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  #1  
Old 02-24-2013
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End Plate Effect

Can someone either describe this to me or point me towards a website offering a good explanation? I haven't been able to find a decent source of information (and maybe a diagram or two) to satisfy my curiosity. Thanks
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Old 02-24-2013
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Re: End Plate Effect

End plate effect is about making the outer end of a foil (wing) more efficient. End plates are typically associated with keels. End plates offer theoretical advantages in two ways. First is the minimization of loss of pressure difference between one side of the keel at the keel tip and the other is in reducing tip vortex and therefore reducing drag.

In a heeled boat, the pressure differential at the bottom of the keel is reduced by water slipping off the tip (bottom) of the keel. This reduced pressure makes a proportion of the lower keel area less effective. The end plate caps the bottom of the keel directing water aft so by adding a plate at the end of the keel the pressure cannot 'leak' as much.

But also, as a boat makes leeway water slips off of the high-pressure side of the keel to the low-pressure side of the keel and creates a turbulent swirl know as a tip vortex. This mass of swirling water is towed behind the keel creating drag. The longer the keel, the bigger the vortex, the greater the drag.

The end plate reduces both the pressure drop and reduces the size of the vortex. These are also related to the reasons that a deep-high aspect keel is more efficient than a shallower fuller length keel. It is rare to see a boat with a true end plate, but it is very common to see newer boats with bulbs, Scheel keels and so-called wing keels, all of which do two things, lower the center of gravity of the ballast in the keel, and provide an endplate for the keel. At typically designed, none of these act as wings.

I hope this is helpful,
Jeff
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Last edited by Jeff_H; 02-25-2013 at 08:56 AM.
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Old 02-25-2013
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Re: End Plate Effect

Quote:
Originally Posted by Jeff_H View Post
End plate effect is about making the outer end of a foil (wing) more efficient. There are two parts of this. End plates are typically associated with keels. First of all a keel works like a wing with the difference in pressure between one side of the keel and the other creating lift. The end plate caps the bottom of the keel directing water aft.

In a heeled boat, the pressure differntial at the bottom of the keel is reduced by water slipping off the tip (bottom) of the keel. This reduced pressure makes a proportion of the lower keel area less effective. By adding a plate at the end of the keel the pressure cannot 'leak' as much.
...
Well.... Almost. As Jeff points out, there is a difference in the pressure in the water flow around a keel, similar to that of a wing/a foil, wherein the fluid flow across the top of the wing/foil is faster than the flow across the bottom. As the speed of the flow increases, the pressure in the flow decreases (so long as the flow remains "attached", i.e. follows the curvature of the wing/foil), hence the pressure on the "top" is lesser than the pressure on the bottom of the wing/foil creating "lift". This pressure differential remains consistent (although not necessarily constant) over the entire length the wing/foil. At the "tip", particularly of a "square ended" foil, the fluid--air/water--attempts to flow around the tip to the low pressure zone on the opposite side of the foil in a path that is "normal" to the direction of the flow. (This normal flow that curves around the end of the tip is what gives rise to the vorticies seen streaming off the tip of the foil in wind-tunnel and tank testing.) To the extent that the flow around the tip can reach the opposite side of the foil, the pressure differential there is diminished or eliminated entirely, (aka "stalling") and the "lift" of the foil in that area reduced or eliminated. The addition of an end plate, blocks this transverse flow to some extent increasing the efficiency of the foil. Such end-plat treatment can be seen on some of the higher performance jets with vertical foils on their wing tips. Other approaches to this problem include curving the back edge of the foil, and to a lesser extent the leading edge, to a point at the tip as one sees, for example, on the elliptical shaped trailing edges of high performance aircraft propellers and, in yachts, rudders.

Whereas the "top" of a keel foil varies, depending upon the tack one is on, if one is attempting to use "end plate" effect on the keel to improve efficiency, the end-plate must be symmetric, extending outward on both sides of the keel (which also concentrates weight at the tip of the keel improving righting moment). The search for "end plate effect" also extends to sails in some cases, accounting for the evolution of "deck sweepers" or head sails that actually reach and "sweep" across fore-decks as the yacht is tacked (although the frequency with which these sails also scooped up water in rough head-seas limited their effectiveness and eventually put paid to their wide spread use).

It is an interesting but rather esoteric subject and whether it measurably improves performance aboard a yacht really is debatable. For more, Google "wing tip vorticies" and related terms.

FWIW...
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Last edited by svHyLyte; 02-25-2013 at 12:01 PM.
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Old 02-25-2013
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Re: End Plate Effect

HyLyte,

Its funny....both you and I were correcting my poorly written version of my explanation (written last night but corrected this morning) at the very same time. I think that a simple end plate would not do much for performance over a mixed course (beating, reaching and running) but these carefully modeled bulbs which improve stability as well as providing some vortex reduction and lower keel efficiency have clearly resulted in some performance gain. Neither of us touched on the fact that the hull serves as an endplate at the keel and rudder root, and in the case of the rudder, having this end plate can delay aeration of the rudder.

Jeff
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Old 02-25-2013
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Re: End Plate Effect

You may be too young to remember it, but the great debate about Australia II's winged keel, and its performance during the 1983 AC competition involved, in part whether the performance enhancement resulted from the yacht's lowered CG or end plate effect:



The lack of end-plate effect on the top end of rudders where a portion of the rudder extends above the water in old IOR style yachts, can make aeration a serious handling problem. I once proposed installing winglets--similar to Doelfins--on the top of the rudder, with a positive dihedral angle, just below the point where they would have touched the hull with the rudder at its stops, on a boat we raced on in the mid-80's, to prevent aeration but the owner dismissed the idea. "Just watch for bubbles in our wake" he instructed (to warn us of when we were about to loose it and needed to ease the pressure on the yacht). I think it would have worked but...
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Old 02-25-2013
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Re: End Plate Effect

'Induced drag' is a good search term for learning about tip spillage and end plates.

Here's a false-color model of induced drag:



And this photo gives you an idea of the energy dissipation (in the form of drag) entailed by tip vortices:



Some planes have added winglets to try to limit air from leaking around the foil's tip. They work, but the effect is most beneficial in planes with stall-prone wingtips, or in commercial applications where small inefficiencies can add up to millions in fuel costs. In the low-speed, high-wetted-surface world of recreational displacement sailboats, induced drag off sails & foils mostly vanishes in the noise.
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Old 02-25-2013
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Re: End Plate Effect

When I read the title to this thread, I thought it would be a decussion about the effects of a loose footed main vs. the foot attached to the boom.
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Old 02-25-2013
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Re: End Plate Effect

Quote:
Originally Posted by Jeff_H View Post
End plate effect is about making the outer end of a foil (wing) more efficient. End plates are typically associated with keels. End plates offer theoretical advantages in two ways. First is the minimization of loss of pressure difference between one side of the keel at the keel tip and the other is in reducing tip vortex and therefore reducing drag.

In a heeled boat, the pressure differential at the bottom of the keel is reduced by water slipping off the tip (bottom) of the keel. This reduced pressure makes a proportion of the lower keel area less effective. The end plate caps the bottom of the keel directing water aft so by adding a plate at the end of the keel the pressure cannot 'leak' as much.

But also, as a boat makes leeway water slips off of the high-pressure side of the keel to the low-pressure side of the keel and creates a turbulent swirl know as a tip vortex. This mass of swirling water is towed behind the keel creating drag. The longer the keel, the bigger the vortex, the greater the drag.

The end plate reduces both the pressure drop and reduces the size of the vortex. These are also related to the reasons that a deep-high aspect keel is more efficient than a shallower fuller length keel. It is rare to see a boat with a true end plate, but it is very common to see newer boats with bulbs, Scheel keels and so-called wing keels, all of which do two things, lower the center of gravity of the ballast in the keel, and provide an endplate for the keel. At typically designed, none of these act as wings.

I hope this is helpful,
Jeff
Great technical explaination ( for an ametuer like me), I always wondered the reasoning for them also. I knew we kept you around for a reason. The vortecies part I wasnt quite sure of how they affect the boat, now I do understand. Hylytes addition helped also. I do remember when the Aussie dropped that secret winged keel; on the AC races and how it improved their performance so much. Was a credit to Conners sailing skill to remain competiitive in that racing series with the newer technology advantage of the new keels they had.

Thanks for your post Jeff/ Hylyte...I learned something useful today
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Last edited by chef2sail; 02-25-2013 at 12:05 PM.
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Old 02-26-2013
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Re: End Plate Effect

Thanks for the great explanations. When I posted, I was thinking of end plate effect as pertaining to a keel, but lancelot brings up a good point about a loose footed main: Is the radius of the boom on an attached foot main enough to limit or eliminate the end plate effect?
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Old 02-26-2013
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Re: End Plate Effect

Quote:
Originally Posted by obelisk View Post
Thanks for the great explanations. When I posted, I was thinking of end plate effect as pertaining to a keel, but lancelot brings up a good point about a loose footed main: Is the radius of the boom on an attached foot main enough to limit or eliminate the end plate effect?
Not really but at that point there is very little impact on the performance of the overall sail. To give yourself a sense of what's going on with the sail, when close hauled take a long thin woolen yarn to the clue end of the foot and let it stream aft. To the extent you're getting vorticies, it will "pin-wheel"
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