You can test for hub slip by scribing line that mate up on the prop body and the prop nut. Re-create the problem, then stop the engine, trim up the drive and see if the marks are still alaigned. If they are, replace the coupler, if they are not re-hub the prop.
Couples fail for two reasons:
1. Improper alignment. The engine and drive need to be perfectly aligned to avoid stress on the rubber part of the coupler. Misaligment makes the rubber overheat and fail,. You can immediately diagnose this problem because of the smell. Alignment needs to be checked once a year.
2. Inadequate lubrication on the coupler splines. The stern drive input shaft is made of steel and has splines. These splines mate with the aluminum splines in the coupler. The two surfaces are never supposed to touch each other and rely on a thin film of high pressure grease to prevent metal to metal contact. When the grease wears out or gets squeezed out, the steel shaft splines hammer the aluminun coupler splines until they fail. If memory serves, the coupler needs to be greased every 50 hours under normal operation, or every 25 hours if you often operate at low speed. You can't over-grease a coupler.
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