Ive just had the same problem with a Volvo D2-75, a 73 hp engine with sail-drive.
After too long diagnosing the problem we found it was the sacrificial rubber bush in the propellor hub.
However, the issue differs depending on the type of prop you have. Some props don't have this sacrificial bush. In those cases there is a sacrificial component in the clutch
So it is likely your diagnosis is that it is either of these sacrificial components: in the propellor hub or in the clutch.
The first is easy to check, Take the prop off (if it is a Flexofold prop, the hub will come off with the prop) and if you are careful you can do it in the water
, without lifting the boat.