Are you sure? The steel is pretty hard, and I think the prop or the threads would give before you snapped the shaft. Whatever. If you're using one of those 4 way glow plug wrenches, they aren't long enough to tighten the nut securely. Get a box end wrench, which for a 10mm nut will be about 4 inches long. For a wood prop, you should see the washer sink into the wood visibly. For plastic props, you can't compress them too much.

Has anybody out there ever damaged a prop shaft by tightening a prop nut too tight? Just curious.