Paul,
There is a basic design fault with the keel winch, the drum seizes on the axle, this causes the axle to turn and this chews up the casing allowing the drum to move. As a result of this the teeth on the drum no longer mesh with the drive gear and it slips, or worse.
If you look at my post from 3 years ago (titled 275 keel winch modification) you will see that I modified my winch to stop this happening. Three seasons on this has been a total success.
There should be enough details in the post to enable you to make this modification yourself, if not I'm happy to advise you on how to do it.
Alternatively I have modified two other winches in the same way and they are sat in my garage doing nothing, I'd be happy to let you have on of those.
I hope this helps.
Phill