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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel > water flow GPM per HP of an engine?
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Thread: water flow GPM per HP of an engine? Reply to Thread
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Topic Review (Newest First)
05-06-2012 12:31 PM
HUGOSALT
Re: water flow GPM per HP of an engine?

I believe my Yanmar 3GM 22.5 hp raw water cooled is supposed
to circulate 4 gallons per minute @ 1000 rpm's and 8 gallons
per minute @ 2000 rpm's.
Test that may be helpful is to run motor for 30 seconds collecting
exhaust water into large bucket and measuring amount of water.
Then at same rpm's do same but with hose exiting water pump directed straight into bucket and measuring again. Will give you
idea of degree of blockage if any.
Suggest checking out basics first such as any blockages at intake, seacock, strainer, shells/barnicles in water pump and as suggested
checking for blockage in mixing elbow.
05-05-2012 10:19 PM
SlowButSteady
Re: water flow GPM per HP of an engine?

Quote:
Originally Posted by JimsCAL View Post
More importantly, you forgot the rather significant heat in the exhaust. Basically the heat rejected is roughly split equally between the cooling water and the exhaust. So your cooling water estimate is about double what's needed.
Actually, I did think about it. But I assumed that the water and the exhaust gases would be at equilibrium by the time they got to the end of the system. That may not be true, strictly speaking. However, unlike a car, truck, or anything else with a dry exhaust, we're talking about a wet system. As such, the exhaust gases must be pretty close to thermal equilibrium with the water. If that weren't the case the hot gases would cause localized heating of the seawater. That would, in turn, cause precipitation of the calcium salts (i.e., "scale") all along the exhaust hose. While this definitely does happen at the mixing elbow, it's usually much less of a problem downstream (particularly after the water lift muffler). So, the heat of the exhaust gas may take some of the energy away from the engine, but not all that much.
05-05-2012 09:52 PM
RichH
Re: water flow GPM per HP of an engine?

Forgot to add in the 'slab rust' post. The LAST thing you want to do with such an engine that has the potential to develop 'slab rust' is to use a 'straight' acid to remove any possible scale if it is so fouled. The acid will quickly turn the ferrous deposits into ferric deposits ... and then you have to begin all over again with 'heat soaking' the engine back to 'ferrous'.

Such engines should really only be descaled with inhibited commercial boiler descaler compounds ... such as Ryd-Lyme and once done the engine still should be run hard and long to redevelop the protective black ferrous rust.
05-05-2012 08:59 PM
JimsCAL
Re: water flow GPM per HP of an engine?

Quote:
Originally Posted by SlowButSteady View Post
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.
More importantly, you forgot the rather significant heat in the exhaust. Basically the heat rejected is roughly split equally between the cooling water and the exhaust. So your cooling water estimate is about double what's needed.
05-05-2012 04:48 PM
RichH
Re: water flow GPM per HP of an engine?

The old Faryman and Universal atomic A4 EXHAUST manifolds are notorious for 'slab rust' which partly or 'occasionally' block the water flow. Only way to check for 'slab rust' is to remove the manifold go inside with a stiff wire and break up all the 'slabs' while the manifold is inverted.
Only way to prevent slab rust is to never ever allow the manifold to drain ... and you must run the hell of the engine so that the stratified cast iron develops an internal thick deposition of 'protective' Black/Blue 'ferrous' rust. If the passage is drained, Red 'ferric' rust will form and if it forms in the 'stratifications' (laps) in the casting the red rust will continue to push-the-casting-apart .... as 'slabs'.

When 'storing' any engine with a cast iron exhaust manifold or cast iron block the block/manifold should be filled with 50:50 anti-freeze with RUST INHIBITERS to retard the 'growth' of red/ferric rust. The US Navy hardly ever 'shuts down' an engine that is made of cast iron ... as soon as the temp. drops the ferrous rust begins to change to destructive red/ferric rust.
05-05-2012 02:52 PM
deniseO30
Re: water flow GPM per HP of an engine?

Thanks Rich and Slow! Knew someone would know!
05-05-2012 01:45 PM
SlowButSteady
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.
05-05-2012 01:20 PM
RichH
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'.
05-05-2012 12:59 PM
deniseO30
Re: water flow GPM per HP of an engine?

His is a very old and strange engine V2 design
05-05-2012 12:49 PM
Faster
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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