I have a 1978 fiberglass 17 ft cc wellcraft, bought a used 70 Johnson 2 stroke for it and it only is getting to 3600 rpms and 17 mph. Tested everything on the motor and everything runs good with it. Talked to a couple people and came to the conclusion that the boat is too heavy for that motor. It is an old boat had may be slightly waterlogged. The prop on it right now is. 13 3/4 x 15. if I decrease the pitch on the prop to get more rpms would t really increase it or does it matter because it's underpowered anyway. What would be a good size to try?