Originally Posted by sailingdeacon
I agree with EO32: There has to be a paddle wheel effect if the pitch is too great. for sure if there was no pitch the prop would be like a paddle wheel with definite yaw. See the photos at the bottom here, which might indicate a paddle wheel effect more than other props.
Right now I think the boat/rudder is a subtle problem but made considerable by the two blade efficient prop. I think there is a good chance the reduced pitch will help at least. My thinking is that a reduction in pitch of an inch will reduce the yaw or paddlewheel effect proportionately more.
Just to chuck something else into the mix:
I noticed exactly the same thing on a Catalina 28 we took out recently for a spin. What the owner told me lined up with Idiens posts - that it was caused by the wash from the top blade striking the rudder side-on and has absolutely nothing to do with rudder design.
Apparently, this effect is significantly reduced by installing a small skeg immediately aft of the propeller - exactly like the ones fitted to outboards for the same reason - to straighten the flow.
If this is indeed the case, reducing the pitch should reduce the effect to a tolerable level - but will not completely eliminate it.