You're right - it does push on the back of the boat as well - but the back of the boat is hooked up to the water (hopefully) by the hull and (especially) the skeg or rudder, so it doesn't move to any real extent. (Unless you're driving through one of those big, turbulent washes that tugboats create as they are trying to push a supertanker into the dock..... ) Then, EVERYTHING moves.......a lot.
And yes......lawn darts behave the same way. Physics is physics. If the wings start flapping, you need to go see your local composite repair shop. Fast.
BajaBob38's .38 Special, on the other hand......Well, it seems to wander all over the place at any speed, current, or wind condition.....
Madcow - sorry we hijacked your thread. So far the prop / drive alignment answers seem to make the most sense. Any signs of a bent or (worse) twisted skeg?