Flexible blade pump impellers should be a thing of the past

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njthake

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Can someone please tell me why we are all still using raw water pumps with impellers that wear out?
With a good strainer, a pump with ceramic blade would last for years. Engines below the water line are self-priming, and above the water will self-prime with an anti syphon device so that is not an issue
 
Can you provide a link to the type of pump you mean? I am not aware of a small positive displacement pump that does not rely on flexible parts in some way, other than a piston pump that would be extremely complex and expensive for the duty. Vane pumps as used in hydraulics are of broadly similar design and have straight rigid blades, maybe of carbon, but they would not cope with fouling or other debris.
 
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Seagull engines have a rigid bladed pump, as does our washing machine, car and central heating pump, all radically different functions i know but the pretty effective and don't need regular replacement. All need good filters (except the seagull), but we have those on a boat. I'm sure there's a reason why they wouldn't work and await to see what those more expert than i can add.
 
The rigid blade type are centrifugal circulation pumps. They will not self prime, and have a very poor lift, so they're just not practical for a raw water pump. I don't know much about seagulls, but if they have a rigid blade pump then it must be at the bottom of the leg where it is fully immersed and doesn't need to lift or prime on the inlet side...
 
Mandarin331 said:
Seagull engines have a rigid bladed pump, as does our washing machine, car and central heating pump, all radically different functions i know but the pretty effective and don't need regular replacement. All need good filters (except the seagull), but we have those on a boat. I'm sure there's a reason why they wouldn't work and await to see what those more expert than i can add.
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All are in a position where they are flooded and do not need the self priming capability of a flexible vane pump.
 
I see this issue from another perspective.
A flexible vane pump is ideal for a marine engine, self primes , tough and tolerant of seawater and debris.

My take on this is that with mofern polymers the life of the impeller is much better than say 30years ago.
Like many I replace mine once a year bit it is never trally necessary.

Seals with stainless steel springs and you have reliability and longevity.
 
RichardS said:
I don't understand that phrase at all. An engine below the water line is only self-priming if the pump is self-priming ..... and if the seawater filter is above the water line, as it always is I believe, then a centrifugal pump is never going to work.
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The seawater filter on my boat is integral with the inlet seacock, and therefore definitely below the waterline.
 
RichardS said:
if the seawater filter is above the water line, as it always is I believe
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Definitely isn't always. I've fitted them above the waterline on both my boats, but Kindred Spirit's was originally below the waterline and Ariam didn't have one at all.

Pete
 
vyv_cox said:
Can you provide a link to the type of pump you mean? I am not aware of a small positive displacement pump that does not rely on flexible parts in some way, other than a piston pump that would be extremely complex and expensive for the duty. Vane pumps as used in hydraulics are of broadly similar design and have straight rigid blades, maybe of carbon, but they would not cope with fouling or other debris.
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The first one that comes to mind is the Stuart Turner 12/50 , this has acetal (plastic) or brass impeller. 5 bar pressure, 62.5 lits/min should be plenty for most engines. I am thinking of engine driven so the capacity could be increased. My pump in my water maker has been going strong for 8 years now.
 
njthake said:
The first one that comes to mind is the Stuart Turner 12/50 , this has acetal (plastic) or brass impeller. 5 bar pressure, 62.5 lits/min should be plenty for most engines. I am thinking of engine driven so the capacity could be increased. My pump in my water maker has been going strong for 8 years now.
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Are these centrifugal pumps? For the reasons stated above they are rarely suitable for seawater engine supply. They must be below the waterline to prime, not always possible especially for motor sailing.
 
nigelmercier said:
Fit a SpeedSeal Life, keep one spare. Replace when it fails or looks naff.
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May be I have missed something, having watched the video I understand the front plate being fitted with the brass and PTFE disk to allow the front side of the impeller to turn freely but what about the back surface of the impeller inside the pump housing? The rubber of the impeller is still rubbing directly on the pump housing. So if the pump runs dry the impeller will still heat up on the inside surface.
 
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Big John said:
May be I have missed something, having watched the video I understand the front plate being fitted with the brass and PTFE disk to allow the front side of the impeller to turn freely but what about the back surface of the impeller inside the pump housing? The rubber of the impeller is still rubbing directly on the pump housing. So if the pump runs dry the impeller will still heat up on the inside surface.
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It's a good point John which has been raised before. There has to be some play across the impeller, and if there wasn't it would develop, and I recall some speculation about whether the inlet and outlet are slightly offset from the centre line to tend to gently drift the impeller outwards towards the face plate as it's much better that the plate wears rather than the body.

I don't recall whether this was ever proven either way ...... :o

Richard
 
RichardS said:
It's a good point John which has been raised before. There has to be some play across the impeller, and if there wasn't it would develop, and I recall some speculation about whether the inlet and outlet are slightly offset from the centre line to tend to gently drift the impeller outwards towards the face plate as it's much better that the plate wears rather than the body.

I don't recall whether this was ever proven either way ...... :o

Richard
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Hi Richard, you could be right about the inlet outlet being offset forcing the impeller towards the front plate but I know from experience when I change my impeller every year I see a rubbing mark on both plate and on the inside of the pump housing. The impeller has to be flush fit to enable it to be able to draw the water. Incidentally I fitted the quick change Speed Seal front plate some years back which does make checking and replacing the impeller a lot easier with only 4 thumb screws but I will hold back on the Speedseal for life.
 
On my Jabsco pumps the rear of the housing is replaceable too.... it wears just as much as the front plate...... It comes out easily once the cam is out.
I replaced both plates and the cam in my pumps last year.... no noticeable change, but they prime instantly, even after being out on the slip.
 
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