Is the belt a V-belt or is it a ribbed serpentine belt. I had an issue with my Sanger which had a Mercruiser motor. It constantly burned up belts no matter how tight it was. We never really got to the bottom of the issue, but we changed the Alternator pulley to reduce low rpm load and we eventually changed the tensioner pulley mount. The belt would still shed material and squeal lightly on occasion, but the last belt lasted better than 200 hours. The first belt was thrown at around 20 hours.
The belt is going to slip on the crank no matter what. If you were turning the crank that meens you are turning over the motor with very little force and that meens you have bigger problems. I would try two things. Belt conditioner, or replace the belt.