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Old 08-20-2012
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Diesel engine compartment ventilation

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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Lightbulb Re: Diesel engine compartment ventilation

Quote:
Originally Posted by MarkSF View Post
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?
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.

Last edited by olson34; 08-20-2012 at 01:00 PM.
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Re: Diesel engine compartment ventilation

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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Re: Diesel engine compartment ventilation

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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Re: Diesel engine compartment ventilation

Quote:
Originally Posted by misfits View Post
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?
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.

Last edited by olson34; 08-22-2012 at 11:12 PM.
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Re: Diesel engine compartment ventilation

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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Re: Diesel engine compartment ventilation

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Originally Posted by MarkSF View Post
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?
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 100ļ 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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Re: Diesel engine compartment ventilation

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

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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Re: Diesel engine compartment ventilation

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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