Re: Some thoughts about Twin Keels

DELFTship forum Hydrostatics and stability Some thoughts about Twin Keels Re: Some thoughts about Twin Keels

John R. Coil

Then there’s something I just read elsewhere where a forumite was relating his problems with a twin keeled yacht he brought. Seems his 11.5m boat has brakes on after about 5kt and he thinks it may be because of “turbulence” between his 2m long keels that are only 0.8m apart.

In the past I’d read about how having a catamaran’s hulls far enough apart was necessary to avoid increased drag from their combined wave systems … the ratio of the distance between hull centerlines and LWL being recommended at something like 2.2 (i.e. a 22′ LWL cat should have around 10′ between keels). Now, waves systems at water’s surface are one thing but there’s still gonna be pressure built up between twin keels in motion, right?

Do you think that, not entirely unlike a cat with hulls too close together, there may be a distance that’s too close for twin keels so that the water, past a certain point, just has trouble moving between the keels because of built up pressure?

If so this may be WHY having a bit of toe in is good, since that would help to relieve pressure farther aft. As for the issue of closeness: 2m / 2.2 = 0.9m … which is more than the 0.8m given … maybe the same desirable ratio applies in this case too?