What's normal temperature what's not

[WayneH]

Side question: If your thermostat, impellor, and exhaust elbow are setting the engine temperature, why do people say you have to run at 2/3 to 3/4 throttle to get the engine to temperature?

LO,

Here's my non-mechanic answer, FWIW. The thermostat STARTS opening about 15° below design temperature. If you are running low rpms, this opening can provide sufficient coolant flow the the HX to reduce the temperature of the coolant to the point that the thermostat actually starts closing again. Especially with very cold raw water.

And to clarify (because sometimes I miss things) Coolant contains antifreeze and raw water comes from outside the boat. Some engines are raw water cooled. My engine has a HX that uses raw salt water to cool off my antifreeze coolant. And the two flows never mix unless I have expensive problems.

 

[Ron20324]

It would be interesting if someone with a gauge to read degrees would see how long it take the engine to cool down to "acceptable" (read tolerable) level. Anyone?

 

[Tally Ho]

Greg,
A bit of background first. A gasoline engine runs a compression ratio of about 10:1 while a diesel engine runs about 18:1 to get the air hot enough to ignite the diesel fuel. To withstand this higher pressure and for durability, the diesel engine is much beefier metalwise. Once you get this mass hot, it does take a while to cool off. You can slightly speed up this process by idling the engine several minutes before shutting it down. But it still takes a while to cool it off. When we are cruising, we would run the engine blower for about 30 minutes after anchoring to help cool off the engine. And we would leave the aft cockpit hatch open for another hour or so to vent the heat.

Thanks Wayne.
That all makes sense to me. I don't have an engine room blower, but I typically will let the engine run at file when I dock to cool things off some.

On this weekend trip, I opened the doors to the engine room (I use this term very loosely), opened the ports in the adjacent cabin to try and dissipate the heat before sleeping next to the hot mess.

I just got to thinking after observing how cool the exhaust water was, could my thermostat be stuck closed, so there is no real heat exchanging going on...the engine alarm does not go off, but my hot potable water is hot enough to scald you ( no mixer valve), but my exhaust water is cool.

Maybe I am over thinking this. But I will probably replace the T-stat this winter.

Greg

 

[Davidasailor26]

It would be interesting if someone with a gauge to read degrees would see how long it take the engine to cool down to "acceptable" (read tolerable) level. Anyone?

Probably depends on what you mean by “acceptable”. A warm engine in April is a lot more tolerable than August. But as an example, here’s my engine temperature for several hours after a long run yesterday. We shut down at about 4PM around 165 degrees. My chart only shows the last 24 hours, so the first few hours of decline aren’t visible, but this gives some idea as to how long it takes for the block to cool. 5 hours after shutdown the block was still around 120. Temperature sensor is mounted on the outside of the closed loop water pump. We have a blower tied to the ignition. The cabin was in the high 70s most of the time.