See if you can find a 2:1 crank pulley. Most car pulleys are a 3:1 ratio with the alternator. For a car that's fine since a car motor typically spins at between 2000 and 3000 RPMs. This will spin the alternator around 6000 to 9000 RPMs. However, at boat RPMs around 4000 to 5000 RPMs, that would spin the alternator around 12,000 - 15,000 RPMs. This will eventually kill the alternator, or you may or may not have a problem with throwing belts.
With a 2:1 ratio this will put the alternator at around 8,000 - 10,000 RPMs given a 4,000 - 5,000 engine RPM. This puts it closer to what it would spin in a car.
Some people even get a direct drive kit. These place the alternator right in front of the crank and couples it directly to the crank, giving it a 1:1 ratio so that it spins at exactly the same speed as the engine. Plus, no belts to deal with. With a 1:1 ratio your alternator will spin slower than it does in a car, which will greatly reduce the wear on the alternator.
