Impellers

I have just pulled apart my freshwater pump, to find the offending part is the rubber impeller.About six months ago my genset stopped, impeller again.

This may be a dumb question but why don't they make them out of brass, or something more durable than rubber.

I'm sure there is a good reason why they don't, and I am sure a more gifted mechanical type will tell me why. I just don't get it.

An impeller pump won't pump without the compression/release cycles of the vane as it turns. Make it out of metal, it bends once and stays that way.

If you replace the impellers annually you won't have a problem. If you run the pump regularly, but not many hours you can probably get 2 years.

But I'm with you, there ought to be a better way. We just converted our Westerbeke genset to use a centrifugal air conditioning pump, a March Series 5 pump that matches our A/C pump. Powered by 120VAC, it runs when the genset engine runs. I've read where these work well for many years. I can't yet attest too much about the longevity of the genset application, other than it works. But the exact same pump on our A/C system has thousands of maintenance free hours.

I think the question is more about why they don't make it a hard-vane centrifugal pump like the belt-driven fresh water pump on the front of the engine instead of the flexible-vane pump they have now. And that is a good question. Perhaps the amount of dirt and goo that often gets ingested would quickly destroy a pump with hard vanes. As it stands now, hard material that gets by the strainer has a chance to get thru the system. Any major damage to a rubber impeller just requires a quick impeller change and you are back underway. But this is ONLY speculation.

My previous boat used to eat impellers for breakfast. I rebuilt the pump with a new cam and still could only get 30 hours on an impeller. My marine supplier suggested switching from the black Jabsco impellers to a blue imbeller. I think this may be the brand: https://www.marineandrvpartssupply.com/globe_rubber_parts_List.html

It lasted at least two seasons. (by two seasons I mean 500+ hours on a commercial boat)

Rubber impellers are self priming,metal ones are not.
Bluetide

Unfortunately, the SpeedSeal Life didn't fit my Yanmar (extends impeller life and can run dry for a while without destroying the impeller) but the original SpeedSeal raw water pump cover did, which should make changing the impeller easier.

Here's a quote from the SpeedSeal site;

"The main UK agent for Yanmar, EP Barrus, carried out tests which showed Run Dry periods without the bearing covers of 2.38 minutes before impeller failure. With them in place this increased to 10 periods x 5 mins without failure PLUS a further 10 periods X 10 mins without failure after which the tests were terminated. This exceeds the SOLAS (Safety At Sea Standard) requirements by far and has been further tested by two independent agencies."

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Good question Andy,
I'm not a mechanic but I work with a lot of pumps in my job. The impellor pumps are a displacement type pump. They rely on a fairly tight seal in each chamber between the lobes of the impellor, and achieve good flow rate at relatively low rpm. A soft bodied impellor gives a good seal as it allows contact with the pump body without serious wear issues. And, as Tom said, a bit of sand or crud won't be a big problem because the soft impellor rather than bind or gouge.

A brass impellor would be much less efficient. More clearance would be required to ensure there is no contact with the body, allowing water to escape from each chamber.
This could be compensated by making the pump twice the size. Also adding a prefilter to protect it from solids, but that ends up to maybe 5X the cost.
You'd also have more air lock problems with a non-contact impellor as the ability to "suck" would be greatly diminished.

Another option might be a turbine type pump. They have larger clearance and dont require a tight seal, so they use a brass or steel impellor and much higher rpm to achieve a similar flow rate. So rather than impellors, you'd be replacing bearings. They also self destruct quickly if they cavitate from inlet blockage.

I think the flexible impellors pumps are fairly well suited to the job. Just try to find the best quality impellor possible (and don't forget to open your seacock).

Bluetide said:

Rubber impellers are self priming,metal ones are not.
Bluetide

Click to expand...

I've always believed this to be the reason for rubber impellers.

Simply mount a centrifugal bronze pump below the water line , where it will always be primed , no lift capacity required .

These do wear slowly from sand in the water , but last for years (maybe decades), unlike rubber impellers.

FF said:

Simply mount a centrifugal bronze pump below the water line , where it will always be primed , no lift capacity required .

Click to expand...

Or a self priming centrifugal pump wherever you want.

RickB said:

Or a self priming centrifugal pump wherever you want.

Click to expand...

Interesting! This is the first time I've ever heard of this. So, after mounting & plumbing the new pump, I just remove the old impeller and put the cap and gasket back on?

Its a good idea FF & Rick. There's a big choice of electric centrifugal pumps ot there, although the flow rate wouldn't change with engine rpm, as an engine driven pump would. Maybe that's a good thing?
I guess you could measure maximum water flow per minute out your exhaust at max rpm and then match an electric pump to that.

In marine applications on our small engines:

An engine driven standard impeller pump is sized by the manufacturer to provide variable flow for both engine and exhaust cooling as the engine speeds up. An electric constant speed pump would have to be sized for the max flow + a safety factor, very problematic in a high speed diesel operating at 80% load hauling sport fisherman offshore in say a 42' Bertram.

Why not just regularly change out the impellers and the other parts or pump too - BEFORE they wear out? Seal problems are easily detected by raw water leakage from the weep hole. After many decades (too many) of maintenance on marine engine pumps, it seems pretty simple to me.

As an aside, the biggest pumps I have used are 5000 KW vary speed centrifugal in slurry applications, the wear parts all had a very rigorous replacement date.

The question was for a noisemaker water pump, most of which only operate at 1 speed.

An air cond system would be similar , only one speed required.

For cruisers that wish to make the main engine not rely on an impeller pump, keel cooling is the option.

On a budget change (no dry stack) some type of water pump would still be required to cool the exhaust hose/system.