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.
Feetup: Yes it is trying to yaw... I have to hold the wheel under some force to keep the WHEEL from spinning to to port When the wheel is released under high power, the wheel turns on its own quickly to port and the boat immediately turns hard left..
Unfortunately I can't tell for absolute certain the straight ahead rudder position based on the wheel position. Remember that with a wheel it is hard to know exactly how centered the rudder is based on the wheel position.
HOWEVER, after much recent careful observation under power and coasting, if, under speed, I take it out of gear and hold the wheel in the same position, the boat very slightly go to port.. If I turn slightly to starboard the boat goes straight. (I think I have been misled due to thinking I knew where the wheel-rudder position was )
This amazes me. I think this is saying that the boat needs a slight starboard rudder position to go straight while NOT under power. THe hull/rudder is causing the boat to yaw left. Yet when under power with this prop, the rudder needs to be straight ahead, while the boat is trying to go to even port more because of the high pitch of the prop and higher speeds. Jeepers! Wild!
In other words it seems I have to turn slightly to starboard when going from power to coasting. And when going from coasting to power I have to turn slightly to port (apparently to get a straight ahead rudder).
If on the other hand, we were to assume that while not under power (coasting) the rudder is straight ahead, then under power as the boat yaws left, the rudder is turned to port in order to go straight. This makes no sense since the boat is trying to go to port already.
Here are photos of the prop.
(for a limited time).. The prop size appears larger due to some photo distortion. In other words it appears larger than 15" diameter, but it is not. The rudder is aft out of the photo.
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.