Re: destroyed impeller
If your Yanmar is like my 2GM20F, the exchanger doesn't have coils, but simple straight-through pipes arranged in a cylinder--looks like a Gatling gun barrel. You should be able to clear the front of them by removing the forward cap on the exchanger. Four bolts I think.
2000 Beneteau 331
Northern Chesapeake Bay
Ambition is a poor excuse for not having enough sense to be lazy ~ Steven Wright