When I have my motor in neutral and rev it up to full throttle it will climb just fine, but when I shift into forward it won't climb all the way up even though it's at full throttle. It will only go at half speed.
Could it be a spun hub, or the friction screw? A couple of times it has reached full throttle in forward, but when I slow up and speed up it is only at half throttle.
A spun prop hub is like a slipping clutch. The engine will rev up, but the boat will not speed up, and may even lose speed as the motor speeds up. Is that what you are seeing?
