Anyone know what the average Engine compartment temperature

Rich Stidger · Jan 17, 2018

I have no idea what the average is but I can tell you that my compartment with my Volvo MD22L-B 50hp 4 cylinder is about 120-130 F. This is with a 3" vent fan running and exhausting the hot air around the alternator.
If you have a compartment that is at 170 F or more, you are very likely to trip an automatic halon extinguisher if you have one installed. The automatic extinguishers trip at about 170-175F. Don't ask me how I know this......

FastOlson · Jan 17, 2018

We run our engine space exhaust blower all the time the diesel is running. Helps to somewhat cool the area and with the vent hose pickup attached near the alternator also especially helps cool the alternator a bit.

Our mechanic told us, many years ago, that these small engines are designed to shed about 5% of their waste heat via direct radiation. That's not much, but still a good reason to keep the block clean and the blower on.

dlochner · Jan 18, 2018

I wouldn't expect the temperature to exceed the operating temperature of the engine, except for the area immediately adjacent to the exhaust. The hot part of the engine, i.e., the combustion cylinders are enclosed in a coolant bath that should maintain the engine temperature at the desired temperature. I think most diesels run at about 180 degrees, yours may vary, but will certainly be below 212 degrees other wise the hot water tank would over pressurize and the raw water cooling would boil.

kloudie1 · Jan 18, 2018

Dave is correct... but the temp in the box is probably right at 180 F.. in the summer, with our ambient in the mid/high 90's, the box gets very hot inside ater extended motoring.. hot enough to trip the temp trip on my air conditioning raw water pump.. The 34 did not have a blower originally.. I have a new one in a box but haven't gotten around to installing it yet..

CaptKimble · Jan 18, 2018

I am in the process of insulating my engine compartment. This is good food for thought. I am thinking that I should install a vent hose closer to my alternator and always run the blower. I always noted that the engine compartment got quite warm during extended operations. Thank you for raising the question.

Rick Webb · Jan 18, 2018

I've always noticed it gets warmer in the engine compartment after the engine is shut down. Running the blower for 20-30 minutes after shutting down helps keep the rest of the boat cooler.

SEMPERAVANTI41 · Jan 18, 2018

On my Hunter 50 the blower fan is wired to the ignition. It is always on when the engine is running. I think Hunter wired it this way as the engine compartment is extremely tight. There is also a blower switch for when the engine is off.

Gunni · Jan 18, 2018

Pick your insulation for best acoustic performance, heat is mitigated by ventilation.

jibes138 · Jan 18, 2018

When the motor is running all the hot air in the compartment is being gobbled up by the engine and replaced with cooler air. When you shut it down that air heats up to the engine temp so it will get hotter when the motor is shut off. The motor will radiate heat to the surrounding environment either while running or when shut down. The reflective properties of the foil backed insulation likely help in this regard. Not sure how big your motors are but some quick math, maybe 950 CC's displacement for a three cylinder. So spinning at say 2500 rpm's and the intake cycle once every two rpm's for each cylinder. So it is eating 1250 intakes per minute x 950 CC's = 1,187,500 CC's of air per minute. Converting to cubic feet = approximately 42 cubic feet of air per minute. An engine compartment might be say 4 feet wide, x 7 feet long x 4 feet high = 112 cubic feet. So the air in the compartment is being completely exchanged by the engines' operation every 3 minutes.

dlochner · Jan 18, 2018

Rick Webb said:
I've always noticed it gets warmer in the engine compartment after the engine is shut down. Running the blower for 20-30 minutes after shutting down helps keep the rest of the boat cooler.

This is a short term effect. Once the cooling water stops circulating any residual heat in the combustion chambers has to go somewhere and out through the engine block it goes. Since the water is under some pressure and it is not circulating it does not pop the safety valve on the HW tank. The water in the cooling chambers will get warmer and then decline as the stored heat is released.

I'm not sure that running the blower to cool the engine compartment is all that efficient. Air is not all that good at removing heat and the energy expense of moving that air may be prohibitive. On my old Sabre 30 the garbage can sat over the diesel, on warm days we would remove the can and let natural ventilation cool the engine without burning up electrons and listening to the blower motor. A major side benefit to this approach was avoiding the unpleasant consequences of baking garbage.

SG · Jan 18, 2018

I would advise not assuming adhesive will hold the insulation in place. There are stick anchor systems which work. If the adhesive let’s go, and a “slab” of foil faced lead sound insulation let’s go, it can be more than a just an appearance issue.
Of course you want to use a system which is designed for engine room sound (and, to an extent, thermal insulation).