Wing keels

[Dallas]

Hi

I hear so much being talked about keel shapes, length, draft etc. Most of which I understand but can anyone explain what is the advantage (or disadvantage) of a bulbed wing keel?

Cheers


Dallas

 

[jamesjermain]

This is a very complex subject because bulbs and wings are designed for, and serve a range of different pusposes.

When Australia 2 shook the world by beating the Yanks at their own game many people thought it was entirely due to the extra speed gained by their wing keel and suddenly wings were all the rage. It got so the wings were longer than the keel was deep.

In fact it is more likely that the wings were simply a rule cheating device which allowed the boat to have more of other things which are what actually made her go fast. It has been argued that the wings actually slowed her, or that they only served to change her motion allowing the hull to perform more efficiently in the seas off Newport.

In the wake of her win all sorts of keels were tried and a bewildering number of trials were conducted all with conflicting results.

What seems to have shaken out of it all is that wings can be very good for dampening pitch and heave thus giving a boat the motion of one heavier and bigger. In heavy or difficult seas a wing is particularly effective at stopping hobby horsing and helps the boat drive through the water. It is not so good in light airs when its greater wetted surface casues extra drag.

It is also an efficient way of reducing daught while retaining a good ballast ratio and a low centre of gravity because so much of the weight is in the wings which are low down.

One of the great claims of the wing was that it made the keel more efficient by creating an end plate thus increasing the pressure difference between the two sides of the keel by reducing the migration of water round the tip. This made the keel better able to resist leeway.

Modern thinking is that this effect, plus keeping weight low and ballast ratio high can be achieved equally well, if not better by a flattened bulb on the foot of the keel which trails off into a 'tail' . This has lower wetted surface and creates less drag than a wing and therefore performs better in a wider range of wind and boat speeds.

This is a pretty cut down and gap-filled version but I hope it gives you some idea. I am sure our technically expert contributors will give you a more scientific reply

JJ

 

[davidhand]

You can get the weight down lower, if you look at the current AC boats they have incredibly slender keels with a large bulb at the end. Beneteau (I know thats a dirty word to many on this board) had at one time cast iron keels with lead wings, I guess with wings you get the weight low plus some aerodynamic effect.

 

[PeteMcK]

Nope. That just about says it all. The end plate effect is significant. I've recently conducted wind-tunnel tests on a half scale model of a low aspect ratio keel fitted with and without a range of winglets and small, simple end plates. It appears that lift-to-drag gains of the order of 10% can be expected (and that's with simple plates). Upwind, the keel generates about 35% of total drag so at first glance this might appear to be a worthwhile mod. It would be, if we could ensure that the boat always sails at a constant, and pretty exact, angle of fore/aft trim. Pitching can add considerably to the wing/plate drag too: on the other hand, in certain conditions, and with well designed wings, the pitching motion can add substantially to the drive. Off the wind, you're almost certainly quicker with an ordinary fin (ballast and damping considerations apart).

 

[jamesjermain]

Pete,

Am I right in thinking that end-plates only work within very narrow speed bands as well?

JJ

 

[Twister_Ken]

One disadvantage

They're harder to unstick after an accidental encounter with the sea bed. A 'plain' keeled boat can usually be heeled to get off her off. A winged keeled boat doesn't respond to the same treatment.

 

[Dallas]

Thanks for all the replies. Certainly gives me something to think about.

The boat I have seen has a bulb keel with the bulb extending (and tapering) some 18 to 24 inches aft of the back of the vertical part of the keel. The wings stick out about 12 to 18 inches each (pointing down slightly) and are connected to the sides of the bulb but only run the length of the keel (not right to the end of the bulb)

So a keel as described above kind of gives the boat the best of both worlds but doesn’t excel in any.

Or have I completely missed the point?


Dallas

 

[Chris_Robb]

Re: One disadvantage

And may get stuck in an unstable upright position - and then fall.........

 

[Mr Cassandra]

Re: One advantage

One advatage is that you are closer to the beach when you do go aground !!!!!
cheers bob t

Bob T

 

[PeteMcK]

In the simple steady state of a wind tunnel test, no. At all speeds of interest to us, the end-plate reduces, quite markedly, the radius of the tip vortex (which is caused, as you say, by spillage from the high pressure to low pressure side of the fin) and this reduction is associated with very promising efficiency gains. The real situation complicates things rather a lot, and differences in speed, for example, are inevitably associated with small, but significant, differences in boat trim, and so on. You can easily end up having negative gains, so to speak. I'm not entirely convinced that it's been done to death though: the published research in this area is surprisingly sparse by comparison with what's out there on sails and hulls.

 

[Peppermint]

Re: Boats important to!

I've sailed a couple of production boats with wing keels and I've found them a bit duff.

I suspect that the benefit of wings is not achieved through a shallow draft keel on a wide beamed boat, yet this is the avenue often explored by manufacturers. The problem of most shallow draft production designs is compounded by shallow rudders.

If your looking for shallow draft and adequate windward performance the "Scheel Keel" seems a good compromise Or some of the twin daggerboard designs.

 

[Dallas]

>>> a bit duff. <<<

Obviously a technical term

Dallas

 

[Peppermint]

Unfit for there purpose then. Or C**p.