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Old 05-09-2007, 21:05   #1
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Do raw water pumps suck?

I have to plumb the gearbox oil cooler into my raw water system. This uses a rubber impeller Jabsco type pump.

I know that most fittings are plumbed into the high pressure side of the water pump">raw water pump but on the John Deere 4045D that is most inconvenient as the engine fittings are rubber hose and stainless steel pipe (difficult to alter) and in the wrong place.

However the raw water strainer is right next to the oil cooler (which is below the water line) and I think that I can take 1.25" hose straight there and then reduce to 1" hose to the raw water pump. The oil cooler would offer some resistance to water flow but (hopefully) not that much.

Is this practical or would the pump cavitate or otherwise misbehave?
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Old 05-09-2007, 21:22   #2
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What size is the inlet to the raw water pump???

And what is the # of the pump???

Each pump has a flow rating at certain rpm's. If the inlet is restricted slightly a lower flow may be permissible. But if your max'n out the pump's rpm's then is will cause the impeller to bypass a little but unlikely to do any damage.

It's when the water stops flowing or no water at all that will damage the rubber................................._/)
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Old 05-09-2007, 22:29   #3
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Impellor pumps will suck, but they don't like restriction for too long. This can cause blades to fail and break off prematurely. It is always best to keep the intake as least restricted as possible.
However, if I understand correctly, you have 1.25" hose form the cooler and a 1" pump intake. This may give you enough unrestricted flow. The only other issue I can think of, is how hot will the water be as it enters the pump. You don't want it too hot as the blades will flex too easily and not give the correct head of pressure.
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Old 06-09-2007, 02:18   #4
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Pump data...

I don't think that the water will get very hot at all. It is a small oil cooler and I think that it is only there because the gearbox is continuous rated for the full output of the engine.

I think that the previous gearbox became degraded because it was a bit too small for the engine, but it hardly even got warm when I used it.
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Old 06-09-2007, 06:44   #5
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Just installed a new Beta engine, and that was the way it was plumbed at the factory--raw water goes first to the transmission, then to the pump. So far (0.4 hours total running time) its working great.
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Old 06-09-2007, 06:47   #6
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How has the performance of the boat improved from the 4108 to the new BETA engine????? In terms of speed etc....
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Old 06-09-2007, 08:24   #7
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Running the raw water through the oil coolers to the pump is the usual way of doing it. It then goes to the heat exchanger and sometimes the exhaust manifold and then is injected into the exhaust.
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Old 06-09-2007, 21:55   #8
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Same here - Bukh (Denmark) 3cyl marine diesel with raw water intake; goes through oil cooler, then raw water impeller pump, then heat exchanger. Entire sequence at 3/4" ID.

As an aside, somewhere along the line it was suggested to me to consider installing a temperature gauge to monitor engine oil temperature (already have a coolant temperature transducer).

Any opinions on the real value of knowing engine oil temperature - ie. will this be an additional gadget with minimal practical value?

Martin


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Running the raw water through the oil coolers to the pump is the usual way of doing it. It then goes to the heat exchanger and sometimes the exhaust manifold and then is injected into the exhaust.
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Old 07-09-2007, 01:23   #9
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There is actually a reason why it is sometimes done this way around. If there is a leak in the cooler, instead of water being pumped into the oil, the oil is more likely drawn into the water. Not sure which is worse, but neither is a good thing to have happen. I have the pump first and the engine oil cooler second, the transmission cooler third, the water cooler fourth and the manifold last.
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Any opinions on the real value of knowing engine oil temperature - ie. will this be an additional gadget with minimal practical value?
No not really. Not for small engines like we guys play with anyway. The water temp tells you all you really need to know.
By the way, the oil does about 40% of the cooling, the water does mostly the rest. A small percentage is air cooling.
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