When to replace water pump impellers?

Trundlebug

Trundlebug

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How often should the rubber vane type impellers in the raw water pumps be renewed?
When I took out the impellers at the end of last season there were some slight marks on the end of the vanes but no real major wear or anything. How far gone should they be before replacing them?
I can't really believe they should be renewed every year regardless. There must be a more scientific rationale!
Engines are twin Volvo Penta MD31A.
 
Excellent question. A high proportion of yachties change their impellers annually. Others take them out in the winter and put them back in the season if they look OK.

One problem is that impellers can be hard to remove without stressing them so I would argue that unless the removal was completely stress-free it is better to fit a new one if you have removed it.

I don't see the rationale behind playing around with it...the manufacturer should quote a service life and the impeller should be replaced after that. My Yanmar 110hp manual gives a service life of 500 hours so I do mine at a convenient time before we get to that number of hours and I don't remove it or look at it in between.

I think that this is one of those subjects which generate strong feelings - if so, I don't mean to give offence to anyone. I don't really see a 'right' or 'wrong' with an item like this that doesn't wear or deteriorate in any measurable way.
 
I change mine when I service the engines and filters, 500 hours. can get at mine easily and have a puller for them, also carry four spares, plus I have quick removable covers, so see no point in replacing them every year.
 
The life depends on the engine and size of pump. The impellers in larger engines (45 HP and above)with multivane pumps seem to last for years.
On the other hand, the life of impellers in the popular 2020, 2030, 2040 VP engines varies from a month to perhaps 2 years if very lucky. I think it depends on having a flaw free impeller. My engine has done 600 hrs and I have used 6 impellers. The worst lost 2 vanes in under a month. The best was removed at 1 year undamaged but had nicks from the removal. The others had lost 1 vane or had more than one vane with a serious tear. Even the RNLI video showed a damaged impeller on a 2030. IMO the pump for these smaller engines is probably not engineered well enough.
 
Small marine deisel salt water pumps have a really hard time of it. I've had one stripped of all it's rubbery vanes (no I don't know what hapened, probably sucked something in, but the pump had no debris in it, not even rubber granules). Replacing the impellor when you're in the water is a swine (on my boat anyway), except at sea when it is a swine squared. I do mine every year with the engine service "whether it needs it or not" and as above have a number of part worn ones in my "come in handy" box of engine spares.
 
I have a VP 2030. I renew mine every year. At the end of last season 2 blades were missing - recovered culprits, fitted new and I'm away again.

Maybe the question is how often should I remove the cover and check ? From some of the replies above it looks like once a month !
 
I inspect them very carefully, using a magnifying glass. Bend each vane in both directions, looking for cracks in the roots. If they are sound, look for excessive wear of the flanks - the impeller should be a tight fit in the pump housing. A little flattening of the vane tips is perfectly OK. If no problem is found I refit them - it seems to me that once you have one that has proved itself to be reliable it makes sense to continue to use it.

Don't forget to inspect the housing cover. If you motor a lot in muddy waters it can become quite worn, preventing a good seal and giving initial suction problems. After transiting the Canal du Midi we found that the pump drained when sailing and would often not re-prime. The cover was worn by about 1 mm.
 
It seems to me the more you use your boat the longer they last. I think they suffer from standing in one position for long periods especially when it's cold. I use my boats all year round and have probably replaced 4 impellors over 35 years. I just replace them if the pumping seems to be less than normal or the engine temperature starts to increase. the worst I have had is 1 broken vane.
 
I have a small raw water cooled engine and have never needed to change an impeller. Eventually, I changed one at (at least, to my knowledge!) 6 years old and even then, aside from a small amount of wear on the tips, it didn't look at all bad. The engine doesn't do that many hours either so it is often left in one place and I never used to take it out over the winter.

Another thing I have noticed is that everyone always strongly advises tha tthe vanes be only bent in one direction (to suit the direction of rotation of the engine) when fitting the impeller. Whilst this is sound advice and, clearly, good practice, I have often noticed that when I stop my engine, it often "bounces" backwards as the piston comes up on the compression stroke at the last revolution. Clearly, this must leave the vanes facing the "wrong" way for when the engine is started again but it doesn't seem to bother it!
 
Absolutely...

Since they don't wear out at a directly proportional rate to the number of hours in use but tend to just "go" by one blade breaking up first which damages the rest of them or by the central hub unbonding from the rubber, it isn't too easy to determine a standard service life.

It could even be argued that by installing a replacement of unknown competence for what has been up until then a perfectly reliable and serviceable component that you are introducing uncertainty and could be doing this to an even greater extent by systematically replacing at one year intervals - the "the more you take from the supplier's store room, the more chance you have of being given the duff one" - sort of philosophy.

I replace mine at around 500 hours as by that time I believe that it is fair to assume that deterioration has or isn't far from, taking place.

If yours has a soft metal backplate, this too MAY need replacing to maintain the lip seal.

Steve Cronin
 
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