Our 20” Max Prop. (3 blade VP) fell off today, when put into reverse very gently to stop the boat at about 1 knot approaching a mooring. Amazingly, I found it on the very murky bottom, apparently intact.

Blades feather as normal, easily

I dismantled the prop, and can see no damage inside. The nut had of course unscrewed from the shaft, letting the prop fall off. The two nut-retaining pins (#5 in the PYI installation drawing) seem undamaged

The shaft feels as if there is no keyway in it, but the water is too murky to inspect it properly. It seems that the key sheared and let the prop turn on the shaft, unscrewing itself. However, we did not find any key debris inside the prop hub. Of course it could have fallen out. There is no visibility below the boat, and the bottom is VERY soft mud (the prop had sunk 90% into it. I was very lucky to find it by feel) No point is looking for debris from the key.

We have run over 900 engine hours since last dismantling the prop in Tromso, and have sailed from Northern Norway to our present location in St Lucia, with various detours and two winter haul-outs on the way.

Any advice on how to avoid a repetition? (we found the tiny SS locking pins on the nut intact)