Because hotter air can hold more moisture, the air inside a sealed boat on a summer day will be about three times as wet as the air outside-air that in the summertime at the shoreline is likely plenty humid to begin with. This steamy air permeates everything inside the cabin, fostering rot, corrosion, and mildew that are damaging to boat, gear, and locker contents.

Moisture trapped inside the boat even contributes to saturation of the fiberglass-the cause of hull blisters.

The most obvious source of interior moisture is water in the bilge, but even when the bilge is bone dry, the air inside an inadequately ventilated boat will still be wet. The daily heating and cooling cycle 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 soggy as a rain forest. This is doing damage to your boat.