Twin keel research

Quote:
Take a V-hulled, single chine sailboat with twin keels. Inside, the area between the keels is sloped and narrow, reducing its usefulness. Now, what if, just for the length of the keels, the hull shape between them were flat athwartships but with the same rocker longitudinally as the standard design? The usefulness of the interior would be enhanced by allowing a lower and wider cabin sole with more headroom and less awkward stowage. The hull would have less wetted surface and slightly more volume. The forward and aft ends of this section would have to be faired into the rest of the hull, making the keels a little longer at each end.
Would there be any predictable effect on the hydrodynamics or performance or handling of the vessel?
The way I am reading this (correct me if I am wrong) it that you want to eliminate the V section between the keel blades and create a flat spot in there. I also suspect that you are suggesting that the bottom of the keels is the bottom of the flat between them creating a box between the keels? If this is so...then you are creating a massive box keel with a tremendous amount of volume. You would need to bolt massive amounts of lead to it to get it to settle on its lines. It would be a pig to sail with all that weight and you would have created a very movement un-friendly shape. Great for sitting at a mooring or dock but crappy for sailing.

Apparently I have misled you, or you have misread me. The draft of the hull (exclusive of the keels) would be unchanged. The area between the keels (but not to their full draft) does become a shallow box, the bottom of which has the same line (rocker) as the bottom of the original V. By my calculations for the example in mind, the displacement is increased about 10%, so the ballast would have to be increased accordingly. Because of the more convenient shape, part or all of this extra ballast could be in the form of a significantly increased battery bank, something which doesn't fit as readily in a V-bottom.

 

I make the V a built in water tank. The top of the tank is what the floor rests on. It also adds a huge amount of strength to the keels attachment.
The keels are a foot below the bottom of the V, so eliminating the V would have no draft advantage.
Brent

 

An explanation of a potential problem with twin keels that I had not thought about before...basically, the author contends that they are more susceptible to being rolled over.

http://origamimagic.com/articles/twin_keels.htm

Once there, also click on "home" then "articles" for some more viewpoints not commonly seen.

 

Twin keels may have more of a tripping action, but this is offset by their being shallower and the tripping action being higher up, and thus having less leverage, as well as their reduction in rolling rythmically, before being hit with a wave. I think it is a non issue.
Brent

 

I gotta go with Brent on that one
( granted Im stuck inland for the time being )
but nothing in all the research Ive read nor the first hand experiences suggests a greater incidence of roll overs
tripping

the big advantage is they roll less in a swell and stand up when grounded
I think there is however a lot of validity to there pointing differently and being slightly slower
but
there is something to be said for the two keels not needing to be exactly equivalent to the area of the one large keel
and so the additional fluid resistance can be about the same

( that will cause a ruckus )

I think that article that discussed that is actually in this thread somewhere

best
B

 

Great thread, don't know how I missed it for so long.

Twin keels, like the wings of a biplane, are going to deliver lower lift/drag performance, partly due to lower aspect ratio and partly, I suspect, interaction between the flow around the keels.

The subject of toe-in was addressed earlier in the thread, primarily as a tool to reduce/eliminate leeway. This could have a benefit if the angles were chosen so the weather keel was at zero lift therefore zero drag and least flow disturbance while the lee keel did all the lifting. End plates could increase the effective aspect ratio also by reducing end vortices. This must have been tried somewhere.

I have experimented with a single Bruce foil in the past for a very small craft to allow it to carry more sail. Although not very eficient due to its low aspect ratio it was effective and cancelled heeling 100%, but the slow turn rate of the craft I tried it on, a kayak, caused trouble when changing tack, and leeway severely reduced velocity made good to windward.

The single Bruce foil relies on leeway to develop the counter-heeling moment. I currently have a project to try two such foils, with a higher aspect ratio, a proper profile and toe-in per 3rd para above, which hopefully will reduce both drag and leeway. It's been delayed by health problems, building a canoe, and now a small sailboat has got in the way but I plan to get back to it.

There's not much about the Bruce foil on the net that I've found so I'm interested in any comments you may have.