Could be so many different things. Is this boat driven off the flywheel or the crank? (L or RH prop?) How is the steering rigged? Any play?.....Is it a trailing rudder? Does the boat have Cav plates?......Are they adjusted properly?......Any hooks in the bottom??.......What speed are you refering to?......Full or part throttle?.......All the time, or just when you get on it/let off?.......MP