Hydro, just spitballing here and as you know boats aren't really my thing but flight trimming an airplane certainly is. I can't help but think that the fluid dynamics between a boat and airplane are very similar. I recall you stating that CG is crucial to boat performance so my theory may be at least partially correct. As with anything powered by a reciprocating engine there are forces created by the rotating mass. Obviously on an airplane you have the crankshaft, prop and spinner. Props themselves produce an uneven thrust vector. These are just a couple things that need to be considered when trimming a pattern model. From my limited experience with boats I don't see why the same things would apply however the values would be different. The way we compensate for these phenomena is the use of offset thrust angles and sometimes offset lateral balance. The lateral balance offset I would guess would be the biggest benefit due to the gyroscopic forces imposed by the heavy flywheel. Or I could just be all wet ( ha ha pun ) and this has already been thought of ( most likely ) and I will learn a little bit more about boats when I read your reply.