Engine bay ventilation

[Alexis]

Hello there,

Have a sailing yacht fitted with an 18hp Volvo diesel. The engine bay is fitted with an extracting fan and flexible that leads to skin fitting on the transom.

Question: when should I use this extractor and what is it's real use? Before starting engine to extract diesel fumes, while the engine's running, or once it has stopped to evacuate the heat?

Any information or point of view welcome.

Cheers, Alexis

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[ccscott49]

Use it before starting the engine and after the engine has stopped to draw hot air out of the compartment. Not when the engine is running, as it will have to "suck" against the engine drawing in air for combustion, and will probably burn it out. However, if your engine intake is outside the compartment, you can use the extractor fan all the time the engine is running. Hope this makes sense!

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[bendyone]

Hi
Normally fitted to petrol engins to vent the compartment before starting the engine. cant do any harn to run it before starting. If it is the proper sparkless fan it would be safe to use. Also if the inlet was low down it would also suck out any gas from the boat.

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[snowleopard]

large engines put out a lot of heat and the air sucked in by the engine intake is not enough to keep the engine room temperature down, an 18hp engine doesn't come into this category. use the fan to cool the compartment after running and before starting if there's a risk of inflammable fumes in the compartment.

inflammable vapour in an engine compartment can be dangerous with a diesel as you can't stop it by turning off the ignition. if a diesel is sucking in an inflammable vapour it will rev out of control and possibly blow itself up!

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[Keith]

I run mine all the time (18hp yanmars) and there is a school of thought that it cuts down on deterioration of engine bay components, i have noticed on my own boat that it has significantly cut down on the radiant heat inside the boat, if you can wire the fan to a thermostat of some type then the fan can auto cut out after engine switched off,still pursuing this on my own boat, but you know how it is,... time and money!!!!!! and you never have the two together!!!........keith

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[Avocet]

I have a little 12hp diesel and the manual says the maximum permitted ambient engine "room" temperature is 40 centigrade. Don't know if that helps or not! My engine has a fan built into the flywheel and this is connected to two shell vents in the cockpit (handy for keeping feet warm)! I don't think the amount of air the fan extracts competes in any way with the engine's intake requirements although it might in a very small, well-sealed engine bay.

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[eastmed]

a diesel engine needs fresh air to run properly. too hot air can cut by 20% of the engine performance.if you force hot air out fresh air will go in.
normally,in summer time, i put on the air extractor from the engine bay after two hours of continuos operation till the engine is off.in cooler seasons it will go on later.if you sail a lot you can not avoid motoring for long periods.
forcing the hot air out also helps to mantain a "living temperature" in the yacht and this is important in warm waters as the med in summer time.
i fitted also a pipe that feeds fresh air in to the engine bay.
stay cool an enjoy your boat !!


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[andy_wilson]

500cc (approx) running at 3000 rpm, drawing a fresh charge of air every other revolution (4 stroke cycle).
.5 (litre) X 3000/2 X 60 (minutes) = 45,000 litres of air per hour.

Not sure of any aux. fans that can match that.

Best used as a precautionery suck before engaging starter motor, then reversing polarity and used to supply forced air whilst running, particularly if engine box is well sealed (though most yachts supply limitless air via the bilges), then in hot weather back into suck mode after engine stopped.

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[ccscott49]

Totally agree, this was the point I was trying to make, unless the engine breaths from outside the compartment, (via a sealed hose) no fan available for a yacht will supply the air ness, for combustion. The fan is there to get the hot air out of the engine space, when the engine has stopped and get the fumes out before it starts. If the engine breaths from outside the compartment, the fan can be left on all the time, to lower the engine space temperature. A lot of the larger motor boats, have an external engine air intake.

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[Alexis]

Dear all,
Many thanks for your experience and wisdom. I will re-design the ventilation of my engine bay accordingly:

1° link extractor fan to a waterproof switch on the engine control panel.
2° change the size of the intake flexible pipe, it currently is about one 3rd of the area of the extractor fan flexible pipe.
3° Follow your inputs as to when use the extractor fan.

Many, many thanks,
Alexis

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[Marsupial]

I ve heard some rubbish but this lot takes the biscuit!

You should have two pipes of roughly equal diameter each connected to fresh air outside the boat, usually they are connected to skin fittings mounted on the transom if its a yacht and are about 4" dia. One sucks or blows cos it has a fan on it, the other one will therefore do the reverse. Normally the fan is arranged so it sucks air out of the boat so new fresh hopefully cooler stuff can come in through the other pipe. This arrangement makes the engine more efficient and the engine compartment cooler. Most installations have a thermostate mounted on the fan which is high up in the engine bay so it cuts in when the engine bay gets hot.

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[ccscott49]

wouldnt want 4" fittings on the transom of my boat, conneccted directly to the engine compartment, thats for effin sure!

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[Alexis]

Dear Marsupial,

Thank you for your input.
If I may say so, I was rather put off by your lack of respect for other peoples' inputs as well as your bragging in the first sentence of your reply. Here's a place where people give their opinion and share their experience. I don't think any of the persons who replied to my post think they detain the absolute and single truth about engine bay ventilation. You may well be a qualified marine engineer, this doesn't give you the right to look down at people the way you did here.

Wish you a nice day,

Cheers, Alexis

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[ccscott49]

Alexis,
Dont worry about it, he is full of crap! The egine will draw in cooler air, for its combustion, the engine compartment gets hot, but the fan cannot draw air out of the compartment, if the engine is drawing more air in than the fan can draw out. (which believe me it will) If you want the hot air in the engine compartment drawn out, whilst the engine is running, the engine itself, needs to draw its air from a seperate air intake, outside the compartment, then the fan can draw hot air out of the compartment, as it no longer has to work against the engine and it has an inlet to draw the cool air in. I checked with the engine room staff here, that is the way it works here. We have effin big engines and effin big fans!

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[Avocet]

It's not that simple.

Most extractor fans are axial fans (like a car radiator cooling fan). They can't work against much of a pressure difference. In other words, they can flow massive amounts of air (for their power) as long as they don't have to "suck" or "blow" against any kind of resistance. The engine, on the other hand, is a "positive displacement" air pump. They can't shift anything like as much volume of air (again comparing like with like) but boy, can they suck! If you don't believe me, turn your extractor fan on and put your hand over one end of the tube - you'll easily stop the flow of air. WHATEVER YOU DO, DON'T TRY THE SAME THING WITH THE ENGINE AIR INTAKE WITH THE ENGINE AT FULL CHAT!!!!

I think what I'm saying is that it really doesn't matter what your extractor fan is doing. If the engine wants air, it will jolly well get it and it really won't have to try too hard to suck it backwards past the extractor fan if that's the easiest way to get it - regardless of whether the fan is running or not!

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[jfkal]

Don't be so harsh on people who try to help here. But I must agree that your post made the most sense. It his funny that people always assume that an engine compartment is as airtight as a cookie jar. The engine has plenty options where to get its air from. My setup is:

1. There a two louvered vent opening of 3" next to the companionway.
2. Two 4 " clam shell covered and goose necked feeds from the transom (and no I do not mind those openings). One has an extractor fan fitted.
Result: Less heat radiated into the boat. Higher engine performance. No diesel smells (I shudder at the suggestion to pump air into the engine room. Potentially lethal with petrol engines).
3. The fan is connected to the "ignition" switch. So it runs a few seconds before starting and I keep it running for a few minutes after engine shutdown.





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[ccscott49]

I do believe thats exactly what I'm saying, the fan will not "pull" against an engine.

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[Trevor_swfyc]

Alexis,

I also have a Volvo 18 hp diesel which does not have any engine bay blower. The engine bay does not get hot due to the enormous for the volume air change by the engine intake. My engine bay is not a sealed system if it was the engine would stop or maybe the stairs would be sucked in, the air gets in easily on my yacht via the large cockpit locker. I would suspect that if a fan was fitted to blow air out, it would even with the engine running at 2000 revs as the air supply is such that only a partial vacuum is created by the engine.
I think it would be unlikely although not impossible for you to get an explosive mixture with diesel in the engine bay, as diesel has a flash point of about 70 centigrade. Unlike petrol which has a flash point below room temperature, this is one of the reason why we should fight to keep the tax concession on diesel.
Is it not possible that your boat was re-engined at some time from petrol (a wise move) and the blower has been left, even though it is now really redundant.

Trevor

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[Nauti Fox]

Er, yes you were.On our arrangement we had engine intake air drawn from the lazzerette with inlet & exhaust fans driven by the engine.Result was the saloon was always smelly.So I disconnected the engine driven fans and fitted two electric (vetus) fans both extracting.Result was lovely. No more fumes.
Agree with what the majority of people on here have said, that if the engine intake is in the engine room fans will not pull air out while the engine is running,if its outside the engine room, no problem.
Al

<hr width=100% size=1>No dear,the water goes in the other one.

 

[Avocet]

Yes, I wan't disagreeing with you! Yours was the last post in the thread so I just replied to it! If anything, I was disagreeing with MY earlier post which, having read it again, suggested that the extractor fan might compete with the engine in a very small, well-sealed engine room (whereas in fact, the engine wouldn't run in any case in a small, well-sealed engine room!)

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