Bobs analogy with the weedwacker is incorrect. turning the weedwacker at high rpms is difficult, this is due to gyroscopic forces. If what Bob had said was true(and what you are saying is true with this boat), the weed wacker would try to twist out of your hand when it's throttle is gunned(you say it would twist perpendicular to the driveshaft - it will torque in your hand, but along the same axis as the driveshaft), It does not; it only tries to stay in the same orientation (like a gyroscope). An engine, unmounted, and sitting on the ground, will vibrate and 'walk' due to the vibration, but not due to the force of the piston being 'blown' one way and comming back the other