<<The only person I know of who argues against the superior
hydrodynamics of under hull rudders is Frank Bethwaite>>
I won't really touch the efficiency debate because it can be a
religious argument, and I'm not expert enough to take a definitive
stance.
For my purposes, I'll accept that an in-hull rudder is more
efficient, and then not choose that design because the difference won't
be enough to matter. Plus the other obvious benefits of a rudder that
kicks up.
<<Can't see how a Seacart or Dragonfly daggerboard could kick up.
>>
It's a neat little trick. The line that holds the centerboard down
in its sailing position travels through a cam cleat to hold it in
place. That cam cleat is then mounted on what amounts to a large
hinge. The the pivot point of this hinge setup is forward of the
cleat. On the aft/open end of the hinge sits a spring-loaded ball
bearing that sits in a divot, like and old-school cabinet door stop, to
hold the hinge closed.
Normal sailing loads will never pop the hinge open, so the cleat
holds the downhaul. But since the line in the cleat is about 2-3 cm
above the top of the hinge, grounding out will put a huge force on the
cleat with enough of an upward component to pull the aft end of the
hinge out of the friction hold. Once the hinge pops open, the downhaul
automatically slips out of the cam cleat, which can no longer hold the
line due to the new angle.
It would be nice to use it on the proas, but it really only works in
one direction.
I must be mistaken about the seacart. I must be remembering another
boat.
- Mike
On 6/15/2010 7:37 AM, Rob Denney wrote:
Mike,
The only person I know of who argues against the superior hydrodynamics of under hull rudders is Frank Bethwaite, who found, when testing small scale rudders that the drag was way higher than it should be as the rudders were operating in the turbulent zone attached to the hull of the runabout they were testing from. Obviously this is less on a highly polished hull, but still exists. No idea whether it overcomes the benefits of the end plate effect which is often dubious with loosely fitting daggerboards. The other benefits of the off hull mounted rudder certainly do.
Can't see how a Seacart or Dragonfly daggerboard could kick up.
Look forward to some feedback on Arttu's set up.
rob
,_._,___