A dock mate of mine had a single cylinder Farryman diesel that would regularly break impeller vanes off. He had a mechanic come to look at it and the explanation was that one cyl. engines will rotate backwards a bit on shutdown causing the vanes of the pump to reverse, then reverse again on startup. The fix was to install a smaller cam in the pump; That worked OK since he was only in the cool water of Lake Ontario.

Not sure if this might apply in your case ?