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Old 03-10-2011, 19:06   #16
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Re: Raw Water Pump vs Fresh Water Pump

Actually Centrfugal pumps are much more forgiving to being run dry than impellor pumps. The centrifugal "vane" doesnt care, just the bearing needs to stay cool enough. I have accidentally run one a full day with no ill effect. An impellor may pass a little bit of debris but not much. One broken blade takes them all out.... Only reason I brought this up is the impellor pump seems to be the most often occurring failure on a diesel engine.... Aren't trash pumps big centrifugals?
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Old 03-10-2011, 19:14   #17
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Re: Raw Water Pump vs Fresh Water Pump

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Originally Posted by delmarrey View Post
As I said above. or been damaged by debis, or run dry (thruhull valve closed).
Not true..centrifical pumps can run dry longer than your engine would without water flowing...with a strainer...debris aren't too big of an issue and the larger ones pass a lot of debris anyway.

I know people and have done it myself...used washdown pumps to get home without problem...properly sized and plumbed could just as easily replace an engine driven pump.
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Old 03-10-2011, 19:16   #18
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Re: Raw Water Pump vs Fresh Water Pump

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Originally Posted by Cheechako View Post
Actually Centrfugal pumps are much more forgiving to being run dry than impellor pumps. The centrifugal "vane" doesnt care, just the bearing needs to stay cool enough. I have accidentally run one a full day with no ill effect. An impellor may pass a little bit of debris but not much. One broken blade takes them all out.... Only reason I brought this up is the impellor pump seems to be the most often occurring failure on a diesel engine.... Aren't trash pumps big centrifugals?
Some just never think outside the box...

Yep.... trash pumps are big centrifugals...but they still don't like chunky, hard debris or rags....
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Old 03-10-2011, 19:26   #19
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Re: Raw Water Pump vs Fresh Water Pump

Quote:
Originally Posted by Cheechako View Post
Actually Centrfugal pumps are much more forgiving to being run dry than impellor pumps. The centrifugal "vane" doesnt care, just the bearing needs to stay cool enough. I have accidentally run one a full day with no ill effect. An impellor may pass a little bit of debris but not much. One broken blade takes them all out.... Only reason I brought this up is the impellor pump seems to be the most often occurring failure on a diesel engine.... Aren't trash pumps big centrifugals?
Self priming centrifugal pumps, like trash pumps, are as far as I'm aware self priming by having a reservoir that you fill with water. They're made such that the water from the reservoir gets into the pump, pumps, poorly, the mixture of air and water, then the air goes up and continues on past the pump and out and most of the water falls and returns to the pump. This makes the pump work well enough to provide suction to pull the water from below the pump. Once you fill it the first time, it doesn't need to be primed again unless you empty it or it sits long enough to leak out or evaporate.

How a Self-Priming Pump Works | PumpStoreUSA.com

John
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Old 03-10-2011, 20:06   #20
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Re: Raw Water Pump vs Fresh Water Pump

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Originally Posted by cal40john View Post
Self priming centrifugal pumps, like trash pumps, are as far as I'm aware self priming by having a reservoir that you fill with water. They're made such that the water from the reservoir gets into the pump, pumps, poorly, the mixture of air and water, then the air goes up and continues on past the pump and out and most of the water falls and returns to the pump. This makes the pump work well enough to provide suction to pull the water from below the pump. Once you fill it the first time, it doesn't need to be primed again unless you empty it or it sits long enough to leak out or evaporate.

How a Self-Priming Pump Works | PumpStoreUSA.com

John
Or in the situation the OP wants/suggests...just mount it below the waterline and no priming necessary...

The only real challenge is to ensure the pump is sized to provide adequate flow...and as speed increases..the thru hull scoop should actally help a centifugal pump quite a bit.
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Old 04-10-2011, 07:29   #21
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Re: Raw Water Pump vs Fresh Water Pump

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Who cares if the pump sucks or blows...as long as enough water goes through the heat exchanger...it will work...
I agree.
Think of a well pump on a house. I have one that can suck and it's centrifugal.
If it sucks without being submerged it has to have some water in the housing.
I always hated rubber impellor pumps. They take a lot of engine power to turn vs centrifugal pump (IMO). And are not as reliable and cost more and need more repairs.
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Old 04-10-2011, 07:34   #22
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Re: Raw Water Pump vs Fresh Water Pump

3/4 HP High Pressure Jet Pump - A&H Turf & Specialties


I have something like this. I dont use it hardly at all, but it does suck up water and is centrifugal no rubber rotor.
I thought I could use this on the boat as an emergency pump.
It is cast iron. I could coat the interior maybe to stop rusting.

What about all those large gas powered dewatering pumps? They suck water.

One issue is rpm pump speed, if you run a centrifugal pump slow, it may have to be below the waterline to work.
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Old 04-10-2011, 08:43   #23
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Re: Raw Water Pump vs Fresh Water Pump

Centrifical pumps have been used as raw water pumps. There was a particular Onan generator that when the seal leaked water would get into the engine. Owners got smart and replaced the engine driven pump with a 110 volt centrifical pump mounted below the water line.
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Old 04-10-2011, 11:24   #24
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Re: Raw Water Pump vs Fresh Water Pump

Quote:
Originally Posted by psneeld View Post
Not true..centrifical pumps can run dry longer than your engine would without water flowing...with a strainer...debris aren't too big of an issue and the larger ones pass a lot of debris anyway.

I know people and have done it myself...used washdown pumps to get home without problem...properly sized and plumbed could just as easily replace an engine driven pump.
I use to rebuild centrifugal pumps when in the Navy in the 60's. The first thing that would go were the wear rings between the impeller and the case (lower and lower flow). Next would be the blades on the impeller, and then the shafts. And in a couple cases I had to fill the eaten away interior of the pump case, from cavitation, with a Belzona filler then paint on the Belzona ceramic coating.

I find it hard to believe that one could run a centrifugal pump dry for very long w/o damaging the seals or packing/shafts. As well, if one were to run these with the valve closed or the inlet restricted it would get hot and start to cavitate, ruining the impeller and seals.

So answer this;
Why is it that ALL manufactures of marine pleasure boat engines us the rubber impeller pumps?

Are there not small enough centrifugal pumps to do the job?

BTW Centrifugal pumps also have to be run at a constant speed, not variable like on the belt/gear driven rubber pumps. That means one needs a separate motor (electric) to run the pump. (no battery/ no pump)

So when the engine is at idle a centrifugal pump may just pump in too much water cooling the motor too much. And at full RPM's, it may not pump enough especially if it's getting worn wear rings.

Rubber impeller pumps on marine motors have prevailed for longer then I I've been around for good reason. And I'm sure in the past 50 years someone has had a brain fart thinking the same thing as the OP above.

Whats been proven, is proven and that's that!

Effect of Internal Clearances on NPSHR | Centrifugal Pumps

http://www.maintenanceworld.com/Arti...fugalpumps.pdf
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Old 04-10-2011, 11:28   #25
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Re: Raw Water Pump vs Fresh Water Pump

Quote:
Originally Posted by HopCar View Post
Centrifical pumps have been used as raw water pumps. There was a particular Onan generator that when the seal leaked water would get into the engine. Owners got smart and replaced the engine driven pump with a 110 volt centrifical pump mounted below the water line.
Generators run at a constant speed/rpm.
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Old 04-10-2011, 11:39   #26
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Re: Raw Water Pump vs Fresh Water Pump

Quote:
Originally Posted by sdowney717 View Post
3/4 HP High Pressure Jet Pump - A&H Turf & Specialties


I have something like this. I dont use it hardly at all, but it does suck up water and is centrifugal no rubber rotor.
I thought I could use this on the boat as an emergency pump.
It is cast iron. I could coat the interior maybe to stop rusting.

What about all those large gas powered dewatering pumps? They suck water.

One issue is rpm pump speed, if you run a centrifugal pump slow, it may have to be below the waterline to work.
Yeah, I've got a couple of those around, didnt realize they are centrifugal....? They'll build up to 80 psi!

"BTW Centrifugal pumps also have to be run at a constant speed, not variable like on the belt/gear driven rubber pumps. That means one needs a separate motor (electric) to run the pump. (no battery/ no pump) "
Hmmmm.... the two pumps I posted earlier are sold by Marine supply stores with V belt pulley configurations... Having trouble logic-ing out why a centrifugal would have to be run at a contant rpm...?
Also, isnt the fresh water pump on your engine a Centrifugal?
"So answer this;
Why is it that ALL manufactures of marine pleasure boat engines us the rubber impeller pumps?"
Yeah.. most do. There is likely a reason, probably due to the issue of the Centrifugal needing a head or prime to work..... too iffy for a universal engine application I s'pose.

Still, it's an interesting question.
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Old 04-10-2011, 11:43   #27
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Re: Raw Water Pump vs Fresh Water Pump

Delmarrey, I hadn't thought about the speed issue with centrifugal pumps. That's why it will work with a generator and not with a propulsion engine. What is Belzona? We have a problem with cavitation damage in one of the Oberdorfer pumps we sell for air conditioning water supply.
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Old 04-10-2011, 12:08   #28
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Re: Raw Water Pump vs Fresh Water Pump

Humm! All these years and the only centrifugal pumps I've ever seen run by belts were on the front of combustion engines, which are a closed system. And there is very little pressure involved, just for circulation.



Industrial Protective Coatings and Repair Composites - Belzona
Products - Belzona

This stuff is an epoxy type coating that is impregnated with different fillers. Some are a metal and others ceramic. Different grades have different purposes.

I've used Belzona for repairing pumps from centrifugal to bellows types, as well as coating the inside of large steel valves for saltwater use.

It's not cheap though. The last time I purchased a 1 ltr tub w/catalyst of the 1321 it was about US$250. It comes in colors too (blue & gray), so when you put on more then one coat you can see the color difference for full coverage.
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Old 04-10-2011, 12:19   #29
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Re: Raw Water Pump vs Fresh Water Pump

Centrifugal pumps don't have to be run at constant speed. Their output and delivery pressure is directly related to pump speed and impellor configurations.
The worst thing you can do to a centrifugal pump is not allow it to pass water forward, as Del says.
Spent 17 years in the Fire Dept, latter years designing and specing pump/ power plant combos for fire trucks.
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Old 04-10-2011, 12:39   #30
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Re: Raw Water Pump vs Fresh Water Pump

I skimmed through and don't see any mention of the fact the most pleasure boats send the exhaust out with the raw water, which acts as a muffler. Its crucial that the raw water pump turn only when the engine turns, as its the exhaust pressure that keeps the water from getting into the engine. Water in engine=(
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