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Old 22-09-2014, 11:37   #1
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Engine compartment ventilation

Some boats have very little ventilation air leaving the machinery space. The air consumed by the engine air intake is pretty much the only way air can exit the space unless there is some active ventilation. Auxiliary diesel engines on sail boats are not "required" to have these powered ventilators but gas/petrol boats are required to have positive ventilation in most jurisdictions. But in the case of petrol the ventilators are not for cooling but just to reduce the concentration of explosive vapors.

If you have an automatic fire suppression system the ventilation system has to have some means of automatically shutting down the influx of forced air if fire is detected along with an automatic engine shutdown. So it is a bit of system engineering to have forced ventilation but still avoid making a fire worse.

I have heard the argument in the case of diesel sail boats that it's best to not actively ventilate the machinery space due to increased risk of a fire more quickly getting out of control. The theory being that the engine's own air consumption will be replenished with fresh cool air and that's good enough as far as cooling goes. Any fire suppression system will starve the fire (and engine) for O2 thus shutting the engine off without any particular change to the engine controls.

I have also heard the argument that forced ventilation is mandatory to reduce the temperature in the machinery space.

What is the conventional wisdom here? Is high temperature in the machinery space a greater hazard than the risk of a fire that is force fed air by a ventilator fan? Are some engine types less affected by ambient temperature or are all engines negatively impacted by high ambient temps?

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Old 22-09-2014, 12:06   #2
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Re: Engine compartment ventilation

My engine room has a blower just like a gas boat, except that it is wired into the ignition and not on a separate switch and it's intake is located at the top of the compt, so when the ignition is on, the blower is on, it pulls air from the compt. It was factory installed.
I also have a Halon automatic fire bottle, the assumption is if there is a fire, turn off the engine and if the heats gets high enough, the fuse on the auto bottle will melt and blow Halon into the area. Also factory installed
I have not yet added a little cover to allow me to discharge a handheld fire extinguisher into the engine compt, but I do think those are a good idea.
If you have a big alternator it might not be a bad idea to have a duct that directs cool air to the alternator

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Old 22-09-2014, 12:36   #3
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Re: Engine compartment ventilation

Hi transmitterdan,

I've almost always been proponent of cooling/ventilation.Coupled with fire extingish system-automatic vent shutoff.Not a to dificult wiring.

Excessive heat anywhere is never good for anything except for converting
to hot water!

All the Best
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Old 22-09-2014, 12:44   #4
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Re: Engine compartment ventilation

I cant think of any boat I had that didn't have a fan for the engine space. I always run it when the engine is running.
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Old 22-09-2014, 13:52   #5
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Re: Engine compartment ventilation

This has been discussed many times on the forum (check the bottom of this page). My belief is that you need extra ventilation. An exhaust fan and an inlet air duct large enough to allow in the exhaust air as well as combustion air. A fan directing some cool air on to the alternator, as stated above, is also a good idea. Many engine rooms now are also equipment rooms with a lot of expensive electronic equipment installed that needs to be kept below certain temps.

By the way, what sources are you using.
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Old 25-09-2014, 14:02   #6
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Re: Engine compartment ventilation

When we motor for long distances in the tropics, we leave the starboard lazarette open/secured to the lifeline to allow the engine to vent heat and circulate fresh air. Since it always occurs on light air days with no wind, there is no concern about water getting below. Also, every hour, we run our engine compartment fan(which vents aft in the cockpit) for 15 minutes to remove moisture and recirculate the air. There is another concern that few mention in regards to heat/moisture in your engine compartment and that is that it accelerates the disintegration of your engine soundproofing material. This was one of the first recons I did on my boat when it was only five years old. However, the PO in Florida motored more than sailed and never vented/aired the compartment. Ventilation? Yes, and as much as you can manage. Good luck and good sailing.

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