# Diesel engine compartment ventilation



## MarkSF (Feb 21, 2011)

Hello,

I have two clamshell vents in the cockpit, but it looks to me like they are connected together with one length of white hose! Surely this can't be right.

What is the best way to connect them? Someone mentioned convection, how to best optimise airflow?

Thinking that one clamshell should have a hose that goes to the bottom of the engine compartment, and the other to the top, does this sound right?


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## olson34 (Oct 13, 2000)

MarkSF said:


> Hello,
> 
> I have two clamshell vents in the cockpit, but it looks to me like they are connected together with one length of white hose! Surely this can't be right.
> 
> ...


If a gasoline engine, the hose goes to the bottom, for all the usual safety reasons. For a diesel engine, hose goes to the top, and preferably near the alternator.

That exhaust hose should go thru a "bilge blower" suction fan like the venerable Attwood product.
Run it all the time the engine is running. Diesels are designed to have a small % of their waste heat removed by radiation, so getting the heat out of a tight sailboat engine compartment is a good thing. Also, if your engine has any crankcase blowby, that will be removed rather than smelling up the cabin a bit.

Sounds like someone re-hosed your boat in a rather strange and silly way.

Don't worry about the inlet clamshell hose. As long as the engine compartment is open to the area serviced by that clamshell, it will pull in plenty enough air. Diesels have a voracious appetite for air supply.


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## misfits (Dec 9, 2011)

Funny because I was thinking about this last evening, why do sailboats with diesel engines have a blower in the engine compartment & should I remove mine. 

Unlike its gasoline counterpart there are no explosive fumes that need to be ventilated before turing the key. I was curious as to the competition of air needed for correct combustion & the negative pressure created within the engine compartment by the exhaust blowe. Does running the exhaust blower have an effect on the amount of combustion air avaliable?

Olson34, you bring up a good point, getting rid of heat rejection & odors. I'm not sure if it needs to run whenever the wheel is turning though. 

Any thoughts?


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## MarkSF (Feb 21, 2011)

Misfits,

To me it seems that as long as there is a reasonably large vent into the compartment, an extra extraction blower will not significantly change the pressure in the compartment. You will just have more air circulation and lower temps.

An extraction fan / blower for an inboard diesel is not compulsory according to USCG regs, unlike on gasoline engines. That is not to say it isn't useful or recommended.


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## olson34 (Oct 13, 2000)

misfits said:


> Funny because I was thinking about this last evening, why do sailboats with diesel engines have a blower in the engine compartment & should I remove mine.
> 
> Unlike its gasoline counterpart there are no explosive fumes that need to be ventilated before turing the key. I was curious as to the competition of air needed for correct combustion & the negative pressure created within the engine compartment by the exhaust blowe. Does running the exhaust blower have an effect on the amount of combustion air avaliable?
> 
> ...


I know a master diesel mechanic with over 40 years in repair and servicing, and he advises to run the blower for the reasons mentioned. Chance of lowering the relative air pressure in the engine compartment is nonexistant, unless an owner has somehow sealed up _every_ opening going into it. The factory normally leaves ample square inches of open access to the (usually) lazarette where the intake ventilator is located.

I would agree that, in theory, running the blower fan while the engine is slurping in air seems superfluous, but in fact if we shut off that fan we do get some mild blow-by odor into the cabin shortly.

Good to let it run for another five minutes after shut down, on a hot day, too. That block radiates heat for a while.


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## MarkSF (Feb 21, 2011)

Higher engine compartment temperature means hotter engine oil, alternator, starter, fuel, fuel pumps, drive belts, and extra load on the cooling system - all of which are undesirable.

The only good news is hotter intake air should mean lower fuel consumption - but lower max. power output.


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## svHyLyte (Nov 13, 2008)

MarkSF said:


> Hello,
> 
> I have two clamshell vents in the cockpit, but it looks to me like they are connected together with one length of white hose! Surely this can't be right.
> 
> ...


We have a single 4" diameter vent line that runs from the top of the engine compartment to a radial discharge blower mounted in the bustle that discharges hot air through the transom whenever the engine is operating. We also have 3-3" diameter "static" vent lines that run from the transom to the bottom of the engine compartment to admit relatively cooler air. The air discharged by the blower easily exceeds 100º on a hot summer day. That will make quite a difference to the operation of the engine here in southwest Florida. In the Bay area, your static vents may be enough considering how much cooler it is there.

FWIW...


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## MarkSF (Feb 21, 2011)

These are static vents I'm dealing with - so yours go to the bottom of the engine compartment?


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## fryewe (Dec 4, 2004)

Perhaps they were connected together to prevent shipping water into the engine compartment when the cockpit took on water.

I have two boats of the same model with small Yanmars. One was delivered with an engine compartment vent in the bridge deck locker with a clamshell cover on it. The other has no engine compartment vent to outside of the boat...it had numerous one inch holes bored in the engine compartment bulkheads to allow free flow of air into the space from inside the boat and ventilation into the boat was through the hatch(es) with or without boards in place and through the cracks around the sliding hatches if they are closed.

The strongest argument for ventilation is for cooling if you have a small diesel because combustion air goes through the air filter housing intake which is only a bit larger than a one inch cross section...much less than the cross section of the combined openings into the engine room for most boats through which air can flow. The engine shouldn't lack for air for combustion even in the absence of a vent.

The history of my two boats is that the one with the vent is being restored after sinking due to "unknown reasons"...but it's not improbable that the bridge deck locker vent contributed when it became submerged (other minor leakages building up in the boat to lower freeboard to that point).


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## H and E (Sep 11, 2011)

I have a universal diesel in a Catalina that has two vents. One is intake and the other is exhaust with a blower. I do not remember any instructions regarding the running of the blower. I check it occasionally to be sure it works and have not run it during normal operations. So far I have not smelled any diesel in the boat. I like the idea of using it to remove heat and may start running it when the motor is operating. If I remember one hose with the blower is up high and the intake hose is down low.


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## Ritchard (Aug 15, 2011)

I like the idea of running the blower when under power to both remove any diesel stink, and also to keep things cooler on a hot day. However, my blower is LOUD! I wonder if I can somehow soft-mount the thing to quell the vibrations.


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## Barquito (Dec 5, 2007)

I wonder how much air actually moves on a static set-up with two clam shells. Mine both have screens too.


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## MarkSF (Feb 21, 2011)

Probably about the same amount that the diesel engine is pumping out of the exhaust


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## Gunga Din (Jul 24, 2012)

Or the same amount the engine is taking in? If they're connected and the compartment is sealed. 
Anyway, the hoses help to direct the flow to desired areas - like the alternator.


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## MarkSF (Feb 21, 2011)

Gunga Din said:


> Or the same amount the engine is taking in? If they're connected and the compartment is sealed.
> Anyway, the hoses help to direct the flow to desired areas - like the alternator.


The amount of air the engine is taking in, and the amount going out of the exhaust, are essentially the same.


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## Gunga Din (Jul 24, 2012)

Hi, MarkSF
I didn't mean to correct you or be rude but you're wrong. Not literally if you wanted to differentiate exhaust gases from air but, then, there's no air in the exhaust.
What I meant it's because the whole idea of an diesel/internal combustion engine it's the expansion thru burning/explosion.
Again, don't get me wrong. Just a comment.


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## AirborneSF (Dec 14, 2010)

MarkSF, I have the same thing, one facing forward ducked along the top for about a foot supplying fresh air into the eng compartment, the other faces to the rear and it's hose goes to the bottom, and pulles air out of the compartment.


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## MarkSF (Feb 21, 2011)

"there's no air in the exhaust"

Air is 80% Nitrogen and 20% oxygen. Exhaust is 80% Nitrogen and most of the rest is C02. It entirely depends on semantics, but you could say that exhaust is at least 80% air.


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## MarkSF (Feb 21, 2011)

AirborneSF said:


> MarkSF, I have the same thing, one facing forward ducked along the top for about a foot supplying fresh air into the eng compartment, the other faces to the rear and it's hose goes to the bottom, and pulles air out of the compartment.


Thanks... I think that's exactly the info I needed to correct my plumbing.. the boat's that is...


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