I changed spark plugs then took my boat out for a test drive it ran fine I anchored at the beach on the way back I tried opening it up and couldn't get above 3000 rpmsm
I have been all through the engine without finding the problem than I noticed the prop has fresh nicks on all 3 blades. Could this be the problem?
I would think a damaged prop would raise the rpms and lower your speed. As Bond-o indicated, you'd have to of added pitch through the damage -- not likely if you didn't feel such an impact.
I'm still learning, so my input is from my gut, not exerience.