: Hi Mike,

: Have to remember that water density is much different than that of
: air, so the reynolds number changes considerably for foils in
: water. For air a fat foil works great at slow speeds, not the
: same in water.

: Bill H.

Hi Bill,
I suppose a supercavitating craft would work in air as well, the Blackbird is a bit like one. Though supercavitation does drop the medium density to that of air around the object.
Normally I don't think of the two, water and air, in connection with each other in relation to foils. I treat them as seperate sciences with a lot of commanalities. :)

Still, even with the greater density of water, the speeds we do a 25%-30% from leading edge for the thickest part of a foil would give the better results. That doesn't always give a 'fat' foil as the leading edge can be sharper than round, though the sharper the leading edge the more 'twitchy' the foil becomes.

I had a look at the foil on my plastic yak and even for a cheapie, it has a distinct foil shape. I wouldn't use it as an example for testing effectiveness or efficiency as it would only do as one for a poor example of a good idea, it's too flexible. I'll probably make a strip foil and replace it.

Mike Savage
South West Cork