"Martin Baxter" wrote in message
...
Bob Crantz wrote:



The Flettner "trim tab" improves stall characteristics of the rudder.

That may be true, but if you have to push the rudder to one side to
"balance" the boat, (doesn't drag increase with angle of attack?), then
you're going to increase drag.

It's the L/D ratio. If L increases faster than D the rudder is more
effective.



Better to trim the boat properly with
the sails and not have to shove bucket loads of water around.

This is only true if the boat sails in a straight line down its centerline.



Cheers
Marty
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That may be true, but if you have to push the rudder to one side to
"balance" the boat, (doesn't drag increase with angle of attack?), then
you're going to increase drag.


Bob Crantz wrote:
It's the L/D ratio. If L increases faster than D the rudder is more
effective.


Not so. If the L/D ratio increases, then it is possible to
have less rudder drag with the same turning force (or to
generate more force with the same drag) but it not the same
thing as reducing drag overall, which can be done wuite
easily by minimizing the turning force needed.

Some designs have the underwater foils set up so that the
lift of the rudder is a significant factor going to
windward... Hobie 16s are a good example of this. Then, the
L/D ratio should be optimized because the lift is getting
you somewhere. In all other cases, the "lift" from the
rudder isn't helping anything other than to prevent the boat
from going where you don't want it to.

Fresh Breezes- Doug King