On Mar 11, 10:35 am, "Roger Long" wrote:
... There was a rash of rudder stock failures that had an outer layer of carbon
fiber for stiffness. Turned out the plain fiberglass part of the stock was
plenty strong enough. ...

Funny you should mention that. The guy working just up the bench from
me was building the rudders and stocks for the cat and he had built
carbon/glass stocks which failed exactly as you describe. He was
building the cat's stocks out of S-glass/epoxy around small foam cores
(for forming). He'd also built successful carbon stocks (including
the one on his own boat) but felt strongly that mixing materials in
rudder stocks was a bad idea. He also thought that S-glass stocks
were nearly as good as carbon because carbon had to be over-built to
absorb shock loads that were not a problem for S-glass of the required
stiffness. Carbon that's durable enough ends up being stiffer than
needed. There's still an advantage but not nearly as much as you
would guess just looking at the materials. Adding a bit of spring
into a system that takes sudden loads can reduce the peak forces by a
lot. As you say, the engineering gets complicated. Some materials
suppliers will provide engineering help to builders and designers for
surprisingly reasonable fees (even gratis for small stuff)...

-- Tom.