Not trying to be a pill here but I think the shaft loading is something that could be of interest.

When the engine is stopped the belts will pull upward on the sheave and place a bending force (called a "moment") on the shaft. This places the top of the shaft under compressive (pusing) stress and the bottom under tensil (pulling) stress. As the shaft rotates a point on the shaft goes from top to bottom so it goes through a tension, to zero, then compression, to zero and back to tension with each revolution. It would be the same as grabbing the end of the shaft and forcing it up and down at 2000 times / minute (at 2000 rpm of course). This cycling is what causes the fatigue. Here is photo of the results of a fatigue failure. A ductile (bendable) steel shaft will appear to have snapped the same as if were cast iron, glass or some other non-ductile material. Grandpa would say the steel "crystalized" and broke. Make the force large enough and the shaft will break, it's just a matter of time.

Sorry, couldn't help myself. The weather is lousy and the boat is 1 hour away....