Annually, in April we have our yacht pulled out to clean the bottom, apply antifouling and inspect all the below waterline components. The anodes that protect the propeller, shaft and P-bracket from galvanic action. Also inspect the cutlass bearing that supports the propeller shaft either at the P-bracket, or where it exits the hull, for wear. There shouldn’t be more than about 1mm of lateral or vertical movement of the shaft here.
If your yacht has a saildrive transmission unit, inspect the anode that sits just ahead of the propeller, as it needs regular inspection and replacement. Skipping this part can cause the entire outer casing of the unit to succumb to corrosion.
Saildrive units should have the oil inspected for evidence of water ingress while the boat is ashore, we renew the oil annually. If any water is present in the oil, a milky substance, it means the oil seals around the shaft that exits the drive underwater need to be replaced.