If you have the Onan genset (
https://www.cummins.com/generators/onan ... -generator), I can provide some insight. I've done this several times. My last boat had the same exact genset.
Remove the front panel and upper panel (by tilting the cockpit bench seat forward). You can probably get it (the water pump) out with out removing the side panel. If you find you need more room, instead of trying to remove the entire side panel (which has several complications, just remove the upper and front side bolts, to give you a little more wiggle room).
From the front panel side, remove the water pump entirely from the engine. You will have to loosen or remove bracket and hoses. You will find yourself climbing in and out of the engine compartment a few times. Replace impeller and reinstall. I use a zip tie to compress the impeller so that I can more easily slide it into the housing. Have your wrenches and sockets within arms reach. I lay an old towel on the main engine and spread my tools on top. I sit in a ball, sideways to the genset, while doing this. The hardest part is getting back up and out of that position each time. This is the worse of it.
As a side note, be sure to check the main engine seacock when you are all done. I found that my body was somehow pushing that over each time I exited this tight space.
For then next few days it will be even more difficult to stand up straight in the morning and you will have a few random black and blue marks. That being said, I consider this a DIY job.
Let me know if have any questions.
Best,
Bobby
