I have been following the OBERDORFER pump discussion (s) and have decided to go with one as compared to replacing our impeller again on our Sherwood. We have 400 engine hours currently and will wait to replace the pump when it starts to go, I know no one has a crystal ball. The question is about how many engine hours does the Sherwood last or when I replace my impeller next should I just do it all? We are in freshwater and have the M25XPB engine. About how long does the pump change over take and I can do that in the water as long as I close the sea cock? Much Thanks.