Alternator bracket bolts broke

We are out the other night and the belt starts to squeal a little. So we head in and I looked at it today thinking I just need to adjust the tension or even get a new belt.(i have no idea how many hours are on it) I open the boat up and find that both lower alternator bracket bolts are missing. I took the alternator out and found both of them had snapped off of course in the block. I only found one on the bottom of the boat. Which is very clean. I managed to extract booth broken off pieces and they were not the same bolt. One was a grade 8 and in newer condition. The other some cheap zinc piece. I am thinking that some one along they way broke one off and put a grade 8 in the good place to help make up for only having one bolt. What do you guys think? And why would they break off like that?

belt too tight? Torqued too tight?

I wouldnt' worry about it, jsut replace them with the right stuff.

I had that happen on my car. I have a 72 charger and the torque from the engine rocking at take off snaped the bolt in the block. Unfortunatly I could not remove mine for my head. So i had to fab up a new mount for it to go into.

Make sure they are torqued to the proper foot lbs and the belt is not too tight.

I had it happen on my boat when I 1st bought it but then the dealer replaced the bolts and 375hrs later....no problems.