Return to Winged Keels

Crash

Super Anarchist
5,020
968
SoCal
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 

 

Panoramix

Super Anarchist
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...

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Elegua

Generalissimo
4,385
1,952
Lower Loslobia
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. 

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

 

JMOD

Super Anarchist
1,184
101
Netherlands
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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Santanasailor

Charter Member. Scow Mafia
1,344
701
North Louisiana
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.  

 

weightless

Super Anarchist
5,607
582
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.

 

Voiled

Member
391
284
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.

 

Panoramix

Super Anarchist
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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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!

 

SemiSalt

Super Anarchist
7,752
269
WLIS
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.

 

pschwenn

New member
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,

 

Zonker

Super Anarchist
9,292
5,225
Canada
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.

 

Panoramix

Super Anarchist
[...] perched on the top of the wall of a tidal barrier.  [...]
Always entertaining...

MjAxMzA5YWY5YjliNDM1M2UwOGY2OGUyMjkyZjYyMTg3NTJhOGI


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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)!

 

Panoramix

Super Anarchist
@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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Zonker

Super Anarchist
9,292
5,225
Canada
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.

 

pschwenn

New member
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,

 

Panoramix

Super Anarchist
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):

twin tandem keel.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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DDW

Super Anarchist
6,317
1,012
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. 

 

pschwenn

New member
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.


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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