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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Old 05-04-2012
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water flow GPM per HP of an engine?

My Friend Hank is having trouble with his old V2 faryman diesel that's running warm to hot. I've finally convinced him that engine temp is affected by water flow more then anything. His is sea water cooled. I think it's around 20 hp. there's a formula I'm sure. or a rule of thumb; flow / HP?

thanks in advance !
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Re: water flow GPM per HP of an engine?

Sorry, no formula on this one because engines have thermostats. It isn't hard to figure out how much power goes out the cooling system since it is typically equal to the amount of shaft power the engine produces (another roughly equal part goes out the exhaust). Then, you can assume an inlet and outlet water temp and you can calculate a flowrate. The trouble with this is that the flowrate is actually very small and the inlet water temp and the power output of the engine fluctuate based on the usage. To deal with this, they install a thermostat so that the maximum water flow exceeds the cooling needs of the engine in the worst case scenario and the thermostat closes down to deal with other situations. Most of the water that you see in an exhaust is actually going through the bypass loop to keep your pump happy except under really high throttle situations. If the flow of water gets to be so low, then this will make the engine overheat but up until this point, the thermostat will just open more and take care of it.

If you really suspect that the problem is with the flowrate, I would recommend finding another person with the same engine and doing a comparative bucket test at the same engine rpm with the engine cold (the thermostat will be closed so the flowrates should be the same). Just time how long it takes to fill a couple of gallon bucket and compare.

Can you see the thermostat opening on the temp gauge? Most real gauges will let you see it open and close if you watch really carefully.
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Re: water flow GPM per HP of an engine?

I've not even been to his boat yet this season... I'm guessing 3-5 gpm based on known the pump on my boat's engine.
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Re: water flow GPM per HP of an engine?

Your guess of 3-5 GPM is probably pretty close. However since his engine is an older raw water cooled engine, I would suspect scaling or clogging of the cooling passages is more likely the cause. An oxalic acid flush may help. When I changed the head gasket on the Universal 5411 on my previous boat, I found 5 of the 8 engine passages completely plugged. Flushing had not been able to open them.
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Re: water flow GPM per HP of an engine?

Seconds the idea of blocked cooling passages. My saltwater cooled Yanmar SB12 kept burning up head gaskets. The cooling through the head was blocked solid with some black crystal that I couldn't even chip.
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Re: water flow GPM per HP of an engine?

On of the easiest places to start looking for blockage is at the mixing elbow water discharge.. but it's entirely possible that engine passages themselves are compromised.
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Re: water flow GPM per HP of an engine?

His is a very old and strange engine V2 design
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Re: water flow GPM per HP of an engine?

Standard cooling water estimate based on max. fuel consumption =
approx. 1 GALLON water PER MINUTE per 10 HP with engine a WOT.

estimate based on consumption rate of high heat value of #2 diesel fuel @ ~138,700 BTUs/gallon

Has 'nothing' to do with 'thermostats'.
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Re: water flow GPM per HP of an engine?

Here's a very rough estimate:

Twenty hp is about 15 kW. If we assume about 30% efficiency, the engine would need about a 50 kW input. The difference, 35 kW, ends up as heat (one way or another). Converting 35 kWHr to kcal we get about 30,000 kcal. It takes 1 kcal to raise the temperature of a liter of water 1˚C. If we set the intake water to 20˚C and the output water to 40˚C, each liter of water will conduct 20 kcal away from the system. So, getting rid of 30,000 kcal/hr will require 1500 l/hr = 25 l/min = 6.6 gal/min. Of course, the efficiency of the engine may be somewhat higher or lower; the water temps may be higher or lower; and we haven't accounted for the heat transfer from the engine to the air surrounding it (remember, your engine compartment heats up quite a bit, particularly with the engine running at or near full load). But that's my rough estimate.
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Re: water flow GPM per HP of an engine?

Thanks Rich and Slow! Knew someone would know!
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