Impeller Schedules

I've seen some worn impellers in my time but nothing as bad as the one above. The muffler looks like a muskrat or otter was making a nest.

Are there impellers made of some kind of metal? If so how often should they be changed?

Only in circulation pumps and never. The pump seal will fail first. The metal impellers are not changeable.

Impeller from a previous boat I bought with 8.1s. 12 years "professionally maintainted by a yacht maintenance company". The water pumps still had the factory paint on the bolts. Backflushed, and got the impeller vanes out. Eventually opened up and looked through the heat exchangers, to make sure I had all the pieces.

My Commander lives on a hoist, the impellers run dry every startup, filling the glass strainers before getting water. At two years, the impellers looked like new, I might go three years next time.

Something else to think about. I had an impeller go bad in the middle of the lake on a cold day with friends on board I didn't notice temp gauge both motors died opened engine hatch and the rubber collars on one motor were on fire right by 100 gallon gas tank 4 fire extinguishers still on fire finally several buckets of water out!! Called may days and talked about jumping overboard tough decision to go ahead and fight the fire!!

Robert Bradley . . . similar experience this past summer on the lake . . . port engine was running hot, with little warning, and a the plastic sheathing on a shift cable melted from the heat because it was near an exhaust riser. Smoke, no fire.

I've always had the habit of checking the exhaust for water while warming up, but that's not enough. Lacking any records of pump service, the marina replaced the impellers on both engines the next day. Not certain about the boat's first 20 years but I know for certain that they had lasted throughout my 30 years. New ones won't last that long.

This is an important post. Your experience is cause for concern. Glad it worked out for you.

Yes it was a learning moment,I have lots of learning moments in 34 years and thanks to the club I'm still learning.cant get enough!!

I change mine every 2 years before the start of the season. If we were putting a ton of hours on the boat and boating year round, I'd probably change at 300 hours and assess condition to determine if I should go longer. At the 2 year mark (and after 2 winters soaking in antifreeze) the impellers are usually intact, not cracked or worn, but starting to get a bit stiff (which can cause issues with failing to prime if they suck any air). I figure if they're always in good condition when I change them, my schedule is appropriate and I should have a very low chance of needing to do an un-planned change (which is good, as the Merc pumps would be a pain to remove and open up on a hot engine).

Rubber and fiberglass exhaust parts will melt down fast with a loss of water flow, especially if the engine is under any significant load. If there's an easy way to add it on your setup, a water flow or exhaust temperature alarm is a good idea. My generator has an exhaust temp sensor in the safety shutdown circuit, but I have not added exhaust temp alarms to the engines. However, my exhausts have 2 stage water lift mufflers, so in terms of noise, it's extremely obvious if water flow becomes significantly restricted or disappears.

Personally, the only sudden impeller failures I've seen (on any engine or generator) have been on startup. While running I've seen loss of water flow happen due to sucking junk against the intake or other issues, but I haven't seen an impeller just come apart while it's pumping water.

A 30 year replacement regimen vs a 2 year routine is quite a range. I suspect that an annual engine hour average could be a factor. My 427's only run 30-50 or so hours a season.

Hours of use definitely are a factor, although with us boating seasonally and storing for ~5 months every winter, I don't factor engine hours into my change interval. Different water pumps on different engines are also a factor, as some pumps are harder on impellers than others. And not all pumps use impellers made from identical material either.

I also don't really care to know how long my impellers could possibly last, as I don't want to ever see one fail due to age. An earlier change avoids unplanned changes, trouble with not priming after spring launch, and also avoids having to flush broken off impeller pieces out of the cooling system (although that's minor in my case, as they'd only make it to the trans cooler 6 inches from the seawater pump).

You'll get more engine hours from an impeller on a boat used year round, as the impeller doesn't soak in antifreeze over the winters (which can harden them a bit) and also doesn't spend months sitting in the same position without moving.

So would you recommend replacement in spring before first outing when dewinterzing?

Yes, I'm waiting 'till spring to replace my impellers.

In Michigan, where boats are stored on the hard for 6 months of the year, it is a good idea to pull the impellers out in the fall and store them in a plastic bag. It keeps them from taking a set and forces you to inspect them.