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Old 03-10-2011, 11:06   #1
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Raw Water Pump vs Fresh Water Pump

I've often wondered why all the engine mounted circulation pumps are reliable centrufugal pumps and the raw water pumps are rubber impellor pumps? The Engine circulation pump has to push water/af mix through all sort of holes, crevices, thermostat etc, most of which appear pretty flow resistant. Why wouldnt a centrifugal work for the raw water....?
They are available from Oberdorfer or Groco in Bronze or 316 stainless....
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Old 03-10-2011, 11:22   #2
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Re: Raw Water Pump vs Fresh Water Pump

Your engine circ. pump is not positive displacement and requires flooded suction to work. Unless below the water line the water pump">raw water pump will have a dry inlet and has the ability to lift water to a certain height.
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Old 03-10-2011, 11:30   #3
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Re: Raw Water Pump vs Fresh Water Pump

I also think some of this has to do with the R/W pump mostly used in salt water, which is not friendly to bronze or stainless. Even though the housing is bronze. The rubber impeller pumps are also more efficient at pulling/pushing water rather than a closed circulation. Chuck
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Old 03-10-2011, 11:33   #4
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Re: Raw Water Pump vs Fresh Water Pump

Yeah, but couldnt most RWP's be below the water line? It's just a theoretical exercise but seems like a lot of boats could go there. I have a non impellor pump on a spring here on my property. Inside it looks like a centrifugal but with tighter clearances. It is mounted about 6 ft above the water collection tank and pumps 150 ft up the hill. It cost $39.....
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Old 03-10-2011, 11:41   #5
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Re: Raw Water Pump vs Fresh Water Pump

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Originally Posted by Cheechako View Post
Yeah, but couldnt most RWP's be below the water line? It's just a theoretical exercise but seems like a lot of boats could go there. I have a non impellor pump on a spring here on my property. Inside it looks like a centrifugal but with tighter clearances. It is mounted about 6 ft above the water collection tank and pumps 150 ft up the hill. It cost $39.....
Absolutely....most boating systems are a 100 years old in their thinking..if you are willing to think it through and be vigilant to the possible design limitations...you have all sorts of options...

Just remember...lots of intelligent people have gone the same route....so change may be a long road of frustration if you don't design things just right.

Me.... I rewrite the book all the time with boats...I think most boat builders are marketing geniuses and design idiots.
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Old 03-10-2011, 12:20   #6
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Re: Raw Water Pump vs Fresh Water Pump

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Originally Posted by Cheechako View Post
Yeah, but couldnt most RWP's be below the water line? It's just a theoretical exercise but seems like a lot of boats could go there. I have a non impellor pump on a spring here on my property. Inside it looks like a centrifugal but with tighter clearances. It is mounted about 6 ft above the water collection tank and pumps 150 ft up the hill. It cost $39.....
Does your $39 pump have a foot valve To keep the impeller flooded?
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Old 03-10-2011, 12:22   #7
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Re: Raw Water Pump vs Fresh Water Pump

There is a word related to pumps called "head". The ability of a pump being able to suck up water and it's ability to raise water. The rubber impeller types over come that problem by being able to suck up water even with restrictions like sea life in the thruhull fitting or strainer.

See this article >>>> HOW TO design a pump system

BTW it's easier and cheaper to repair a rubber impeller type after it has cavitated. >>> http://en.wikipedia.org/wiki/Cavitation
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Old 03-10-2011, 12:41   #8
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Re: Raw Water Pump vs Fresh Water Pump

just naysayers...work in commercial boating operations and you will see all sorts of practical solutions not recommended by Lloyds or ABYC...and WAY CHEAPER to install maintain...some pass USCG standards...some don't.
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Old 03-10-2011, 12:44   #9
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Re: Raw Water Pump vs Fresh Water Pump

A pumps "head" is limited by atmospheric pressure unless the pump inlet is below the water. Centrifugal pumps are better at pushing water than lifting it. Deep well pumps are cascading multi impeller pumps located at or near the well bottom. Some types use some of the discharge water through a venturii to give added lift. The drawings in the article cited all show flooded suction pumps.
How's that for straying off topic?
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Old 03-10-2011, 13:08   #10
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Re: Raw Water Pump vs Fresh Water Pump

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Originally Posted by perchance View Post
The drawings in the article cited all show flooded suction pumps.
How's that for straying off topic?
scroll down & Goto fig. 19 in the article ................_/) http://www.pumpfundamentals.com/pump...m#gl11_1system
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Old 03-10-2011, 13:10   #11
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Re: Raw Water Pump vs Fresh Water Pump

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Originally Posted by perchance View Post
A pumps "head" is limited by atmospheric pressure unless the pump inlet is below the water. Centrifugal pumps are better at pushing water than lifting it. Deep well pumps are cascading multi impeller pumps located at or near the well bottom. Some types use some of the discharge water through a venturii to give added lift. The drawings in the article cited all show flooded suction pumps.
How's that for straying off topic?
And your point? Many diesels have very limited backpressure/head in their freshwater systems... as past the pump there is only a heat exchanger and an oil cooler befor exhaust injection....flow is more important than pressure.

MANY cheap/flooded centrifugal pumps are available as many are in the baitwell group/washdown group....
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Old 03-10-2011, 16:40   #12
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Re: Raw Water Pump vs Fresh Water Pump

Circulating (closed circuit) pumps are in a liquid evironment where the removal of atmosphere is not a factor in their degree of efficiency. The impellor can therefore be of open face design where a volute (shape of discharge side collecter), or the stripping edge ( the point where the water exits the actual impellor), does not have to have close tolerances.

The raw water pump OTOH, has to constantly create a negative pressure, (vacumn) to allow atmospheric pressure, 14.7 psig, to push the water upwards into the pump.
Water cannot be sucked, it must be pushed by something.
A rubber vaned impellor, moving past a cammed area, creates the negative pressure area inside the pump.
There are many different designs of "self-priming" pumps, rotary vane, rotary gear, exhaust ejector, reed valve, for a few.
But the flexible rubber impellor has the advantage of passing mildly obstructive material thru.
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Old 03-10-2011, 17:58   #13
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Re: Raw Water Pump vs Fresh Water Pump

Who cares if the pump sucks or blows...as long as enough water goes through the heat exchanger...it will work...

Engine mounted rubber impeller types can be mounted above the waterline...so what if the others are mounted below the waterline like most air conditioner pumps????
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Old 03-10-2011, 18:53   #14
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Re: Raw Water Pump vs Fresh Water Pump

It comes down to impeller pumps being positive displacement meaning they are self-priming provided they do not have to pull against too much of a partial vacuum (pressure below one atmosphere) in order to get primed.

Centrifugal engine coolant pumps are low on the engine block which means the pump is already bathed in coolant keeping it primed.
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Old 03-10-2011, 18:58   #15
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Re: Raw Water Pump vs Fresh Water Pump

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Originally Posted by psneeld View Post
Who cares if the pump sucks or blows...as long as enough water goes through the heat exchanger...it will work...

Engine mounted rubber impeller types can be mounted above the waterline...so what if the others are mounted below the waterline like most air conditioner pumps????
As I said above.
Quote:
BTW it's easier and cheaper to repair a rubber impeller type after it has cavitated.
or been damaged by debis, or run dry (thruhull valve closed).
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