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  #1  
Old 03-03-2007
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Engine ventilation

Had two 3" Dorade vents for air to the engine space on the rear deck. Since these areas where saturated with moisture, took them out, and epoxy and glassed the whole area (was mounting dinghy davits so wanted the whole area strong).

My question is, how much air intake do I need for my 13 hp Yanmar? The whole area under the cockpit is open (no engine enclosure), so any air going in will be available for the engine. I was thinking of putting a flat ventilator grate on the outside of the rear commining. Would this allow sufficent air intake?

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Old 03-03-2007
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Beezer,
I would suspect, as you do, that the engine has access to plenty of air with existing set-up. The question is more of how much ventilation is going to happen down there with the vents removed. You might end up with foul air that lingers, and may even be low on oxygen. Diesel fumes in an enclosed space, aside from being unpleasant, can displace oxygen and cause you to pass out down there. They will accumulate low in the hull first.

I know that does not answer your question as to what to replace and how. Ohers with more experience will surely respond.
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Old 03-03-2007
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SA, I guess you mean exhaust fumes? Shouldn't they be sealed and all go out the exhaust system and never accumulate in the boat? Ok, I know a following wind at the speed of the boat will accumulate exhaust fumes, but under normal circumstances not a problem.

Back on the original question, the engine needs the CFM required by spec for operation with no restriction.
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Old 03-03-2007
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Jouvert--

I believe he is talking about air intakes for the engine space.. Diesel engines require a fair amount of air to operate properly. In one of the books by Nigel Calder or Don Casey, they have a formula for the size of the vents needed based on engine HP.

If the engine space doesn't have enough proper ventilation, it will draw air from the passenger compartment, storage lockers, and bilge, and will not run as efficiently as a properly ventilated engine would.
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Old 03-03-2007
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I think you can find info on how much air is sufficient in the literature for a 13 HP engine. As other said, any air restriction can lower efficiency but another equally important concern is temperature. High temperature is not good for some of the engine components, specifically the alternator and others. Having a decent air intake will have the added advantage of lowering the temperature in the engine room and the ambient temp of the rest of the boat as a result.
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Old 03-04-2007
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Hi PB,

Yes I would think that would work fine.
May I suggest putting one on each side as high on the coaming as you can. It doesn't really matter where.

The reason being is that the engine compartment gets very hot just from the sun. The two vents will allow some cross ventilation.

Last year I fried my refrigerator and new high-tech battery charger just by leaving the boat in the sun. I put a 3 inch round vent on each side (2 feet forward from the Genoa winches) and a little 3" fan behind one and dropped the temperature 30 degrees. My refer and my charger are fine now (got them replaced under warranty).

Cheers,

Todd
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Old 03-04-2007
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The vents you removed were USCG required equipment for bilge ventilation under a gasoline engine. I do not believe they are required for a diesel, but in any case, they were never put there to supply combustion air to the engine.

Your little Yanmar will get plenty of air from inside the boat unless you are VERY well sealed up. Even one of your dorade vents is more than adequate.

Remember also that the diesel is continually ventilating the engine space when it is running (and consuming induction airflow), so there won't be any "fumes" there unless you have an exhaust or fuel leakage problem.

As for diesel "fumes", the great thing about diesel is that it isn't gasoline and does not vaporize very well. If you smell it it just means that you need to fix some fuel connections. It won't blow up the boat.

Last edited by Goodnewsboy; 03-04-2007 at 10:14 PM.
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Old 03-04-2007
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PBeez you'll know right away if your not getting enough air you will start getting black exhaust smoke and the hull of your boat will soot up around the exhaust over board. Bad thing about intakes outside they will get water in them. my boat has a small vent on one side of the engine cover\steps down low its maybe six by six and one small hole maybe 1.5" on the back of the cockpit coaming with a clamshell on it. motor is 56 hp. If you can make your engine compartment draw like a fire place- one vent to ouside that is higher than intake then cool air gets pulled into the compartment as hot air goes out
(with the engine off of course)the high vent. the compartment will stay dryer and cooler. I wouldn't want the vent outside to be very big at all.
just a thought
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Last edited by soul searcher; 03-04-2007 at 11:55 PM.
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