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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance > Engines > Diesel
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Diesel This is a new forum dedicated to diesel engines and their applicable accessories.


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Old 12-13-2011
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How hot should the cooling water temp be?

I was sailing on a friend's boat recently and noticed that cruising along with his Yanmar at 2200rpm his temp guage read about 180 deg. Mine, on the other hand never reads above 150. Usually I run my perkins 4.108 at about 24-2500rpm and my temp guage sits right around 140-145.

Does it even matter? Is coolant temp a good indicator of reaching high enough combustion temperatures? I did take 3 inches of pitch out of my prop 2 years ago due to OVER-heating issues at higher RPM. Now I'm wondering if my temp guage is telling me that I'm under-propped.

So what is the correct temp for coolant and does it even matter?

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My Perkins used to run at an indicated temp of 140 at 2200 rpm's but less at lower rpms. My Yanmar runs at 160 once it warms and stays there. I believe it depends on the thermostat, the Perkins was overcooled to the point it couldn't maintain temp at low speed I think.
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Old 12-13-2011
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A FWC engine should be running near 180°F at near max. rpm. A raw water cooled engine should be running 'under' 150° to lessen the fouling of the cooling water circuit.

180° gives an optimum development of Horsepower.

Of course all this depends on the integrity of the thermostat and the temperature of the inlet raw cooling water. An engine that is below the optimum operating temp. will have a slightly less combustion efficiency ... and an ever so slightly greater wear profile of the mechanical parts due to 'metal to metal clearances' at the lower temperature.
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Old 12-13-2011
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Quote:
Originally Posted by RichH View Post
A FWC engine should be running near 180°F at near max. rpm. A raw water cooled engine should be running 'under' 150° to lessen the fouling of the cooling water circuit.

180° gives an optimum development of Horsepower.

Of course all this depends on the integrity of the thermostat and the temperature of the inlet raw cooling water. An engine that is below the optimum operating temp. will have a slightly less combustion efficiency ... and an ever so slightly greater wear profile of the mechanical parts due to 'metal to metal clearances' at the lower temperature.
Hmmm.... me thinks I should get an infared thermometer to check the accuracy of my gauge. Then, if the engine temp is still too low I could use that to assume that I'm under-propped could I?

BTW my engine is FWC and max rpm is, unfortunately, a debatable issue on the 4.108. From what I can tell max is 3000rpm. Any idea what temp it should be at cruising RPM? By cruising rpm I mean where the max HP/Torque/efficiency curves meet which is about 2300rpm for me.

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Old 12-14-2011
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Quote:
Originally Posted by MedSailor View Post
Hmmm.... me thinks I should get an infared thermometer to check the accuracy of my gauge. Then, if the engine temp is still too low I could use that to assume that I'm under-propped could I?

BTW my engine is FWC and max rpm is, unfortunately, a debatable issue on the 4.108. From what I can tell max is 3000rpm. Any idea what temp it should be at cruising RPM? By cruising rpm I mean where the max HP/Torque/efficiency curves meet which is about 2300rpm for me.

Medsailor.
I'm more familiar with car engines but my understanding is that it's the thermostat that determines operating temperature, not load - unless the thermostat is fully open which would indicate a cooling system with insufficient capacity.

If you think the engine is running too cool the solution lies with changing the thermostat, not adjusting the prop.
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Agree with Mark .... probably a faulty thermostat. You can descale or replace.

With an IR thermometer you should 'profile' the temperatures. Inlet raw temp., all the inlets/outlets of the Hx, the outlet temp. of the exhaust manifold, the exit temp. (and quantity of water) at the overboard throughhull. Its the 'terminal differences' between the Hx in/out connections that are most important when checking. BTW - all such readings should be taken with the engine under 'full load', max. cruising throttle and after the engine has 'fully' warmed up (heat soaked).
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Quote:
Originally Posted by RichH View Post
Agree with Mark .... probably a faulty thermostat. You can descale or replace.

With an IR thermometer you should 'profile' the temperatures. Inlet raw temp., all the inlets/outlets of the Hx, the outlet temp. of the exhaust manifold, the exit temp. (and quantity of water) at the overboard throughhull. Its the 'terminal differences' between the Hx in/out connections that are most important when checking. BTW - all such readings should be taken with the engine under 'full load', max. cruising throttle and after the engine has 'fully' warmed up (heat soaked).
In my case, max. cruising throttle is the setting where the engine is on the verge of being too hot.
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Some of the comments on this thread linking props to engine operating temp. are a bit off IMHO. Engine operating temp should be regulated SOLELY by the thermostat - that's what it's there for.

A marine engine should be able to reach maximum recommended (operating) RPM, or near it, at hull speed. More specifically, it should reach its peak TORQUE rpm. Prop dimensions control THIS and should not control engine temp.

If you have a functioning thermostat of the correct temp (per the engine manufacturer) and are having cooling or heating problems, the solution is elsewhere. If you overheat you either have circulation problems of one sort or another, you have an inadequate heat exchanger (unlikely if it is OEM) or you are over-propped, in which case your max RPM should be too low.

Over cooling usually just means a thermostat problem (too low a setting or stuck open) or running the engine too slow (not an inability to run it faster).

Many people seem to think an engine is better off running cooler - 150 or even less - but this is not true. Take a look at contemporary cars - they run close to 220 before the fans even come on, racers run way over 200 as well - it gives you more power. Those temps don't leave much margin for problems though.

Engine oil needs to be hot to work properly so I like to see an engine run 180 - 190 in my boats.
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Another issue is condensation in the engine oil. Higher oil temperature means the water etc. gets driven off more effectively and more quickly.
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Quote:
Originally Posted by SloopJonB View Post
Some of the comments on this thread linking props to engine operating temp. are a bit off IMHO. Engine operating temp should be regulated SOLELY by the thermostat - that's what it's there for.

A marine engine should be able to reach maximum recommended (operating) RPM, or near it, at hull speed. More specifically, it should reach its peak TORQUE rpm. Prop dimensions control THIS and should not control engine temp.

If you have a functioning thermostat of the correct temp (per the engine manufacturer) and are having cooling or heating problems, the solution is elsewhere. If you overheat you either have circulation problems of one sort or another, you have an inadequate heat exchanger (unlikely if it is OEM) or you are over-propped, in which case your max RPM should be too low.

Over cooling usually just means a thermostat problem (too low a setting or stuck open) or running the engine too slow (not an inability to run it faster).

Many people seem to think an engine is better off running cooler - 150 or even less - but this is not true. Take a look at contemporary cars - they run close to 220 before the fans even come on, racers run way over 200 as well - it gives you more power. Those temps don't leave much margin for problems though.

Engine oil needs to be hot to work properly so I like to see an engine run 180 - 190 in my boats.
Clearly explained and concise. A worthwhile post, thank you.
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