Ventilation Basics
By Don Casey

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.

Just cracking the forward hatch as this boat owner has done can help move a lot of air through a boat, but for the times when he or she is away and the boat's battened down, another system is required. Hence the cowl vents and Dorade boxes that adorn the cabintop here.

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.

Without the right gear, it's difficult to keep a boat's interior properly ventilated during inclement weather, and that can plant the seed of future damage to both the boat and the crew's health.