Boats need good air flow, particularly when closed up at dock (to expel moisture that will condense to cause mildew, rot, and corrosion) but also when battened down while underway in bad weather (to keep the crew healthy), and when at anchor. Besides discharging moist air, ventilation cools the boat by expelling heat, and this reduces evaporation from the bilge. Few production boats have adequate ventilation. The amount needed is the waterline length, in feet, times the width, in feet, with this product expressed in square inches. It is measured at the mouth of the vent, not at the deck opening, which is why most vents are somewhat flared.
Cowl vents mounted above dorade boxes are best, followed by solar powered fans that include a rechargeable battery to run the fan at night. Lacking dorades, cowl vents can have mushroom shaped valves that screw down to close and will shed water even when open. They are often installed in pairs with one facing forward and one facing aft. Solar fans come with caps that can be fitted in bad weather, and unlike cowl vents that can admit air or discharge it, solar vents are intake only or discharge only. Regardless of type, at least two vents are needed, one to let air in and one to expel it, and they should be as far apart as possible.
At anchor, the flow of air through a boat is aft, moving out the companionway hatch. A wind scoop rigged from the jib sheets above the forward hatch will increase the air flow when anchored. At dock, in contrast, the air flows in the main hatch and moves forward through the
boat. Installing a louvered vent in the hatch, or perhaps making a secondary hatch or weatherboard with a louvered vent, is an easy way to increase ventilation while docked. The vent can be as simple as drilled holes, angled to keep rain out. When hooked up to shore power, running an electric fan inside the boat will keep air moving to reduce condensation and discourage mildew. A small electric heater can be left on in cold weather, with its thermostat set low, provided that it is plugged directly into a shore power cord without being powered through the electrical panel. (Running an electric heater through the panel will destroy the panel eventually and likely cause a fire.) Choose a heater that has its own circuit breaker to avoid relying solely on the breaker in the shore power pedestal. Keeping the forward hatch cracked open perhaps half an inch while docked can improve ventilation in all but the wettest climate. A dinghy or a tarp lashed on deck above it will exclude rain from the hatch. If security is an issue, attach a long bar to the hatch dogs using hose clamps with their screw heads facing aft.