It's the prop
The real answer to this problem is a 4.5-5" pitcjed 3 bladed prop or a 3.5-4" pitched 4 bladed prop. You would then reach WOT, attaining full HP which would increase HP out put and speed (to a point on displacement boats). Thrust would go way up, and fuel economy only slighty down. Another thing to remember, 4 cycle engines don't like WOT for long periods of time, so you would never cruise at 5-5500 RPMS or you would burn up the engine. But then 2 cycle engines don't like being run at below 1/4 throttle or they foul their plugs after a while. My engine does not twist, I have the bolt that holds the collar where the engine turns tight and I do not lube this point ever. If yours is twisting, clean out the grease here with some brake cleaner and it won't turn on you anymore, nor will it steer by the tiller well unless your a gorilla.