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Compass Marine | all galleries >> Compass Marine "How To" Articles >> Changing A Raw Water Pump Impeller > The Old Impeller
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The Old Impeller
23-JUL-2008

The Old Impeller

If you been reading, and paying attention, you'll know that I replaced this impeller only a couple of months ago in April and now, as I write this, it's only late July.


So why is this impeller already cracked and ruined? That's an easy answer. We were motoring back in zero wind on a spring tide day. Spring tides have a nice feature that involves reaching far up onto the shoreline and pulling every last bit of seaweed and deposited junk back off the beach and into the bay. About a mile from our mooring I noticed the exhaust note change like less water was being spit out. I kept a close watch on my water temp and it only went up about 8-10 degrees above normal before we got to the mooring. The next day I went to the boat, cleaned the strainer and reamed out the intake hose between the strainer and seacock. It was certainly restricting flow but it was not running totally dry, just less. Clearly it was enough to heat the impeller and damage it. As always, in a situation like this, where it had limited flow, I make a habit of checking the impeller.


I was very surprised that only a few minutes of restricted flow caused this type of damage. Normally I would have stopped the boat and cleaned the strainer but the temp was not even at 190, it normally runs at 180, and we had a hungry baby on board so I pushed on.


The bottom line is this. If you run your impeller dry, or with restricted flow, at least check on it. If it's in good condition all it will cost you is an $.88 cent gasket. If it's bad, as mine was, replace it. I would guess I probably had another 5-10 hours before this one started throwing chunks!


Globe impellers are reportedly able to run dry for short periods of time however I have had two Globe impellers spin the hub and stop pumping completely. While I think the concept is great I'm a little gun shy, having had two fail me. These Globe impellers may be another option but do know of my failures and that they are almost triple the cost of the stock Johnson impeller.

Nikon D200
1/60s f/4.0 at 24.0mm iso100 full exif

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Dan 02-Apr-2012 06:24
I have had 2 globe impellers completely immolate during routine usage. Not even being run dry and they ate themselves completely. I have gone back to the regular neoprene rubber ones and have had no further issues. DO NOT USE THE BLUE GLOBE impellers.