Mariner's Hardware - Boat Ventilation

(Ventilation Basics Cont)

each day. This will lower the interior temperatures and prevent moisture build-up inside the cabin. Solar vents exclude rainwater, but they will admit green water, making the Dorade a better choice for venting a boat while underway.

Crew Health When you are aboard and you must have the boat closed—either due to weather or sea conditions—it is imperative to still have fresh air flowing through the boat. Few things are more uncomfortable than a sealed-up boat when the rain is coming down in buckets and the cabin temperature is in the 90s. Not only does the boat smell like a locker room and mildew sprout on boxers and bulkheads alike, but also the lack of air makes the crew groggy, lethargic, and even seasick. The danger can be even worse in cold weather if you are trying to heat the boat. Carbon monoxide poisoning is a clear and present danger in these situations.

According to the author, the best way to keep air flowing through the interior when you're out in nasty weather is a cowl vent like the one above, mounted atop a Dorade box.

So how do you keep the air flowing when the weather is boisterous or stinko? The best answer remains cowl vents mounted on Dorade boxes. These admit air but exclude rain and most green water. The question

you must answer is how much ventilation do you need to keep the boat fresh and the crew healthy? There is a simple rule of thumb—authored by Sparkman and Stephens—for determining just how much ventilation is enough. The total ventilation area in square inches should be approximately equal to the boat’s waterline in feet multiplied by its beam in feet. For example, a boat with a 25-foot waterline and a 10-foot beam needs 250 square inches of vent area. We determine the vent area at the mouth of the vent, so even though two vents fit the same diameter deck plate, the one with the larger bell provides greater ventilation.

If you run the numbers for your boat, you quickly realize to provide adequate ventilation for an inhabited boat, you are almost certain to need more than a single pair of vents. For example, four-inch cowls with eight-inch bells provide just 50 square inches of ventilation. We need five of these to ventilate a closed boat. If five Dorade boxes seems a bit much for the deck of a boat with a 25-foot waterline, you can opt for larger cowls. Three five-inch cowls (with 10-inch bells) gets you close to your target amount of vent area. Also, you can likely get away with omitting the Dorade box for cowls opening into the chain locker or the lazarette, where passing an occasional dollop of water shouldn’t be a problem.

The lesson here is that if you are going to be aboard in inclement weather, you almost certainly are going to need more ventilators than your boat has, and probably more than you would prefer to install. But if you cut corners here, I promise you the day will arrive when you will wish you had not.

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