I just recently bought a 79 hunter 30, first sailboat ever so I'm still learning. When I first got the boat the motor would give lots of black smoke and not go over 1200rpms. Since then I have had the prop and bottom cleaned, prop and shaft were pretty bad but bottom looked clean. Also I have done fuel filters, added biocide and a stablizer to the fuel. I have done oil, oil filter, air filter, cleaned raw water strainer, all now hoses, new mixing elbow, and new belts. It now runs good , no black smoke but won't go past 2000rmps. I'm getting 5.5 knots at full throttle at the 2000rmps, no smoke at all. Any help would be appreciated