Boat Health Think about how hot the interior of your car gets when parked outside on a summer day. Similar heating takes place inside your boat every day—a reality you are no doubt well aware of. Sure the interior cools down when you open the hatches, but most days the boat remains closed. The buildup of heat that inevitably occurs—day after day after day—is not doing your boat any good.
Why is this so? Because the hotter the air, the more moisture it can hold. Water in the bilge vaporizes—like cloud formation—and on a hot day the air inside your boat can be as much as three times as wet as that outside. Even if your bilge is bone dry, the heating and cooling cycle of an inadequately ventilated cabin acts like a heat pump. The warming air sucks in moisture from the outside, which condenses out when the cabin cools at night. A few days of this cycle and the interior of your boat is as wet as a rain forest. Believe me, this is doing damage to your boat.
Your boat needs some way of venting that moist air, even when it is sitting in the slip with all hatches and portlights dogged down tight. But just a single vent is inadequate to let the cabin “breathe.” In a house, you'd open two windows to get cross ventilation, and the same is true aboard a boat; you need at least two well-separated ventilators so wet air can flow out as dry air flows in.
Two vents of any type beat no vents at all, but the most efficient passive vent is the cowl vent—a vertical pipe with a bell-like horizontal opening. Standing proud above the deck and facing into the wind, this type of vent funnels a great deal of air below, but it can also admit rain. Facing away from the wind, the cowl vent becomes a powerful extractor. For a boat on a mooring, a single cowl facing aft, used in concert with some other rain-excluding opening—perhaps a louvered hatch board—can do an admirable job of exchanging the air in a closed boat.
A notable alternative for closed-cabin ventilation at the dock or on a mooring is the solar-powered ventilator. Using an integral solar panel to run a small fan during daylight and, in some units, to recharge internal batteries that keep the fan running after dark, a single solar vent, paired with a cowl ventilator or a sizable louvered vent, can effectively exchange the entire volume of air inside the typical sailboat 10 to 20 times 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.
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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