Re: 1985 Glastron SSV151 90Hp Mercury 6cyl
Prop pitch is how far the prop will advance in one rotation - think of it as screwing in a screw. Every rotation advances the screw by some increment - that is the pitch. So turning a 19" prop one time will advance the boat 19". Turning a 21" prop by that same one rotation will advance the boat 21". In doing so, that 21" will also be harder to turn (as it is doing more work), so it will put more load on the engine, and reduce the RPM's. The goal (when aiming for top speed) is to get the max pitch that will still allow the motor to turn it's max rated RPM's.
Now, with all that said, there is another factor called slip that is the inefficiency of the prop. Think of bringing the boat up against a brick wall (not literally - ouch!), and now spinning the prop. Even though you spin it one rotation, the boat doesn't travel forward because of the brick wall. Slip rate will depend on several factors like boat weight, motor height, and other conditions.
Hope that helps.
