When I installed an 80 amp alternator and smart regulator
in place of a 50 amp regulated alternator on my 20 HP diesel
belt slipped and the new alternator got too hot to touch.
I fixed that by having a new double pulley made for the alternator. Importantly I made it 10 mm larger in diameter than the original. That both decreases the load (like a bike with a larger rear cog) and increases the wrap (circumference) of the belt in contact with the pulley. It made no noticible difference to the alternator output and completely fixed the problem. Of course the crankshaft pulley had to be modified; grafting another one beside the original.
The machining was all done by an engineer
and cost around $300 for his work. I removed the crankshaft pulley and reinstalled it so he didn't need to visit the yacht. If you haven't got a puller for the crankshaft pulley it is probably cheaper to buy one than have an engineer
visit your boat. You simply tension the puller and wack the center of it with a hammer and the pulley pops off.
The original alternator pulley was 2 pressed steel
segments held together by the shaft and nut. The new double one was made from solid steel
and fitted the belt better.
(Sometimes competition vehicles have a larger alternator and water
pump pulley to reduce power loss)