As a matter of preventative maintenance, I replace my water pump impellers on all my boats every 2 years. Why change them so often? My boats set for 4-5 months during winter layup. During this extended period without rotating the impeller, the blades that are deformed by the cam in the pump, take on a permanent set. The impeller pictured below has 4 of the 12 blades so deformed that the blades do not even make contact with the housing anymore. This will have a great impact on this pump to move water, that will be even more exaggerated under high loads and engine RPMs. Take an impeller that has been through layup for a couple of seasons, and the efficiency of the pump will have been greatly reduced. Over time, the blades may straighten to some degree, but the impeller will never have strength to pull water as it originally
The waterpump used on the Yanmar 370 is a Johnson F75E-9. The impeller is an 09-821B (or a Yanmar 119574-42550-01 if you prefer to by the Yanmar packaged part.). Getting to the pump on a single Yanmar 370, in this boat is a piece of cake. I always remove the cover in the fall to release any water that may freeze over the winter, so I was familiar with the location and access to the pump before I started. I thought by jogging the engine with the cover off, and pulling on the impeller, I might be able to coax it out. No way!! Rather that damage something, or myself, I started to look at what was involved in moving the pump to a work bench. The pump is removed very easily. Start out by removing the intake hose from the pump. Then using a 12 mm wrench, remove the bolt holding the outlet gooseneck in place. (It is an O-ring sealed fitting with only one screw through through the flange to hold it in place.) Then using a 12 mm wrench remove 2 bolts on the near side and 2 nuts on the far side of the pump mounting flange. Break the paint seal and the pump will slide right out.
After getting it into the shop, I clamped the splined shaft between two blocks of wood in a vise so I could work on the impeller. The impeller developes quite a set over the winter, especially in the area of the cam. No blades were broken or cracked, or shown excessive wear, but I feel the impeller, with the set blades just would not pump as well as a new one .
Being careful not to pry against the thin wall at the O-ring groove, I used a screwdriver to pry on in the low pressure side of the cam area, while taking bites in different spots on the impeller by rotating the housing. If you did damage the cam, you can replace it by removing the screw in the side of the housing. If you damage the housing, you are S.O.L.
After removing removing a cleaning up the housing a bit, I liberally coat the impeller and drive shaft with Quick Silver 2-4-C lube (but any marine grease is probably fine) and gently start the impeller back into the housing while rotating the housing counter clockwise, (the impeller runs clockwise when viewed from the cover end.) Below you can see the set and distortion after just 2 seasons. Grease up the O-ring seal and put the cover back on, and you all set to reinstall the pump.
Inspecting the inside of the cover, you can get a idea of how hard that pump will draw, by the cavitation adjacent to the low pressure side of the cam.