If you sail your boat just on weekends, maybe you don't give much thought to ventilation. Opening the hatches and portlights (if your portlights open) when you are on board seemingly provides plenty of ventilation in good weather. When the weather is bad, you're usually not aboard. However, if you plan to use your boat in more than just fair weather, the level of ventilation found in most production boats will quickly prove to be inadequate. Even if longer-term sailing is not in your immediate future, improving closed-boat ventilation can be significantly beneficial to the health of the boat. Let's look at this latter aspect first.
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.
Water-saturated air is a potent gas that permeates nearly everything in the interior of a sailboat. The visible effect of this dampness is mold and mildew below, but as damaging as these fungi can be, they are not the most serious consequences. The moisture fosters rot in unprotected interior wood, accelerates corrosion, degrades electrical wiring, and even contributes to saturation of the fiberglass—the cause of hull blisters. Simply stated, wet air ages your boat prematurely.
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.
For a boat in a slip, facing the cowl aft will not exclude rain since the wind is just as likely to blow from that direction. In this case, the cowl vent will need to be mounted on a water trap or a Dorade box, either of which will prevent rain from coming below. A pair of cowl ventilators, each mounted on a water-trap Dorade box (with one facing forward and one facing aft), is even more effective. In nearly all conditions, this configuration sets up a beneficial flow of air in one vent and out the other.
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.
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.