In most climates, boats that have been closed for a period of time will become "stuffy." The temperature differential that develops between the boat cabin and the outdoor environment will cause the humidity level inside the boat cabin to rise. When the humidity level in the closed cabin rises, moisture forms, resulting in mold, mildew and musty air. In these conditions, fabrics, boat electronics and even the boat's fiberglass structure can be damaged. Maintenance ventilation is defined as the amount of ventilation required to reduce or eliminate the growth of mold, mildew and musty air in the unattended boat cabin. In general, replacing the air at least once per hour is recommended.
When intake and exhaust airflows are combined, cross flow ventilation is achieved. A cross-flow ventilation system will make the interior cabin more comfortable and pleasurable because air is constantly moving throughout the cabin.
Most boaters are unaware of the hazards that exist in the boating environment. Any number of serious ventilation problems can occur when a boat is left unattended. And even if you are underway, these problems don't go away. Fumes and gasses, ranging from carbon monoxide to hydrogen gas from battery charging, to gasoline, propane or solvents stored below are unpleasant, and can sometimes be dangerous. Also, any open flame devices in the cabin require proper ventilation.
There are two answers to this question – "You cannot have too much," and "Even a little ventilation helps a lot."
Ideally, the air in the interior of a boat should be circulated once every hour. A typical 30-foot boat contains approximately 800 cubic feet of air.
Virtually all boats have hatches, however, in most cases hatches must be secured when the boat is left unattended. Often when underway, hatches must also be closed tight. In both of these scenarios, vents can effectively perform your boat's need for ventilation. One great suggestion is to mount a vent in the hatch. This installation is easy and does not interfere with the normal function of the hatch. Vents can also be installed on the boat deck.