Impeller change 4 a Westerbeke 21
The easiest way to change the impeller would be to pull the engine, like it is with my W27. Short of pulling the engine you could remove the pump but that would mean realigning the drive tang of the impeller shaft so you''ll just have to do it like the rest of us, learn to work backwards. I would strongly suggest that you buy a second shaft and have a your spare impeller pre-mounted to exchange it in a flash if required without the need to remove the snap ring to get at the impeller and don''t forget to replace the paper gasket on the cover.
Depending on the age of the pump you might consider rebuilding it which would require that the pump be removed from the engine anyway. A rebuild consists of two plain bearings and two lip seals. Roughly $15 in bits, a new pump costs approximately $300, thanks Westerbeke!