Jan, Bob, thanks for your input. Re spade rudders, come to think of it, on our planes, flutter of surface controls was a problem at high speeds to the point of the controls actually ripping off. The problem was too much air gap between wing and aileron, elevator... the fix was to seal the gap. This is not possible with a boat rudder standing free in the water. The dynamics of water is much stronger and I could see now how that would present a problem on a control surface surrounded by water.

80 mph, thats moving for any size boat. None of mine even come close, but I do it for fun "relaxation" and speed is not a factor. (Ya, sure, and if you believe that, I have a bridge to sell)