Belt slippage, Volvo 4.3L

Hello I have a 2008 Reinell 185LS with a 4.3L Volvo Penta carbureted engine. Under certain conditions the sepentine belt is thrown off. It occurs at full throttle, when the boat is maxed out. I see that there is belt debris, fine belt shavings in the engine bay. I have replaced the belt and tensioner and the problem is reoccuring very often. Being a HD mechanic I am used to seeing this with misaligned pulleys on certain engines, however I can not see any misalignment in this case. Any input on this would be appreciated. The engine is a Volvo 4.3GL-G 3869358, 4012222180.

I have also checked the pulleys for play, alternator etc.., all the bearings feel good and spin freely without deflection.