V-belt keeps breaking

[Bilko1270]

I have a 1997 Maxum 1900SR with a 3.0LX engine. The engine runs well when it’s soaked in but I’m having issues with the V-belt. I have put in 3 belts in 10 outings in the boat. Belt starts grand with no slipping. Then after 60 mins of running it heats up, expands, I tighten, it’s grand for a while and then same thing happens. Once I get to max adjustment on alternator the belt then starts slipping, smoking and eventually fails. Any ideas what’s going on and what I need to do to fix it? Surely I shouldn’t be going through V-belts at this rate. Thanks in advance for your help.

I’m using the original v-belt for the engine and also tried other belts - they last slightly longer but eventually same thing happens.