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Old 22-07-2012, 16:04   #1
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Engine room ventilation

Well, it's hard to call the space and engine "room" but I'll stick with tradition. I'm curious how others are ventilated, and what the best practice is. The powered vent blower on mine seems like an after-thought. The vent hose sits in the aft end of the space at just about the level of the oil pan (diesel engine). There is plenty of space below in the bilge and keel for gas to collect, and space above this vent going the height of the engine. The room gets really hot.

I can think of reasons why the vent should be much lower - get rid of explosive gasses that sink - but that's not really a problem with diesel I think (tell me if I'm remembering that that wrong). And reason to have it high up - to get rid of heat.

I have no incoming fresh air vent. So air is drawn through any loose spots in the hatches, floor plates etc. That's probably ok.

I'm curious how newer boats might be vented. I'd really like to get rid of the heat. My engine room is well insulated for sound so it's hot for a long time after shutting down.

I'm also curious how people use their blower fans. On all the time when the engine is running? Only at start-up? Never?

Looking forward to input.

Thanks

JR
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Old 22-07-2012, 16:15   #2
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Re: Engine room ventilation

Explosive gasses really aren't a problem with diesel.

Ideally the exhaust system in an engine room should keep a slightly positive preassure as compared to ambient, and provide enough air to keep the engine cool, and to provide for engine respiration.

Are you sure there aren't any inlet paths? Louvered screens, or old plugged up openings somewhere? If not you need to check the specs on the engine and figure out what type of air flow the engine demands. Likely it wants a temprature no warmer than 20 degrees (I think) more than ambient. If you are getting this you are fine. If not you need more airflow.

I run the exhaust system anytime that the engine is on. Plus another hour (id the engine got up to operating temp). Remember that diesels actually get warmer after being turned off as the heat inside the engine moves outward, and the cooling system is turned off. A lot of thermal damage done to engines is during this period, so keeping the blower on will help to cool the engine block, and prevent the rest of the systems from overheating.
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Old 22-07-2012, 16:18   #3
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Re: Engine room ventilation

Hot air should be drawn out of the engine room from the top. Cool air should enter near the bottom, often aimed at the alternator.

l think that the max operating temp of the engine room should not exceed 125 deg. F / 51 deg. C. It is best to have a vent tube for cooler input air. If you want to bring the cool air into the lowest part of the engine space then a fan mounted to blow air at the back of the alternator would help keep it cool. I would recommend leaving the exhaust blower on after engine shutdown until most of the residual heat is expelled, maybe 5 minutes or so.

There are no real standards for diesel powered boats that I have been able to find because, as you stated, diesel is not explosive at normal temps.

Edit: as Stumble stated, maybe an hour would be better, although 1/2 hour should be good I would think.
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Old 22-07-2012, 16:21   #4
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Re: Engine room ventilation

Positive pressure in the engine space...would that force the diesel stench out into the living areas?
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Old 22-07-2012, 16:25   #5
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Re: Engine room ventilation

i have vent hosing to engine room and i use fans in main saloon for dissipation of heat. is tropix here so is a bit more uncomfortable than in san diego --so the fans are important.
on hot days, air conditioning doesnt hurt at all.
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Old 22-07-2012, 16:31   #6
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Re: Engine room ventilation

You are right about having a high exit to draw hot air, but put an intake in and make the hose low for it... that way the blower draws cool air in and hot air out. With the Diesel sucking alot of air as well as the blower, you need more than the "cracks" in the covers....
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Old 22-07-2012, 16:45   #7
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Re: Engine room ventilation

Best of all is to aim an air intake hose right at your alternator to keep it a bit cooler.

The exhaust blower is really only to cool the engine room after the engine has been shut down. You don't want to run it underway as the diesel needs all the air it can get. For example, Yanmar specs that a 38hp 3JHTE should have over 80 sq cm of intake air vent cross section. I think my engine gets most of it's input air from the bilge.

It's a useful experiment is to see if there's any suction on the engine room access panels when the engine is running at full speed. If so, add some more vents.

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Old 22-07-2012, 16:45   #8
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Re: Engine room ventilation

Engine box is not air tight on Shiva. The 36hp diesel with a high output alternator has a 3" duct with a fan exhausting air at the top aft when the ignition key is in the ON position (engine running or not). Above the shaft aft is a large opening that leads to toward the stern and the aft lazarettes... which are also ventilated and so air is drawn in low. But the infliltration is weak and not as concentrated as the exhaust port.

It's probably good practice to let the fan run for some time until the air coming out has cooled a bit... but the block as so much residual heat it would take lots of ventilation to cool it measurably.
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Old 22-07-2012, 16:59   #9
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Re: Engine room ventilation

Quote:
Originally Posted by micah719 View Post
Positive pressure in the engine space...would that force the diesel stench out into the living areas?
Only if the rest of the system isn't working properly.
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Old 22-07-2012, 17:01   #10
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Re: Engine room ventilation

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Originally Posted by DeepFrz View Post
There are no real standards for diesel powered boats that I have been able to find because, as you stated, diesel is not explosive at normal temps.
See


ABYC H-32 Ventilation of boats using diesel fuel.

Requirements:

32.4.4 Ventilation. The changing of air within a compartment by natural or mechanical means. Ventilation may be affected by dilution of contaminated air, by local exhaust of contaminated air, or by introduction of fresh air.

32.5.3 Combustion Air. Ventilation provisions and openings to the machinery space provided for supplying combustion air shall accommodate at least the sum of the maximum air requirements specified by the engine manufacturer(s) for each propulsion and auxiliary engine(s) in that space.

NOTES:

1) Consult the engine manufacturer(s) for combustion air requirements, including inlet air temperature and restriction/depression.

32.5.4 Additional uses of ventilation. Power or natural ventilation may be needed to control compartment temperature. Power ventilation may also be used in the machinery space for odor control and personnel comfort while servicing equipment.

For overall engine room design ventilation, engine and fuel shut down in case of fire, alarms, and fire extinguishing system(s) must be designed to work together.

Many times crew comfort limits engine room under extremes of ambient to 40 degrees C. Also, the engine room temperature needs to be well controlled in order to properly select and size equipment.

ABYC also sets maximum engine room temperature at 50 degrees C.
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Old 22-07-2012, 17:37   #11
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Re: Engine room ventilation

Great input all, thanks. I was suspicious this would be the advice, and tips like point the incoming air at the alternator are a bonus.

Right now my engine sucks air from the engine room, which gets well over 50 degrees C. Will do some measurements next time I run up to full temp for a while, and start planning next winter's ventilation project. Might be able to pull air in from the chain locker/sail locker which has a dorade vent, and move/improve the exhaust venting to the top of the engine room.
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Old 22-07-2012, 19:17   #12
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Re: Engine room ventilation

jr pyder--can ye pull air in thru the lazarette?
both mine seem to gain ventilation that way--formosa via the hold in which the quadrant and rudder are located, and ericson via lazarette both via 3 inch tubing from transom to engine, one each side of boat. ericson via cowl vents , formosa via tubing ending under my transom sofa seat, which should be lazarette
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Old 22-07-2012, 19:44   #13
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Re: Engine room ventilation

Thanks Zee. Will look at all possibilities. Exhaust flex tube goes through lazarette to deck vent. Could do something similar for inlet. All space is tight for more flex pipe of course.
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Old 22-07-2012, 19:51   #14
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Re: Engine room ventilation

mine kinda just lays there--doesnt do anything but get in the way so i leave it in the least travelled locations and seems to be fine--keeps down the diesel smell in house, some. same situation in both boats, except the ericson has cowls on transom for them
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Old 22-07-2012, 19:54   #15
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Simply running my diesel evacuates the engine box from one to two times a minute (50 to 100 CFM) by my calculations. Seems like enough. I run the blower after shutdown, sometimes. With ambient temperatures near 40 I doubt any reasonable blower would keep the temperature under 50.
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