What you are experiencing is the same thing you would experience if you were a pilot and flying a "Taildragger". Something called "P" factor. During take-off and before the tail of the plane comes up the nose of the plane sits high and tail low which means the lower half of the propeller blade is the leading blade and is primarily moving in one direction. This torques the airplane around the vertical axis in one direction and you have to add rudder to counteract it. As the tail comes up the "P" factor subsides as both blades become equal pullers and torque is somewhat negated.
In a boat, with the engine trimmed down all the way the lower blade is the leading blade and will torque to one side. Trim it up and both blades begin to pull equally. Continue to trim up and the steering torque will start to go in the opposite direction as the upper blade swing is now the leading blade and is moving in the opposite direction the lower blade was. Make sense now?
