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Old 03-03-2007, 12:21   #1
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Engine ventilation

Had to remove the 3" Dorade vents on the rear deck because of saturated core. Filled the holes with glass and epoxy. My question at this point is how much air intake do I need for a 13hp Yanmar? There is no enclosed engine compartment, everything under the cockpit is open. Was thinking of a flat ventilation plate on the back of the rear coming, but not sure if that will be sufficent, or if I even need anything. (There are also two 5" cockpit speakers.)

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Old 03-03-2007, 13:35   #2
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The rule is 10-12 CFM per HP.
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Old 03-03-2007, 14:38   #3
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Another rule-of-thumb (I actually hate rule-of-thumbs because they seldon work well) is one square inch of non-mechanical ventilation area for every cubic foot of engine compartment. I know you said you didn't have an actual engine comartment, but you will still want to have enough ventilation to keep air moving in the area the engine is in.

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Old 03-03-2007, 14:40   #4
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Heh, another "rule of thumb" is to keep the Air intake as far away as practicle from the exhaust so as you don't suck black carbon back into the engine room.
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Old 03-03-2007, 14:51   #5
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I would think

that the vents were originally put in to help dissipate the heat a little.

On most boats the air will circulate from the aft to the front internally. With the vents on the transom the air would go in those vents (even if they're facing aft) and come out through the gangway, just as mine did before I fully enclosed the motor compartment. I run a bilge blower now with the motor running. If it goes bad I can still pull a cover loose and go back the way it was.

Motors do tend to put off a little oil vapor when running, even if they are new. Personally I would reinstall new vents to allow the boat to breathe. Otherwise you may start collecting a residue on the interior and if someone were down below while motoring, the only fresh air they would get is what the motor is drawing in. Even with a front hatch open the vapors would be working their way out that forward hatch.

Putting a front scoop on the forward hatch will change the direction of flow but it will gather sea spray as well.

One big problem power boaters have is exhaust fumes coming into the cabin area from the rear exhaust ports. Fortunately, sailboats are fairly aerodynamic and scoop away most of the fumes.
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Old 03-03-2007, 15:37   #6
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