on is simpler in every way than redesign and renovation of an existing wing.

Difficulty & Subtlety of Increasing Effective Span.  Draft constraints on keels can be sharp, or just desirable, so the search for lower CG and greater effective span is quite legitimate.  But very difficult to get right.  Scheel headed in the right direction, but understandably, missed.  If someone other than Burt Rutan designs a Canard airplane, watch out.  If a team tacks winglets on a 12 metre without the insight and analysis of the early Australian boat, hold that bet.  The twin keels are a great alternative either side of the English Channel for sitting down gracefully when the tides out without resorting to stilts, but they are not a match for performance for the same effective deeper single span (and sitting down can be a lot worse than graceful.)

If Scheel had gone further: a gentler, truly progressive increase in foil thickness toward the base of the keel, and a near-sharp edge at the base as virtual fence (with zero surface area) for flow (pressure) escaping below, before reaching the trailing edge.  Perhaps progressing to a foil at the base which is net more effective at higher foil thickness.  It might have worked quite well.  Searching for lower VCG, "Marry it or Divorce it": a keel should lift, a bulb should be as invisible as possible (not easy), not lifting or dragging more than inevitable.

Inherent Disadvantage.  Winglets will only get you a virtual span increase of 1/2 or a little more of their own span.  And they'll generally have twice the surface area (or more), of simple span increase.  They'll introduce interference drags the simple span lacks. 

Advances in other keel shaping to reduce induced drag (the "bad" wake vortices) might change those deficits.  You won't have to wait for an example: Centerboards.

At first sight, a long post wouldn't seem to simplify anything, but sometimes seeing that a situation is simpler than it appears gets complicated.  

 

regards,

 

The wings add no righting moment , only lateral resistance 

hence these un ballasted  wings cause the boat to “trip “

hard to justify wings 

a well drawn scheel  keel works well 

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First, we sail a wing keel boat because that was what was available when we dirt poor horse trainers wanted to get back on the water.  We found one that was in really excellent condition at a price we could afford. We have owned several before and since, but the Wing Keel Santana 20, Anna Eleise is the one that stays in our family.  We sail this boat not because it has a wing keel, but because we found a boat we liked and purchased her Despite the wing keel.  In the end, we do appreciate the advantages this keel offers.  

Second being a student of aviation and a licensed pilot I concur with what GRR said. Winglets increase a wings efficiency by reducing the vortices coming off of the wing tips.  

The design of the wing on the Santana’s keel could offer some lateral resistance when the angle of heel is fairly steep (not the best condition to sail a Santana or any small IOR influenced design.  They like being sailed flat) Also, the quantity of lead in those wings offers a fair amount of righting ability.  All of which was never the idea of having winglets on the end of a wing.  

It is rumored that the Santana wing keel is faster than the full keel under very light wind conditions.  This is rumor, i have seen no data and I suspect the sails and or the sailor would make a greater difference under normal light air conditions.

I see a need for “winglets” on shoal draft keels.  The full wings that seemed to be in such a vogue a few decades ago, are to me really just a marketing fad that seemed to work to a certain degree, just as a bulb would work.    However, anything you have to drag though water is going to be…a drag.  

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6 hours ago, slap said:

It's fairly common for shoal draft versions of boats to have more ballast than their deep keeled sisters to make up the loss in righting moment.    As an example, my boat has about 10% more ballast than the deep keel version.   And the shoal draft keel often has more wetted surface and drag than the deep keel.   So the shoal draft version can be slower on all points of sail.

My impression is that the wing keel fad has died down somewhat and most current production shallow draft keels just use a bulb like or flaired shape to get the ballast low enough for the required righting moment. 

Of course a shoal draft boat is far faster than a deep keel boat if the water is shallow enough.

What, sticking the keel into the bottom does not form an effective end plate??

I think this is really the issue. It's not strictly the efficiency of the keel at it's two tasks, it's how much it enables you to go places that deeper keel would rule out and how much you give up for that. It takes a really well-designed and well-executed wing keel to be better than just sawing off the keel at depth X and adding a streamlined bulb. Most are not all that well-designed.

One benefit of wings that I haven't seen mentioned... they damped motion. They dampen the vertical motion or 'heave' which is one of the components that induce sea sickness the most. And if the keel is forward or aft enough of the center of pitch rotation, they can dampen pitching nicely too. I did some sailing on a mid-1990s Bavaria with a rather forward-set wing keel that was notable for it's smooth motion in a chop.

- DSK

 

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One problem for those of us who sail with a muddy bottom. Is a winged keel gives more chance of getting really stuck in the mud.

I spent hours considering winged keel designs for my little boat, but came to the conclusion.

A, I didn't have access to suitable computing programmes or testing facilities to design one.

B, As you tack off a river bank, a wing would require keeping off the bank further.

C, The boat would need an even shorter keel to avoid ploughing the bottom with the wings while heeled.

So in the end I went for a airfoil shaped thickened by 30% bottom 1/5 of the keel in which all the lead lives..

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4 hours ago, Steam Flyer said:

What, sticking the keel into the bottom does not form an effective end plate??

I think this is really the issue. It's not strictly the efficiency of the keel at it's two tasks, it's how much it enables you to go places that deeper keel would rule out and how much you give up for that. It takes a really well-designed and well-executed wing keel to be better than just sawing off the keel at depth X and adding a streamlined bulb. Most are not all that well-designed.

One benefit of wings that I haven't seen mentioned... they damped motion. They dampen the vertical motion or 'heave' which is one of the components that induce sea sickness the most. And if the keel is forward or aft enough of the center of pitch rotation, they can dampen pitching nicely too. I did some sailing on a mid-1990s Bavaria with a rather forward-set wing keel that was notable for it's smooth motion in a chop.

- DSK

 

Damping hey?

Doesn't the whole boat move with the wave equally? In deep water at least....

And by "the whole boat" I mean the more technical* term, not bows or transoms.....

 

*Center of gravity?

 

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The simple answer the that is the vertical movement up and down of the boat is pulling or pushing water over the wings if they are big enough to notice the effect. The water in a wave does not move equally from surface to the bottom of the keel. The boats vertical movement drags a large amount of water with it.

Big area keel wings like this one are known to have a large damping effect.

 

wing_keel_800.jpg

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I think that >90% of winged keels are very ugly from a hydrodynamic point of view. Most of the wings are just a place for extra ballast and lots of extra wetted surface area. I think a well designed bulb is a smarter way to maximize volume/minimal drag.

#1 All the surface area we can create

#2 very bad bulb shape. wing profile looks suspect too.

#3 way too much squish in the bulb. Little squish good to lower VCG but too much is just too much wetted surface area

#4 basic keel planform shape looks wrong. fin profile looks wrong.

#5 probably does least harm

#5,6  fins too short for any tip vortex reduction effect

image.png.0943f2f21ab83b1c590a3cd24fc0f6cf.png   image.png.6cae7d87f8151d15c1c53cb242715e51.png  image.png.05d904e569d3c7b657bae17214beb118.png   

image.png.7bbd4c2b8bb0c103e26b62832376814d.png   image.png.9ec66f82490cf39e029b90347568690d.pngimage.png.76fa97bfefd48d9d65a317adddb564a5.png   

 

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Yep most wings shown were a fashion statement.. marketing said put a wing on, without a budget for proving the shape..

And the other reason for not having them.. weed and lobster pots. That's why I would never have a forward projection below the waterline , nor wings like that once I'd thought about it. The closest I'd have would be a beaver tail .

 

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1 hour ago, The Q said:

Yep most wings shown were a fashion statement.. marketing said put a wing on, without a budget for proving the shape..

And the other reason for not having them.. weed and lobster pots. That's why I would never have a forward projection below the waterline , nor wings like that once I'd thought about it. The closest I'd have would be a beaver tail .

 

It's like having the bottom of your keel molded to the shape of a Bruce Anchor... who wouldn't want that?!?

 

9 hours ago, Ease the sheet. said:

Damping hey?

Doesn't the whole boat move with the wave equally? In deep water at least....

And by "the whole boat" I mean the more technical* term, not bows or transoms.....

 

*Center of gravity?

 

Yes, the whole whole boat moves with the wave but unless the water is also moving up and down equally, horizontal surface will do something good for once, and dampen the motion thru the water. As Zonker says, it's almost certainly not worth the added drag of that surface area in the first place.

- DSK

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40 minutes ago, Steam Flyer said:

It's like having the bottom of your keel molded to the shape of a Bruce Anchor... who wouldn't want that?!?

 

Yes, the whole whole boat moves with the wave but unless the water is also moving up and down equally, horizontal surface will do something good for once, and dampen the motion thru the water. As Zonker says, it's almost certainly not worth the added drag of that surface area in the first place.

- DSK

It's a wave, it moves up and down. Equally...

 

In practice, there may indeed be a dampening effect. In theory, I'm not so sure.

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5 minutes ago, Ease the sheet. said:

It's a wave, it moves up and down. Equally...

 

In practice, there may indeed be a dampening effect. In theory, I'm not so sure.

Wave mechanics isn't really my area of expertise, but the water underneath and within a wave does not move as much as the surface. If it did, it wouldn't look like a wave. It would be the whole ocean moving up and down. Probably shake the planet.

- DSK

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5 hours ago, Zonker said:

#5,6  fins too short for any tip vortex reduction effect

Have to disagree on this one. The #5 example is certainly too fat. Vortex reduction fins can be quite small. Look at all the airplane examples and this, crappy, illustration from the 2003 AC:

 

image-28.jpg

 

 

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FWIW, here's the keel on my Catalina 50. Draft is 5.6' / 1.7m, ballast is 13,250 lbs / 6,000 kg. Overall dry weight is 36,000 lbs / 16,330 kg. Full specs here:

https://sailboatdata.com/sailboat/catalina-morgan-50

Plenty of wetted area but she's a pretty stiff boat with a nice motion. Decent compromise for a Florida boat. Here in Seattle, a Max Prop and big asymmetric help recover some light air performance. 

large.keelAft.jpg.d24c8576183674588dba0b42e9715ba2.jpg

large.IMG_20151218_094943.jpg.edcb5f144bcbdd0982f761f66d3246c7.jpg

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In aircraft, winglets have probably been more studied for sailplanes than anything else, as tip vortices (induced drag) are all important. Very well designed and modeled by a team of university students, you can get about 0.5% improvement in performance at some speeds, while sacrificing performance at other speeds. Not well designed - well, they can still look cool. On a Boeing, which flies at the same speed and conditions 95% of the time, you can tune them to do some good. I put the factory winglet kit on my sailplane, no measurable difference in performance before and after, I could circle a couple of knots slower so maybe they delay tip stall and spin departure a little. They do look cool on the ramp. Like PHRF you get a rating hit when you install them in handicapped competition. Like PHRF I don't think they make up for the rating hit. 

Dangerous to compare a sailplane (AR = >30) to a boat keel (AR ~ 2). In a boat keel the whole thing is a tip vortex. For most of 'em, I think it is a place to put more lead, makes blocking a little easier, and ungrounding more difficult. The cool factor is only important at boat shows (admittedly important for the builder) since you can't see the wings at the dock. 

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2 hours ago, Voiled said:

Have to disagree on this one. The #5 example is certainly too fat. Vortex reduction fins can be quite small. Look at all the airplane examples and this, crappy, illustration from the 2003 AC:

Hint - look at the chord of the fin. You can't compare the flow around a keel with and without a bulb. Totally different. And airplanes versus sailboat keels.

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Something to remember about AUSTRALIA II, is that the major innovation in the minds of the designers was the inverted keel shape, not the wings. 

By having a longer root chord than tip chord, the center of lift on the keel was lowered.  Getting the low pressure region on the keel further below the surface apparently resulted in a noticeable reduction in wave drag. This keel arrangement appeared to be a significant improvement all on its own in both their panel code simulations and tank tests.  The one downside of the upside-down keel, was that the larger tip resulted in more induced drag.  The winglets were added to try and combat that.

1 hour ago, DDW said:

In aircraft, winglets have probably been more studied for sailplanes than anything else, as tip vortices (induced drag) are all important. Very well designed and modeled by a team of university students, you can get about 0.5% improvement in performance at some speeds, while sacrificing performance at other speeds. Not well designed - well, they can still look cool. On a Boeing, which flies at the same speed and conditions 95% of the time, you can tune them to do some good. I put the factory winglet kit on my sailplane, no measurable difference in performance before and after, I could circle a couple of knots slower so maybe they delay tip stall and spin departure a little. They do look cool on the ramp. Like PHRF you get a rating hit when you install them in handicapped competition. Like PHRF I don't think they make up for the rating hit. 

Dangerous to compare a sailplane (AR = >30) to a boat keel (AR ~ 2). In a boat keel the whole thing is a tip vortex. For most of 'em, I think it is a place to put more lead, makes blocking a little easier, and ungrounding more difficult. The cool factor is only important at boat shows (admittedly important for the builder) since you can't see the wings at the dock. 

That's interesting, but not too surprising.  Gliders are really the wrong platform to see strong benefits from winglets -- they're already so optimized for low induced drag.  Winglets rely on spanwise flow induced by the tip vortices to generate thrust.  When your AR is greater than 30, the tip vortex is small, and you get very little AoA on the winglet, which means very little lift, which means very little thrust relative to their area... which means they're hard to justify.

The original NASA technical report by Whitcomb which kicked off all the winglet fuss in the late 70's demonstrated a ~20% reduction in induced drag for a wing with an AR between 6 and 7, if memory serves.  Modern airliners tend to have wing aspect ratios of around 7-9.  So, compared to your average glider with an AR of approx. 30, an airliner has 3.75x more induced drag.  Even on airliners, winglets only really make sense if you have a fixed span.  The new Boeing 777-9X went with folding wing tips for a higher AR (AR = 11, apparently), rather than winglets -- it will be interesting to see if that becomes a trend.

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8 hours ago, Steam Flyer said:

It's like having the bottom of your keel molded to the shape of a Bruce Anchor... who wouldn't want that?!?

 

Yes, the whole whole boat moves with the wave but unless the water is also moving up and down equally, horizontal surface will do something good for once, and dampen the motion thru the water. As Zonker says, it's almost certainly not worth the added drag of that surface area in the first place.

- DSK

As the boat heaves, the keel is pulled upward through the water, but the effect is lessened because the wave action also moves the water up. Also, the boat is moving forward. The combination means that the angle of attack on the wing can be large, at least briefly. I think it's a pretty common observation, noted above, that the wing is a greater handicap in waves, especially waves large enough to give the boat a pitching action. But still, the boats do get to the windward mark even if the deep keel boats get there first.

But how big is the overall effect compared to other compromises? Is the hit taken by a wing keel boat bigger or smaller than the hit taken by a boat with an all-purpose roller furler in place of suite of sails for every condition?

 

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I was convinced that I could 3d model and print winglets for my AC-4 sailplane. I had seen a lot of the after market tips like DDW mentioned but didn't want to alter the original wing tip shape. I figured that I could get a close fit of the mating surface of the printed tip to the wing itself and use 3M VHB double sided tape to affix the winglet. That would also let me adjust the toe in as well as the dihedral within a narrow range. If a tip peeled off in flight no big deal I could just print a replacement. After a lot of research I came across one of the authorities in the glider world who said something like this, 'Adding winglets to a sailplane has a small chance of resulting in a small improvement but a big chance of screwing up what was a perfectly adequate design in the first place'!

Glasfaser Flugzeugservice

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On 1/1/2022 at 12:59 AM, pschwenn said:

The twin keels are a great alternative either side of the English Channel for sitting down gracefully when the tides out without resorting to stilts, but they are not a match for performance for the same effective deeper single span (and sitting down can be a lot worse than graceful.)

I will take a well designed twin keel over a well designed wing keel every time.

A good twin keel will go upwind very well which IMHO matters on a cruising boat for obvious safety and comfort reasons. They will be a bit slower downwind but who cares if you get say an hour later on a 24h downwind crossing. Plus you can go in places that dry which is a real plus.

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3 hours ago, MFH125 said:

The original NASA technical report by Whitcomb which kicked off all the winglet fuss in the late 70's demonstrated a ~20% reduction in induced drag for a wing with an AR between 6 and 7, if memory serves.  Modern airliners tend to have wing aspect ratios of around 7-9.  So, compared to your average glider with an AR of approx. 30, an airliner has 3.75x more induced drag.  Even on airliners, winglets only really make sense if you have a fixed span.  The new Boeing 777-9X went with folding wing tips for a higher AR (AR = 11, apparently), rather than winglets -- it will be interesting to see if that becomes a trend.

Absent other constraints you are always better off with more span than winglets. The very first NACA papers on winglets state this, and it's been true every since. 

Probably bigger difference than AR is Cl. Induced drag is proportional to AR and Cl. Airliners fly at low Cl at cruise speeds, those speeds being about 4x what their stall speed has to be (so dynamic pressure 16x, and Cl & induced drag 1/16). Sailplanes on the other hand have two primary speeds, thermalling ( at min sink close to stall speed) and high speed cruise between thermals (around 2x - 3x stall speed). It is much easier to optimize the winglets for one condition than two. Add to that the same design sailplane has to be competitive in weak eastern conditions and running at 75 knot (Cl about 0.4) vs strong western conditions running at 130 knots (Cl about 0.1). We fill them with water for strong conditions but still a big difference. 

While you might get a 20% decrease in induced drag on an AR of 6, that will occur only at high Cl. At low Cl profile and parasitic drag dominate. The AoA of a sailboat keel is pretty close to its leeway angle, so likely Cl not very high. 

Actual tank tests on keel designs prove that it is very hard, sitting in an armchair, to guess what the performance will really be, especially at low AR. 

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DDW, I still have a hard time wrapping my head around the practice of 'filling our sailplanes with water' and I know the theory and stats and all but it seems contradictory to me. That is until I think back to skiing down hill and how a heavier skier will reach a higher speed on the same skis and the same slope. I have never had the privilege of flying in a ballasted sailplane but would love for you to show me sometime just what that is all about. Do you have a photo of your aftermarket winglets? It seems like it would be a very hard thing to do to pick up a saw and shop off the wing tip of a 6 figure sailplane just to fit a unknown winglet. That is as ballsy as flying sailplanes in the first place!

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8 hours ago, DDW said:

Probably bigger difference than AR is Cl. Induced drag is proportional to AR and Cl. Airliners fly at low Cl at cruise speeds, those speeds being about 4x what their stall speed has to be (so dynamic pressure 16x, and Cl & induced drag 1/16).

While you might get a 20% decrease in induced drag on an AR of 6, that will occur only at high Cl. At low Cl profile and parasitic drag dominate. The AoA of a sailboat keel is pretty close to its leeway angle, so likely Cl not very high.

Yes, wing loading matters.  If you barely load your wing, there's less induced drag to minimize, and adding winglets is mostly adding parasitic drag.  The rate at which induced drag increases with additional wing loading, though, depends on the aspect ratio. The Whitcomb paper found that:

At the design Mach number of 0.78 and near the design wing lift coefficient of about 0.44, adding the winglets reduces the induced drag by about 20 percent and increases the wing lift-drag ratio by approximately 9 percent.

That's a fairly typical lift coefficient for airliners, business jets, small planes, etc. at cruise.  Values up to around 0.6 are not uncommon.  Whether those are "high" depends on your perspective.

For keels the CL is low, but so is the AR.  The induced drag of low aspect ratio keels is much more sensitive to changes in CL than a glider wing would be.  Basically, the gain is much higher.  A keel is lucky to have an AR > 2.  AUSTRALIA II's keel has an AR = 0.4.

8 hours ago, DDW said:

 It is much easier to optimize the winglets for one condition than two. Add to that the same design sailplane has to be competitive in weak eastern conditions and running at 75 knot (Cl about 0.4) vs strong western conditions running at 130 knots (Cl about 0.1). We fill them with water for strong conditions but still a big difference.

Actual tank tests on keel designs prove that it is very hard, sitting in an armchair, to guess what the performance will really be, especially at low AR. 

100%.  Airliners are relatively easy in this regard. Sailplanes are harder. Keels are harder still.

I dug up the van Oossanen paper about the development of AUSTRALIA II's keel and it's clear that they spent a LOT of time fine tuning the wing geometry.  Different wing dimensions, wing profiles, wing angles, etc. were considered.  The optimum seems to depend a bit on heel angle. The final wings have twist to deal with the changing angles of incidence along the winglet span -- a feature you don't generally see in production wing keels.   The best case scenario seems to have resulted in a 25-30% improvement in lift/drag when going upwind.  A major issue is that if you optimize the wings for upwind work, they generate a lot of lift even when the leeway angle is zero -- which means you're lugging a lot of unnecesssary induced drag around in addition to the extra surface friction when going downwind.  Lots of details to get right/wrong and compromises to make.  It's unlikely that designers without AC budgets, access to panel codes, and extensive tank and wind tunnel testing were getting these kind of details dialed in when designing production keels in the 80's and 90's.

Practically, they seem to work fine.  Well designed ones probably have some modest benefits when going upwind, but most are probably not justified from an overall performance perspective.  But, the wide variety of keels out there makes it clear that there are plenty of ways to get satisfactory windward performance for cruising.

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My (new to me) boat, a Dehler 35 CWS has a wing keel.  She was designed by Van De Stadt who know a thing or two, it's not obvious in the photos but the wings have a fair amount of downward deflection (diehedral?) the standard draft is 2m and in wing form it's 1.5m which will be useful when we get back to the Bahamas and the ICW.

 I haven't sailed her that much, but with utterly knackered sails she points very well and makes little leeway at good speed. On my home river, there are two other sister ships, both with full draft keels.  I'm looking forward to having a tussle with them later this year.  

A happy new year to you all.

 

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Here is an addition to Zonkers catalog of wing keel types. This is an O'Day 272. Not a real good boat for a variety of reasons. Very shoal draft. IIRC, I read somewhere that O'Day, and that would probably mean the Hunt design firm, admitted they know not from wing keel design and brought in a consultant to help out. 

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A boat that might be fun to compare is William Crealock's Columbia 28 design. As you can almost read in the text, there is a flare at the foot of the keel meant to work as an end plate. This was about 1967, shortly after Noah went aground on Ararat. 

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I sailed an MG335 with the winged keel quite a bit. Up wind once you get up to speed she goes like a tortured crab, out of the tacks before you get some flow over the foil it's much worse. The full fin version is a nice yacht 

I agree with the statement that the wings on most cruising yachts are either marketing or misguided.

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On 12/31/2021 at 10:37 PM, Zonker said:

without some pretty sophisticated tank/wind tunnel/CFD most winged keels are pretty draggy and useless. At best they keep the VCG low with shallow draft.

But they make for great anchors when you eventually run aground...

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The venerable Shark 24 by George Hinterhoeller has a flat-bottom flare on the keel. I was interested to see that they are apparently are still being made in Germany.

 

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On 1/2/2022 at 11:28 AM, IStream said:

FWIW, here's the keel on my Catalina 50. Draft is 5.6' / 1.7m, ballast is 13,250 lbs / 6,000 kg. Overall dry weight is 36,000 lbs / 16,330 kg. Full specs here:

https://sailboatdata.com/sailboat/catalina-morgan-50

Plenty of wetted area but she's a pretty stiff boat with a nice motion. Decent compromise for a Florida boat. Here in Seattle, a Max Prop and big asymmetric help recover some light air performance. 

large.keelAft.jpg.d24c8576183674588dba0b42e9715ba2.jpg

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I sailed with a keel much like that (winglets more aft) for years here on the US east coast, where the bottom is shallow and often mucky. The keel provided much less draft than the fin keel version, and since I wasn't racing, it was worth trading a bit of performance to increase range. When I would occasionally "find" the muddy bottom, it never stuck with suction as naysayers predicted. It's fun to pee on concepts that didn't take off, but most people posting here probably haven't sailed on a boat with a winged keel so don't know how they handle. :)  (Okay, I admit I'm sailing a fin keel boat now.)

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7 hours ago, Steam Flyer said:

That's actually pretty close to what I remember of Australia II's keel; I think her wings were angled slightly forward.

- DSK

I believe they were angled downward.  Probably one of the better shoal draft designs for pure sailing.  (Yes, I know, 12’s were not shoal draft boats but its the design). As far as another aspects, maybe not so much, good for gathering weeds and other items found close to the surface…Oh for the days when weeds were short, and the oceans were clean. 

 

D1811A86-7353-4E5C-BC92-85E27C4E98AD.jpeg

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16 minutes ago, Santanasailor said:

I believe they were angled downward.  Probably one of the better shoal draft designs for pure sailing.  (Yes, I know, 12’s were not shoal draft boats but its the design). As far as another aspects, maybe not so much, good for gathering weeds and other items found close to the surface…Oh for the days when weeds were short, and the oceans were clean. 

 

D1811A86-7353-4E5C-BC92-85E27C4E98AD.jpeg

Do they have that block chained to it so nobody steals it?

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On 12/31/2021 at 11:37 PM, Zonker said:

without some pretty sophisticated tank/wind tunnel/CFD most winged keels are pretty draggy and useless. At best they keep the VCG low with shallow draft.

And this is why they are still making wing keel boats.  They allow for a shoal draft version; your average sailor of today isn't as concerned about racing as he is about running aground.

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9 minutes ago, Ishmael said:

Do they have that block chained to it so nobody steals it?

TIL the Aussies invented the blockchain.

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On 1/1/2022 at 10:20 AM, SemiSalt said:

What's the difference in the real world. Here are some comparisons of base ratings for some Catalina models (YRA/LIS data):

Catalina 270 - 198

Catalina 270W - 204

Catalina 34 - 156

Catalina 34 Wing - 165

Catalina 400 DK - 108

Catalina 400 W - 111

 

So the wing is assumed to have a penalty of 6-10 sec/mile around the whole race course. The upwind penalty is higher, but the offwind penalty is negligible, and the wing might actually be an advantage there. Cruisers sail upwind a smaller percentage of the time than racers. It's no wonder that cruisers find the wing keel boats satisfactory.  

Does the PHRF rating reflect drag, or the higher pointing of a conventional keel?  A boat with a nice deep fin keel that points 3-5 degrees closer to the wind on each tack should be *a lot* faster than a wing keel over the same course.  6, 9 and 3 seconds / mile don't reflect the differences I would expect based only on drag differences.  

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3 minutes ago, Lex Teredo said:

Does the PHRF rating reflect drag, or the higher pointing of a conventional keel?  A boat with a nice deep fin keel that points 3-5 degrees closer to the wind on each tack should be *a lot* faster than a wing keel over the same course.  6, 9 and 3 seconds / mile don't reflect the differences I would expect based only on drag differences.  

They point in roughly the same direction. They travel in completely different directions.

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5 minutes ago, Lex Teredo said:

Does the PHRF rating reflect drag, or the higher pointing of a conventional keel?  A boat with a nice deep fin keel that points 3-5 degrees closer to the wind on each tack should be *a lot* faster than a wing keel over the same course.  6, 9 and 3 seconds / mile don't reflect the differences I would expect based only on drag differences.  

FWIW, I believe Catalina increased the ballast a bit on their wing keeled boats vs. their fin keeled sisters to keep the VCG about the same and offset some of the performance loss.

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40 minutes ago, Hukilau said:

And this is why they are still making wing keel boats.  They allow for a shoal draft version; your average sailor of today isn't as concerned about racing as he is about running aground.

Tbh, here they've pretty much disappeared. For staying close to the shore, IMHO a twin keel is superior in about every aspect except may be a few cm of extra draft (10cm or 4 inch may be) but as you can dry the extra depth doesn't really matter.

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1 hour ago, Lex Teredo said:

Does the PHRF rating reflect drag, or the higher pointing of a conventional keel?  A boat with a nice deep fin keel that points 3-5 degrees closer to the wind on each tack should be *a lot* faster than a wing keel over the same course.  6, 9 and 3 seconds / mile don't reflect the differences I would expect based only on drag differences.  

It only reflects drag or greater keel efficiency in that those things produce different finish times.

PHRF is NOT a measurement rule, although a lot of committees want to build in their own personal rocket science-y bullshit so their buddies can win

- DSK

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2 hours ago, Steam Flyer said:

It only reflects drag or greater keel efficiency in that those things produce different finish times.

PHRF is NOT a measurement rule, although a lot of committees want to build in their own personal rocket science-y bullshit so their buddies can win

- DSK

It's seconds/mile around the course. The shoal draft boat, with wing or not, may be faster than the deep keel boat off the wind, so it could be -15 sec upwind but + 7 sec downwind,  and 8 sec on average.

Now where did that 3-5 degree number come from? Sailing a bit lower can reduce the amount of lift required from the keel quite a lot, and is probably the right thing in a wing keel boat. A designer may think this gives him leave to make the spreaders 5 inches longer which can be a good thing.

PHRF has adjustments for all sorts of things like different propellers, different jib overlaps, pole lengths, etc, including keel variants. As far as I know, they are mostly swagged. 

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20 hours ago, Panoramix said:

Tbh, here they've pretty much disappeared. For staying close to the shore, IMHO a twin keel is superior in about every aspect except may be a few cm of extra draft (10cm or 4 inch may be) but as you can dry the extra depth doesn't really matter.

I agree: while a shoal draft keel of whatever design reduces your probability of running aground, that probability is always high when sailing in shallow water. Once you do hit bottom, a twin keel seems a far less dire predicament, even if that means a bit more difficulty in a glancing impact. And in many cases, being able to hit bottom and stay there, drying out, is in fact quite an advantage.

So for shallow water, twin keels seem more sensible than one keel, regardless of the bottom of the keel.

It's not all about raw performance or righting moment. As was noticed above, a deep keel wedged into the bottom is much slower than a shallow keel not hitting the bottom, and twin keels hard aground are much better than any single keel hard aground.

In deep water, the deeper keel essentially always wins. Putting anything at the end -- winglets, bulb, a big fat tip -- is worse than stretching into the depths from a coefficient of lift point of view.

Roll in the trade-offs. There are always trade-offs, there is no achievable ultimate solution.

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On 1/2/2022 at 8:48 PM, Rasputin22 said:

DDW, I still have a hard time wrapping my head around the practice of 'filling our sailplanes with water' and I know the theory and stats and all but it seems contradictory to me. That is until I think back to skiing down hill and how a heavier skier will reach a higher speed on the same skis and the same slope. I have never had the privilege of flying in a ballasted sailplane but would love for you to show me sometime just what that is all about. Do you have a photo of your aftermarket winglets? It seems like it would be a very hard thing to do to pick up a saw and shop off the wing tip of a 6 figure sailplane just to fit a unknown winglet. That is as ballsy as flying sailplanes in the first place!

The winglets I installed (or I should say had installed by a licensed shop) came from the original manufacture (Schleicher) bespoke for that sailplane, so not an unknown. In fact until the end of the model run they continued to mold them without the winglets, if you ordered them from the factory they would cut the tips off and install their kit (probably near 100% were sold that way after they introduced the winglets). Yes, you simply sawsall the tip off, install a tip rib with the connection features, plug it in (they come off in the trailer), and fair and finish. Makes you a little sick when you see the sawsall bite in at first.... I don't seem to have great photos of the winglets before and after but I've added some bad ones. 

Increasing the weight just moves the whole performance curve up the speed range. In fact in the last 3 decades by far the largest increase in performance is associated with increasing ballast capability. A Jonkers JS1 for example can carry about 400 lbs of water. I've flown right next to sisterships for long glides (>50 miles), with our glass cockpit instruments and anti-collision devices I can track the other sailplane's location, speed, and sink rate. One friend flies an ASH 31, basically the same sailplane as mine with 18m wingtips, but optionally 21m wingtips, and the ability to load to 12 lbs/ft^2 (I can only go to 9). If he is flying 21m but unballasted, there is a noticeable difference but not dramatic (claimed glide ratios are 50:1 and 54:1). If he makes a mistake or two I will stay with him (though as a former and current national champion he doesn't make many).  If he is flying ballasted we achieve approximately the same glide but he is going 10 - 12 knots faster and that is a dramatic difference, simply no chance of hanging on. That is why in our Truckee FAI contest, we weigh the gliders and adjust handicap based on weight. 

My glider is only about 50 lbs below max takeoff weight with me sitting in it, so though I do have water ballast capability I do not bother with it on the ASH. When we had a Duo Discus we would fill that thing up and it made a big difference. 

No winglets, just slightly upturned as originally molded:

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Winglets added. They split just inside the tip wheel:

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1 hour ago, carcrash said:

So for shallow water, twin keels seem more sensible than one keel, regardless of the bottom of the keel.

Except for one thing - heeling a single keel boat reduces the effective draft but heeling increases a twin keel or wing keel fitted boats effective draft.    Unless you are able to heel the boat *way* over.     And there have been times where I've gone aground, pointed the boat in the other direction and gotten off.    Seems like a twin keel or wing keel boat may be harder to turn around.

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20 hours ago, SemiSalt said:

Now where did that 3-5 degree number come from? Sailing a bit lower can reduce the amount of lift required from the keel quite a lot, and is probably the right thing in a wing keel boat. A designer may think this gives him leave to make the spreaders 5 inches longer which can be a good thing.

I have a boat that is exceptionally good at pointing and that I race in OD, a J/35.  I'm very sensitive to pointing and VMG, and have heard some J/35 old timers talk about some shorter keel versions that didn't point as well until their keels were brought up to class standards - understanding there is a weight distribution issue and other factors involved.  Have seen this with other shoal draft boats locally that don't quite point with their regular fin keel counterparts - this was a problem in the 105 fleet for a bit until a group of owners got together and got a bulk discount on keel repair.  Never measured how much worse they pointed but have heard tales...

So it's a SWAG and based on second hand info as well.  Not pulled completely out of my ass but it's definitely from the rear left pocket of my jeans... 

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2 hours ago, slap said:

Except for one thing - heeling a single keel boat reduces the effective draft but heeling increases a twin keel or wing keel fitted boats effective draft.    Unless you are able to heel the boat *way* over.     And there have been times where I've gone aground, pointed the boat in the other direction and gotten off.    Seems like a twin keel or wing keel boat may be harder to turn around.

Yes, with a twin keel if you run aground while the tide is going out, the chance are that you are stuck unless you were going to windward fully powered but then you have to deal with the potentially bigger issue of hitting the bottom @ 6 knots! 

If you are taking risks like this with a fin keel, chances are that sooner or later you will end up on the side waiting for the tide...

IMO, the big advantage of the twin keel is that you can go somewhere nice while the tide is up, let it go, dry the boat and enjoy the place while others are anchored in the swell! Thus draft is less relevant, it just means that you dry a bit sooner or anchor a bit further out but still much closer than shallow draft fin keel boats. You can also stay in small ex fishing harbours while others are in packed marinas.

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1 hour ago, Elegua said:

I've gotten the feeling that shoal draft is a bit like 4-wheel drive; you just get stuck in places that are harder to get out of.

Maybe less of an affectation. I've yet to see "shoal draft" painted on the topsides of a boat.

It seems to me that there are a couple of different flavors of shoal draft with different uses. I grew up in an area with a lot of interesting but shallow water. The bottom was stone and the tides were negligible. Keel / centerboards were popular there for a while. When the centerboard started bumping on the bottom is was just past time to tack ;). Shoal draft was nice. And, for a time, 6 feet was considered a very deep keel. Over the years folks ditched the boards and moved on to ever deeper keels. Perhaps the summer cruise went away for most folks and there were enough deep anchorages for a weekend? The ability to dry out wasn't ever much use there. A place with interesting tidal waters would make bilge keels a more appealing option. I spent a bunch of time bopping around the Pacific in a boat with about 1 meter of draft and there were places that I thought even that limited my options a little. I know folks who have cruised with the best part of 3 meters of draft in the same areas who claimed to never find it limiting. I don't think it's strictly an engineering problem. Expectations and the dark and mysterious realms of headology play into it.

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Back a pretty long time ago, Yves-Marie Tanton wrote in a blog that he had been studying the French development of "biquille with bulb" boats. These have two high aspect ration keels with bulbs as seen the picture of a Django 7.70. YMT was favorably impressed. It's interesting to me how the concepts of twin keel and shallow draft have be ripped apart. 

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4 hours ago, Elegua said:

I've gotten the feeling that shoal draft is a bit like 4-wheel drive; you just get stuck in places that are harder to get out of. 

Shoal draft allows you to get stuck in much more interesting places.

- DSK

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On 1/3/2022 at 12:39 AM, Whinging Pom said:

My (new to me) boat, a Dehler 35 CWS has a wing keel.  She was designed by Van De Stadt who know a thing or two, it's not obvious in the photos but the wings have a fair amount of downward deflection (diehedral?) the standard draft is 2m and in wing form it's 1.5m which will be useful when we get back to the Bahamas and the ICW.

 I haven't sailed her that much, but with utterly knackered sails she points very well and makes little leeway at good speed. On my home river, there are two other sister ships, both with full draft keels.  I'm looking forward to having a tussle with them later this year.  

A happy new year to you all.

 

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Downward sloped is anhedral vice dihedral 

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10 hours ago, SemiSalt said:

Back a pretty long time ago, Yves-Marie Tanton wrote in a blog that he had been studying the French development of "biquille with bulb" boats. These have two high aspect ration keels with bulbs as seen the picture of a Django 7.70. YMT was favorably impressed. It's interesting to me how the concepts of twin keel and shallow draft have be ripped apart. 

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This is what I meant by "well designed" twin keel, these sail well to windward but are a bit slower downwind than their fin keel equivalent. A bit of toe in of the keels is necessary for good performance upwind or on a close reach but it creates drag when running. Upwind the toed in keel allows the boat to sail with little leeway and that seems to compensate the parasitic drag of the windward keel.  These aren't the shallowest boats but here it really doesn't matter as we have relatively big tides.

The previous 1970s iterations didn't like going to windward and were draggy anyway...

7921108_20210618081140420_1_XLARGE.jpg&w

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2 hours ago, weightless said:

Maybe less of an affectation. I've yet to see "shoal draft" painted on the topsides of a boat. :D

It seems to me that there are a couple of different flavors of shoal draft with different uses. I grew up in an area with a lot of interesting but shallow water. The bottom was stone and the tides were negligible. Keel / centerboards were popular there for a while. When the centerboard started bumping on the bottom is was just past time to tack ;). Shoal draft was nice. And, for a time, 6 feet was considered a very deep keel. Over the years folks ditched the boards and moved on to ever deeper keels. Perhaps the summer cruise went away for most folks and there were enough deep anchorages for a weekend? The ability to dry out wasn't ever much use there. A place with interesting tidal waters would make bilge keels a more appealing option. I spent a bunch of time bopping around the Pacific in a boat with about 1 meter of draft and there were places that I thought even that limited my options a little. I know folks who have cruised with the best part of 3 meters of draft in the same areas who claimed to never find it limiting. I don't think it's strictly an engineering problem. Expectations and the dark and mysterious realms of headology play into it.

I agree that geography is a big driver. Where I sail, a twin keel or a wing keel is less useful.  I have an olde timey CB that draw 3.3M when down and 1.4M when up.  It's not a necessity where I sail, but I enjoy being able to park it places that deeper boats don't go and I also appreciate the extra buffer when naviguessing, though I've never had the nerve to use the CB as a parking feeler.   I've seen French sister-ships use legs to dry out, but then you are into twin keel/fully retracting CB territory. 

 

2 minutes ago, Steam Flyer said:

Shoal draft allows you to get stuck in much more interesting places.

Also true. 

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some keels work better than others.

Ian Howlett designed a great keel for modern 8mR yachts. some of the moderns that started with different keel designs, changed to his design. most of them work best through the different trim tabs. so currently, moving underwater appendages are limited to 5.

 

first picture is of Yquem 2 a dubois design boat, current world champion. 

the second and fourth one are the old tandemkeel of MissU the former winner of the world cup. she is currently converted to the keel of Hollandia. in picture 3. Hollandia was a dominant yacht for many years. designed by Doug Peterson, with Ian howlett wing keel and rudder.

the last one is the new keel on 8mR Natural

 

 

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I really don’t see the need or purpose of the long wings placed well before the end of the keel.  Seems to me they would do nothing to prevent vortex creation and all they would all is increased wetter area and lots of drag.  

A good illustration of wetter surface drag.  Let 75 yards of 4lb test monofilament drag behind the boat to eliminate twist.  The rad had a serious bend and it needed to be tightly held.  4lb test is quite thin.  

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20 minutes ago, JMOD said:

some keels work better than others.

Nice pics!

The common denominator seems to be that there isn't much volume in the wings. The ballast is in the bulb and the wings are doing wing things. The cruising boat wings look to be combining the ballast and wing functions.

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3 hours ago, JMOD said:

some keels work better than others.

Ian Howlett designed a great keel for modern 8mR yachts.

These are examples of keels that work better than others under the 8mR rules. Those rules are weird and simultaneously specific though. It's hard to draw conclusions about the effectiveness of such keels for other classes or boats not restricted by class rules.

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14 hours ago, JMOD said:

some keels work better than others.

Ian Howlett designed a great keel for modern 8mR yachts. some of the moderns that started with different keel designs, changed to his design. most of them work best through the different trim tabs. so currently, moving underwater appendages are limited to 5.

 

first picture is of Yquem 2 a dubois design boat, current world champion. 

the second and fourth one are the old tandemkeel of MissU the former winner of the world cup. she is currently converted to the keel of Hollandia. in picture 3. Hollandia was a dominant yacht for many years. designed by Doug Peterson, with Ian howlett wing keel and rudder.

the last one is the new keel on 8mR Natural

 

 

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12366104_10156357109070268_1739923788573857306_o.jpg.ac3b03b403c638d3f7bcfbd3967b4f06.jpg

jrt9857.jpg.e84f5129739939483d3ac86b75900398.jpg

162571316_10159494151916514_1251259223298535766_n.jpg

Natural-new-keel.jpg

These might look good on the water, but on the dry - to my delicate eyes - it looks like they've been fed on a fat burger diet!

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16 hours ago, Santanasailor said:

I really don’t see the need or purpose of the long wings placed well before the end of the keel.  Seems to me they would do nothing to prevent vortex creation and all they would all is increased wetter area and lots of drag.  

A good illustration of wetter surface drag.  Let 75 yards of 4lb test monofilament drag behind the boat to eliminate twist.  The rad had a serious bend and it needed to be tightly held.  4lb test is quite thin.  

I don't get it either, but I keep in mind that something I've heard, more often from the airplane guys than the boat guys, is "no vortex, no lift". Possibly the wings reduce the flow under the forward 2/3 of the keel which tends to reduce the pressure difference between the two sides of the keel.

Most of the boats with fancy keels are in classes with draft limitations.

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On 1/1/2022 at 12:31 PM, Grrr... said:

And, to add on against a lot of what the OP said - the 'winglets' at the end of the wings on planes are there to increase overall efficiency and lift by reducing tip vortices caused by lack of an end plate.  Part of the same reason that the foiling AC boats started sealing the bottom of the mainsail up against their hull.

Any wing or sail that lacks a seal (it doesn't have to touch but close, overlap even better - rating aside) on its deck/fuselage side risks halving its aspect ratio -- doubling its induced drag.  On boats and airplanes (leaving aside cylindrical wings and other magic) there's nothing there (without an endplate or winglet or other gizmo) to reduce flow/pressure escaping with turbulent vortex off the tip.

So a winglet may reduce  energy lost to excess tip vortex.  But there always has to be a span constraint to justify them, because adding more span is more effective than the most perfect winglet (I don't pretend that's self-evident, but if it weren't winglets would benefit by winglets, and so on), and they increase surface friction faster than span, and they add interference drag that extra span lacks entirely.

Adding "Structure" to the other span constraints, bi-planes offer good insight:  when the desired span is not possible because of structural limits (look at the struts, shrouds, ..., necessary on early planes), a properly configured bi-plane (and maybe a properly configured Bi-quille [discussed elsewhere by Yves-Marie Tanton]) has much less induced drag than expected on span alone.

This is because the lift circulation is about the pair of wings, with a large effective thickness (the lift circulation is not about each wing alone, it must turn about both wings together (if properly configured: span, chord, separation, foil & lead - easy to get wrong.)  [Structure isn't all that motivates bi-planea - they permit tighter turning (rolling) maneuvers.]  But the added drag, as for winglets, is reduced by substituting span, adding inherent speed - monoplanes.

The bi-quille solves another problem.  They are very common on both sides of the Channel, where tidal range reaches 45 feet (e.g. Dinard/St. Malo), and many boats must either sit down on the bottom or stay behind a tidal barrier, essentially a dam.  The bi-quille is displacing the use of dual, lashed stilts there.  And as their numbers increase their configuration is better studied and get's closer to what's possible - which isn't beating span at the performance game when span is not limited.  Unfortunately it's sometimes better to lie down.  Picture a bi-quille, a boat with stilts, or a keel with a dead flat base, perched on the top of the wall of a tidal barrier.  A little bit like CtrBoardSonar: if you touch, make sure its a good bed, or flee.

[For those who think its difficult or just wrong to transfer shapes, mechanisms, & measurements back and forth between and boats and planes, yes it's complicated, often inapplicable, maybe justifiable as a warning, but it's only wrong when its wrong; dangerous if uninformed, otherwise nearly essential]

regards,

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9 hours ago, SemiSalt said:

something I've heard, more often from the airplane guys than the boat guys, is "no vortex, no lift".

That's a weird way of thinking. Vortexes don't help with lift; they're because you have 2 different pressures mixing.

If the glider and airplane builders could make 100m long wings that wouldn't fall off and fit in airports, they would. They would have small tips.

The only big vortex is at the tip. If the tip is tiny, tiny vortex. Tiny drag.

Short stubby keels have big vortices.  And the 8m should have stayed away from the all you can eat buffett.

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22 minutes ago, pschwenn said:

[...] perched on the top of the wall of a tidal barrier.  [...]

Always entertaining...

MjAxMzA5YWY5YjliNDM1M2UwOGY2OGUyMjkyZjYy

25061-191028171412582-1-960x640.jpg

 

Bear in mind that when the the wall uncovers, the sea is going down at a rate of up to 3m per hour (1m per 20min or 10cm every 2 minutes) or 2 inches per minute, so to avoid being trapped, you need to be very reactive. If you are slow you may make it to the newspaper so that everybody can have a laugh at your expense (these photos were found by googling news)!

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10 minutes ago, Zonker said:
9 hours ago, SemiSalt said:

something I've heard, more often from the airplane guys than the boat guys, is "no vortex, no lift".

That's a weird way of thinking

Perhaps it's a way of thinking that comes from the way lifting line theory links vortexes and lift.

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@Zonker Do you think that a twin tandem keel could be made to work ?

The modern twin keels are not very stable (often they need a third leg), let's imagine that you get back the longitudinal stability by having 2 long skinny bulbs each held by 2 narrow fins. Assuming that a competent naval architect is given means to study and design this. Could it possibly work? He can choose single or twin rudder, whatever works best, the only constraint is that the boat is really stable when dry on a surface that is just vaguely plane and sails as well (or at least nearly as well) as a modern twin keel design.

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I think so, if that was a primary parameter.

You position the keel fin where it needs to be to balance the sail plan and then you position the bulb foot to get the LCG correct. 

Probably something has to give but you could do it to sit on rudder/rudders as the third leg.

Probably design the hull/accomodations so the LCG is pushed further aft than normal so the CG is between the keels and rudder. Right now the CG is more likely to be very close to the keel position. Wedge shaped hulls would be good for this. Even push the rig aft and increase main more. Big locker aft so all the lazarette junk is right aft.

 

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9 hours ago, SemiSalt said:

I don't get it either, but I keep in mind that something I've heard, more often from the airplane guys than the boat guys, is "no vortex, no lift". Possibly the wings reduce the flow under the forward 2/3 of the keel which tends to reduce the pressure difference between the two sides of the keel.

Most of the boats with fancy keels are in classes with draft limitations.

Agreed on why winglets might reach forward.

[BTW: "no vortex, no lift" refers not to the tip-vortex but to the near vertical vortex sheet established by the (relative) circulation about the foil -- which creates the desired lift to windward.  The tip vortices flow helically aft off the tip.  Their rotational and turbulent energy represents the undesirable induced drag.  A winglet can moderate (spread out across area and time) the vortex's creation, reducing the losses due to both rotation and turbulence.]

One can see from the back-n-forth on winged keels, they can be effective (at something), or worse than useless, often puzzling.  If they were generally useful without span restrictions, they wouldn't need so much defending and discussion.  That can't stop me though:

 

One important related effect of SemiSalt's 'draft limitation => shallow keel =>  winglets-reaching-further-forward' is that the center of lift of a shallow draft keel moves forward along the keel as it gets shallower (and longer for area), because as he says, the shallower the keel the more of the keel acts like the tip - desirable pressure differences escape over the base earlier, which means that less of the keel upwards and aft is lifting. 

This in turn requires the rig to move forward (the keel can't ordinarily move very far aft for various reasons) to retain the directional stability.  Two problems: if the designer, owner or builder doesn't believe this, the keel might be moved aft anyway.  Or, with the mast forward, the practiced eye sees an awful lot of empty space.  Something 'not there' might not seem troubling, but just as the disappearance of a long-term odor might startle those who worked long to accustom themselves, the aft deck without the main overhanging may startle.  Something may get added there to please.

 

regards,

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39 minutes ago, Zonker said:

I think so, if that was a primary parameter.

You position the keel fin where it needs to be to balance the sail plan and then you position the bulb foot to get the LCG correct. 

Probably something has to give but you could do it to sit on rudder/rudders as the third leg.

Probably design the hull/accomodations so the LCG is pushed further aft than normal so the CG is between the keels and rudder. Right now the CG is more likely to be very close to the keel position. Wedge shaped hulls would be good for this. Even push the rig aft and increase main more. Big locker aft so all the lazarette junk is right aft.

 

Some are already doing this, rudders is a bit iffy as you will find a smallish rock  inconveniently placed.

I meant this (just one side drawn):

587169670_twintandemkeel.jpg.76e204d47db9f4e11385c200ebd42c48.jpg

 

You would effectively have 2 tandem keels. Altogether that's 4 fins.... That's quite a lot, but they could be made quite narrow to keep wetted area low while still having a longitudinally long contact on the ground.

Getting the third leg out is a faff especially if you dry accidentally at mid tide (never happened to me but I don't play this game!).

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1 hour ago, Zonker said:

That's a weird way of thinking. Vortexes don't help with lift; they're because you have 2 different pressures mixing.

There are several mathematical models of lift, and a couple of popular ones directly couple circulation with lift. No circulation, no lift. The circulation results in the vortex at the tip of the wing. Whether the vortex is the result of lift, or lift the result of the vortex is a chicken and egg discussion. 

On those 6 and 12m keels, you have to look at them heeled at 30 degrees which is how they are sailed. The drooping winglets add quite a bit to effective span. That makes them a lift device, not a drag reduction device. 

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51 minutes ago, Zonker said:

That's a weird way of thinking. Vortexes don't help with lift; they're because you have 2 different pressures mixing.

If the glider and airplane builders could make 100m long wings that wouldn't fall off and fit in airports, they would. They would have small tips.

The only big vortex is at the tip. If the tip is tiny, tiny vortex. Tiny drag.

Short stubby keels have big vortices.  And the 8m should have stayed away from the all you can eat buffett.

 

39 minutes ago, weightless said:

Perhaps it's a way of thinking that comes from the way lifting line theory links vortexes and lift.

What the airplane guys mean is not that there is no lift without the spinning turbulence of tip-vortices, but that the vertical vortex sheet mirroring and caused by the circulation (the source of the desirable lift) about the keel or wing, is a necessary part of the lift.  There's no keel  or sail lift without that vortex sheet.

They might also be being careless with their wording -  A wing with no tip has no tip vortex, but is lifting (a cylindrical wing (Rutan), or a wing between two fuselages, or an abstract infinite span wing).

One thing that makes things seem more complicated is that lift suggests good and drag suggests bad, but it's all drag, there's no inherent good or bad outside the intent of the machine.  Lift is drag with a component you find useful; but there's always a component of it that drag you don't want, for example, it's not possible to escape the turbulence and other flow disturbance in the vortex sheet streaming off the rear edge of the keel or sail, which vortex sheet is part and parcel of the "lift".

 

regards,

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Ah ha. A double tandem keel.

If you have a big CFD budget yeah you can make it work. For the rest of us, you'll have to recognize that it won't be as efficient as a regular keel but it's a good solution. Thanks I didn't understand what you meant at first.

 

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1 hour ago, Zonker said:

That's a weird way of thinking. Vortexes don't help with lift; they're because you have 2 different pressures mixing.

If the glider and airplane builders could make 100m long wings that wouldn't fall off and fit in airports, they would. They would have small tips.

The only big vortex is at the tip. If the tip is tiny, tiny vortex. Tiny drag.

Short stubby keels have big vortices.  And the 8m should have stayed away from the all you can eat buffett.

I agree that tip-vortexes don't help with lift, they can only detract from it.

But the rotational momentum inherent in the greater speed on the lifting side of the keel than the other side cannot exist without the equal and opposite momentum of the "well behaved" vortex sheet flowing off the aft edge of the keel or sail.  Conservation of momentum may sound theoretical but it underlies the real magic of lift. 

You can see Circulation initiate and fully develop in a flow chamber.  What you won't see is flow "bouncing off" the bottom of the wing, or a symmetrical foil like a keel or acrobatic wing forcing flow to go further, faster over the bulge on top of the foil than the flatter underside. 

Among the valid alternative ways of looking at lift you might find quicker or simpler ways to get the answers, even more accurate answers [the turbulence in circulation vortexes is not understood well enough to compute correctly], just as Ptolemy was used in space flight calculations before computers could do any more complex calculation on time.

 

regards,

 

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15 minutes ago, Zonker said:

Ah ha. A double tandem keel.

If you have a big CFD budget yeah you can make it work. For the rest of us, you'll have to recognize that it won't be as efficient as a regular keel but it's a good solution. Thanks I didn't understand what you meant at first.

 

You could double down on the CFD versus "regular keel ... good solution".  Many of the puzzles at the technical edge of design seem beyond the experience+insight+tried'n'true approach, but for many of those puzzles, you get an answer with CFD, and it looks precise and formal and authoritative.

But, for example, turbulence cannot be yet treated correctly in CFD (if ever; like the weather it's not even known whether it is predictable period).  For another, CFD'rs change the manner and level of reducing things toward micro-scale elements that they find better at the reduction of our-scale stuff to, and in the meshing used to represent the sails, water, hull, ... .  The principal reason for this is to correct faults in existing CFD results, which also implies that some of the "successful" use in the past was incorrect. Third, CFD sounds good, like "AI", and the renderings of CFD look good.

 

regards,

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