Confirm that prop and zinc are secure. Many have lost
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.
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