Gt40 is goin through some alternator belts!!!
 

So my alternator belt broke off on my gt40 about a week ago and o replaced it. It was fine for a few days and we smelt a burning In the boat. It snapped again. What could be causeing this? The pulleysnseem to be aligned and the tension seemed pretty good

Classic problem with the GT-40. Some people get modified brackets to help get the tension correct. I replaced my alternator and it stopped eating belts. Sometimes the bearings can get out of whack apparently.

I had the same problem when I got my boat a couple of years ago. I would tighten the belt to what I thought was normal and it would start slipping, heat the belt up then break it. Finally I tightened it as tight as I could get it and it has been fine ever since.

It spins pretty freely by hand as well so the alternator should b fine right? Maybe more tensio

I would say slipping. We use commercial mowers that are belt driven with belts about 10mm thick. If you get a jam under the deck and smoke the belt, it won't last long at all before it breaks.