Originally Posted by erps
Maybe it's just so fouled with marine growth that it's a big clump of mussels/barnacles? If that's the case, have a scraper ready for your diver buddy to correct it on the spot.
In light of the fact that the boat hasn't been sailed all summer, I would bet that the prop is covered in marine growth. When the shaft is turning, you are essentially spinning a barnacle-covered club which can't get a 'bite' to give you any propulsion. Good news is that you won't have to pull the prop or replace it.
If your friend is diving the boat and doing more than observing, he should have gloves and bring along tools to clean the prop and shaft. I use a wire brush and a wide paint scraper, but others may suggest differently. Personally, I would hire a professional to do the whole bottom (your marina or club can recommend someone) -- especially if the boat is staying in the water for the winter.