Keel design questions

[olesprit]

Hi everyone,

I'd really like to hear of some opinions on the shapes of keel bulbs on displacement cruiser/racer yachts. Last year myself and crew faired our lead keel with WEST SYSTEM epoxies, looks fantastic and always comes out looking as good as it goes in, last winter we feathered the trailing edge to a fine point rather than a champfered edge, seems to have been a good modification as the boat steams upwind now.
For next trick i was thinking of reducing any end plate effect by maybe sharpeneing up the foot of the bulb, possibly to the extent of fairing the bulb out so that it has two winglets on either side, any thoughts?
Would be really interested to hear any thoughts on the above.
Oliver

[jehardiman]

For next trick i was thinking of reducing any end plate effect by maybe sharpeneing up the foot of the bulb, possibly to the extent of fairing the bulb out so that it has two winglets on either side, any thoughts?


That would not reduce the end plate effect, but increase it. The end plate effect is the apparent increase in aspect ratio (and therefore lift) of the foil caused by the "plate" limiting flow over the end of the foil. A well faired "bulb" provides a small to moderate effect, while a flat plate with a sharp intersection provides a large effect. The problem is that the "plate" causes interference and wetted surface drag , increasing with the sharper the intersection. As windward performance is a function of the lift/drag ratio, it is important that the proportional gain in lift excede the gain in drag. This is not a trivial problem and even the big boys (i.e. ACC teams) with lots of money and smart people have missed the design point.

Additionally, all the above is predicated upon smooth, steady flow conditions. In a seaway with roll, pitch, yaw, and wave orbitals; too sharp an end plate can actually reduce the lift of the foil due to secondary cross-flow effects.

Since you're happy with the foil now and don't seem to have a do-or-die, one shot, drop dead date; it would be best to not make any drastic, non-recoverable, alterations. Flatening the bulb verticaly would be the first thing I'd try, but not go beyond a 3wx2h ellipse. Keep the fillets large with about the same tangent to tangent included angle. I'd only think about a beaver tail or winglets if you think that in all your sailing you're going have enough stability and sailarea to carry the extra drag.

[olesprit]

Will do as you suggest, will simply add fairing compund to tip of keel bulb on either side. I don't think the extra wetted surface area will be much at all, my intention is to simply remove the radius on the keel tip so that the side is at 90 deg to the bottom. Thanks for advice though.

[Eric Sponberg]

Have a look at my website, www.sponbergyachtdesign.com, and go to "Repairs and Modifications". You'll see a keel repair project on a Cambria 44 called Magic, where we cut off the tip of the keel and replaced it with an equal amount of lead in a bulb shape. My bulb designs typically are oriented at the aft end of the keel tip, as on Magic, and I make the trailing edge of the bulb in a flat shape like a beaver's tail, and the aft edge scalloped shape as in a swallow's tail. Hence I call these bulbs beavertail-swallowtail bulbs, or B-S bulbs for short (pun intended). But they have proven very effective in many of my designs. The owner of Magic reported that the boat pointed a few degrees higher and sailed a tad faster--1/4 to 1/2 knot or so, because of the reduced drag of the keel.

Eric

[Sean Herron]

Hello...

See http://ventureyachts.com/scheelkeel44.htm ...

SH.

[Bremner]

Hello...

See http://ventureyachts.com/scheelkeel44.htm ...

SH.

I'am seeking advice to replace or fix my inner keel board on a 1968 pacemaker
do you recommend this site/