Confirm that prop and zinc are secure. Many have
lost stock
Volvo props. A loose or out of balance prop could cause this issue. You could swap on a known good prop to confirm. This is the best case scenario.
Next best case: Could be a prop shaft/bearing issue. To check, remove prop and see if prop shaft has any play to it using hand pressure (it should not). This test can be done in the
water.
Other possibilities are not pretty. Any internal sail drive
repairs require specialized tools, skills,
parts, and $$$.
Check the sail drive leg itself to make sure there is not excessive play...it should move just a little with firm hand pressure.
Could be a gearbox/coupling issues. That's better than an internal sail drive issue. Open up the
engine room and give it a test run to see if you can hear or feel an obvious problem area.
Safety note: if you are doing this in a marina then disconnect (not just turn off) all
shore power and turn off
inverter if equipped...you will be in the
water grabbing big metal
parts of the
boat...bad way to discover an
electrical issue.