Split centre boards

[Sybarite]

Inspired by a post on a French forum I was wondering at the following scenario.

If you took a centre board and split it length ways and each is profiled like an aeroplane wing so that it provokes lift to windward could one effectively by letting down the appropriate side, improve one's heading to windwards/VMG? Or are there compensating forces?

Just curious.

John

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[Talbot]

This is the principal of the winged keel. The wings create lift and tend to cancel out leeway - seemed to work on a Sigma 362.

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[blackbeard]

Not seen this done with centre boards but the Red Fox series did exactly this (ie 2 assymetric foils, used one or the other according to port/starboard tack when going to windward) with bilge boards.

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[vyv_cox]

I'm no expert on aerodynamics nor hydrodynamics, but surely the phenomenon of lift in aircraft wings only works because air is compressible? High velocity air flowing over the curvature of the upper wing surface causes a reduction in pressure, and hence lift. High velocity water flowing over the same curvature will suffer a reduction in pressure but because the water cannot expand to fill the volume, it will develop cavitation.

Not that this idea has not been tried many times. The hulls of Hobie 14 and 16s are asymmetrical to exploit this supposed theory.

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[qsiv]

Absolutely - that is precisely what the Open boats do - they have two boards (they are also inclined for optimum angle of heel), and are often asymmetric. The quid pro quo is increased drag (you cannot generate lift without drag), and on a cruising boat this may reduce the benefit. On an Open 60 with power to spare it is less of an issue.

In truth though the benefit is small, and your keel or centreboard already generate lift as they operate at an angle of attack (the leeway angle). This small angle will cause the water flow to accelerate on one side, thus generating lift. Some racing dinghies (notably Hornets) allowed rocking centreboards to achieve the same effect.

The winglets on winged keel dont necessarily generate lift, rather they operate as endplates, and maximise the lift already generated by the keel by eliminating (or at least reducing) the tip vortices. The same technique is used on the 747-400 (and other planes nowadays), where they have small endplate winglets. This is also the reason why racing genoas are desinged to meet the deck - it stops the 'leakage' of air from one side of the aerofoil to another.

You also see the same endplate technology on single race cars, both on front and rear wings.

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[Jacket]

There's a 17 foot trailer sailer that does this. Leisure 17, I think. There was one at my old dinghy club. We sailed on a very narrow river, so pointing ability was important. The assymetric centreboards made a noticable difference.

I read somewhere that the main benefit is that by reducing leeway to zero the drag on the hull is drasically reduced, as the hull is moving straight forwards through the water, as opposed to being dragged through at an angle. Some tank tests were done, and while I can't remember the actual values forund, I think a reduction in drag of about 10% was quoted- certainly a worthwhile reductiuon.

Some similar tests were done in Australia during the 80's, but on dinghies, rather than yachts. In this case, there was found to be little benefit. It was felt that dinghies, being faster for their length and with deeper daggerboards than would be found on a typical yacht, made significantly less leeway in the first place, and so benefited less from assymetric foils.

In the 70's trim tabs were used on the back of yacht keels to produce lift to windward, but they seemed to loose popularity after a few years. I've no idea if this was because they were ineffective, or due to cahanges in the rating rules. 12 meters were also fitted with them, and they seem to be making a bit of a comeback on grand prix racing yachts.

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[DeeGee]

Apropos of nothing... try holding a spoon delicately by the tip of the handle, and allow the back of the spoon to slowly come closer and closer to a tight stream of water running from a tap.

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[qsiv]

Not strictly speaking the same effect as far as I recall - particularly as most centreboards operate in a largely homogeneous fluid.

If I recall, the lift the spoon registers is due to the deflection of the water as it leaves the tip - not a pressure differential.

Nothwithstanding that - water is compressible (technically again - it isnt the increase in pressure on the underside, so much as the reduction in pressure on the top that generates the lift).

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[vyv_cox]

<Nothwithstanding that - water is compressible > Not at the pressures we are talking about here, I think. If I remember the Marchaj that I read many years ago there is no such thing as hydrodynamic lift.

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[qsiv]

<<no such thing as hydrodynamic lift. >>

Well that will disappoint the propeller manufacturers and builders of planing boats, and rudders ..

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[Skyva_2]

There is an excellent illustrated explanation of hydrodynamic lift for keels and rudders at:

<A target="_blank" HREF=http://www.vacantisw.com/tutorial.htm>http://www.vacantisw.com/tutorial.htm</A>

The articles are good descriptions of the forces acting on a yacht and how they affect design.




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[Jacket]

If there was no such thing as hydrodynamic lift, then a centreboard made out of a flat plate would be as efficient as a foil shaped one. And if you've ever sailed a Lark or other dinghy with a centreboard cut from a steel plate, you'll know that they're nowhere as near efficient.

You've already said that faster flowing water has a lower pressure. That's all that's needed to create lift. Whether the fluids compressible or not is irrelevent, surely?

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[Jacket]

Actually, I'll disagree with myself about the flat plate bit. They're just rubbish because they stall at a low angle of attack.

But I still can't see why compressability of the fluid matters.

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[vyv_cox]

<But I still can't see why compressibility of the fluid matters.> Because gases obey the gas laws, and liquids don't. PV=constant. If I increase the velocity of a gas, the pressure goes down. Liquids are largely incompressible. If I increase the velocity of a liquid, say by imparting energy into it with a pump or propeller, it doesn't reduce the pressure and, more important, the pressure on each side of the impeller, or foil, or rudder, is pretty much the same.

If I do decrease the pressure locally, by bad design or incorrect use, then I get cavitation precisely because the liquid is incapable of expanding to fill the volume.



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[qsiv]

Actually thats perhaps a good thing, because if ther is zero lift there will be zero drag. So once we are out of displacement mode, and good old Froude can be discarded, it implies that speed can become infinite.

I dont ever remember Marchaj lecturing on the absence of Hydrodynamic lift when I was at Southampton University. Indeed, the lectures on lifting surfaces were common between the Aero students and the Ship Science students as both dsiciplines were concerned with aerofoil surfaces operating in a fluid - the only difference being the Reynolds number at which each operated, which governed the choice of foil sections.

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[Skyva_2]

What's more, if I recall correctly, in aerodynamics lectures at Farnborough, air was considered to be an incompressible fluid at speeds up to 200 knots. Bernoulli's equation depends on fluid velocity, and assumes that the fluid is incompressible.

Marchaj's in Sailing Theory and Practice certainly refers to hydrodynamic lift for hydrofoils.


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[MainlySteam]

Yes, for slow flying aircraft (less than around 200 knots) the compressibilty of air is insignificant in the production of lift over the wings. So to raise the stakes one needs to point out that the compressibilty of air also has nothing to do with the driving force from sails either.

If anyone can present a correct and objective post explaining in less than 250 words and with no diagrams how the pressure difference occurs giving lift on keels and sails (and slow flying aircraft wings), I suggest we give them a prize.

John

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[BrendanS]

I started, but gave up. 250 words without diagrams beyond me at this time of night.

If anyone is really interested, have a look at <A target="_blank" HREF=http://www.av8n.com/how/htm/airfoils.html>this site</A> which is about as far as the average layman will want to take it.

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[DeeGee]

Oy, that V is volume, not velocity. Not saying you're conclusions are wrong, as brain not engaged.

<hr width=100% size=1>Black Sugar - the sweetest of all

 

[vyv_cox]

Yes, I just took two steps in one. The velocity increase around the foil results in a volume increase.

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