There are lots of different schools of thought on impeller replacement. Some people run them for years before replacing, and some even run them until they fail. If you use a boat year-round the impellers can indeed last a long time, but when the boat sits for 6-8 months, the impeller more permanently takes on the shape of it's last position, and begins to stiffen up.
The picture below shows the generator and main engine impellers side by side. You can clearly see how the fins are bent over to varying degrees based on it's position in the eccentric pump chamber. These impellers would probably be fine for another season, but replacing them proactively is an easy way to avoid an unpleasant breakdown at sea.
|Generator (small) and main engine (large) impellers|
If an impeller fails, it can be a real pain in the butt. Now you need to make a repair while at sea, in a hot engine room, while the boat rolls and wallows and everyone pukes. But to make matters worse, when they fail they typically send scraps of impeller bits down the plumbing path where they get trapped and block flow, typically in a downstream cooler. So unless the impeller comes out 100% intact, you need to start taking the plumbing apart to fish out all the shrapnel.
And even if you change them at the dock in the comfort of a cool boat, it can still be a bear of a job. Take another look at the picture above. The smaller impeller from the generator is not too difficult to replace, but it's still a tedious process to get the impeller in the pump housing and on the shaft keyway, get the sealing o-ring to stay in place, and get it all bolted back together with out the o-ring or something else slipping out of place.
Now try doing it with that giant impeller off the main engine. And try it on the engine where the pump is on the outboard side of the engine accessible only via a narrow space. After struggling with this for three years now, I think I've finally refined a technique that works pretty well, or at least as good as it will get.
The first lesson is to remove the whole pump so the impeller can be changed out on a bench. This is particularly important on the Cummins QSC engines since the electronic control assembly is strategically located to interfere with the impeller as it's drawn out of the pump body. It's a design by someone who has clearly never changed the impeller on one of these engines. I think design engineers should be required to personally perform every repair and maintenance procedure on whatever they build so they can see what they have wrought on the rest of us.
Anyway, it's a pain to remove the pumps, but easier in the long run. Once on the bench, the impeller needs to be extracted from the pump body. With the little pump on the generator you can pull the impeller out by hand or by gripping with a pair of pliers. But you will never get this monster out that way. Instead, it has a threaded end which is visible in the upper photo. That's used in conjunction with a jack bolt to pull the impeller out, and beleave me, you need it. That's either a 7/8" or 1" bolt (I don't recall which exactly), and it needs to be run in all the way before the impeller is free enough to pull out by hand. The below picture shows the impeller with the jack bolt screwed into the top.
|Impeller with jack bolt|
Getting the impeller out is the easy part. Getting the new one back in is a whole other story. With the little one, a little twist and squeeze is all it takes. The fins are small and pliable enough to not put up too much resistance. But you can forget about that with the big one. Even if you manage to get the tips of all the fins into the pump body, the moment you let down your guard the whole thing will spring right back out. It's not until the impeller is a good half way inserted that it will stay in when you let go. In past years I used a giant zip tie to compress the fins and help get the impeller inserted. This helped, but wasn't great. This past winter I did a car engine rebuild for the first time in over 30 years and that reminded me of the piston ring compressor I had recently used. It seemed like a perfect way to hold all the impeller fins in place and compressed enough to slide it into the pump body and onto the drive shaft. This compressor is nothing more than a long steel band about 4" wide that is wrapped in a circle like a corset, and it has a ratchet to pull it tight around the piston and compress the rings which otherwise want to spring out just like the impeller fins.
So off I went to get a ring compressor and 30 minutes later and $15 poorer I had the solution to getting these giant impellers compressed and installed. It worked great, and I highly recommend the technique to others.
With the fuel filters changed and the impellers replaced, it was time for the water. The yard finished up the bottom paint, and at 8:00AM Friday, in she went. Once in the water, Laurie started organizing the boat for use, and I went through the start up and check out of all the remaining systems. First was to ensure the sea cocks were open for the main engines, then start them one at a time, ensuring there was raw water flow, oil pressure, and checking for leaks especially around the water pumps which were removed for impeller replacement, test the gears, etc. Then I did the same for the generator, and once running I proceeded to test out the air conditioning, electronics, etc.
Last step, after verifying that everything was working, was to update the software in the electronics and to update the electronic charts. The only update for the electronics was for the Maretron displays. They do a good job with fixing bugs and enhancing functionality, so there are updates a few times a year. I wish there were an update for the Furuno MFDs, but still nothing. So all I had to do was update the electronic charts.
Later this week we'll do a proper sea trial in conjunction with moving the boat over to our mooring, then early next week we are off to Mystic CT to pollute the gene pool at the Nordhavn Rendezvous.