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Boat Ventilation

When I walk the docks on a weekday, I see far too many closed-up boats without any signs of ventilation. If yours is one of them, you should know that stagnant air in the cabin is not doing your boat any good. 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.

In AND Out

Every boat with a cabin should have at least two ventilators to let the cabin "breathe" when hatches are closed and portlights dogged. One ventilator isn't enough. Like opening two windows in a house to get cross ventilation, you need at least two ventilators so wet air can flow out as dry air flows in.

The simplest of vents is the louver. Overlapping slats admit air but exclude rain. You can create a louvered ventilator in a companionway door or drop board with a series of parallel horizontal saw cuts made with the blade set at about a 30° angle. Or you can cut a square or rectangular hole in the same door or board and screw an inexpensive metal louvered vent over this opening to deflect rain.

Another simple vent is the so-called clamshell. In this case, the vent opening is a round hole cut with a hole saw. The clamshell vent fastened over the hole serves as a hood to keep out rain. Clamshells are often used to shield the outlet opening for a bilge blower. Clamshell vents are generally less satisfactory on horizontal surfaces than on vertical ones.

Traditional Boat Cowl Ventilator

The traditional boat ventilator is the cowl vent, a vertical pipe with a bell-like horizontal opening. A large-mouth cowl vent standing proud above the deck and facing into a breeze can funnel a great deal of air below. Unfortunately, cowl vents also admit rain. The solution for a boat on a mooring is as simple as facing the vent aft. While it no longer "catches" the breeze, an aft facing vent becomes quite effective at extracting air. In concert with a second opening — a louvered companionway, for example — an aft-facing cowl does an admirable job of exchanging cabin air.

Mounted Cowl Vent on Dorade Box

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 should be mounted on a water trap or a Dorade box, either of which prevents rain from coming below. A trouble-free method of providing excellent closed-cabin ventilation on any boat is a pair of water-trap-mounted cowl ventilators, one facing forward and one facing aft. In nearly all conditions, this configuration sets up a beneficial flow of air in one vent and out the other.

For continuous closed-cabin ventilation, the ideal ventilator may be one with a solar-powered fan. Powered vents use an integral solar panel to run the fan during daylight and, in some units, to recharge internal batteries that keep the fan running after dark. Paired with a cowl ventilator or a sizable louvered vent, a single solar-powered exhaust vent can extract a volume of air every hour equivalent to the entire interior volume of a 30-foot boat. Not only is this good for your boat, but it makes going aboard on a hot day noticeably more pleasant.

All vent installations are essentially the same, whether louvered, clamshell, cowl, or solar. You determine the appropriate location, cut the appropriate opening, carefully seal the edge of any core material your cutout exposes, and mount the vent over the hole. Where the vent opening is exposed below, a trim ring completes the installation. More detailed instructions can be found with Installing Hatches and Deck Plates. For the most effective ventilation, be sure there is plenty of separation between intake and exhaust vents.

Ventilating lockers

Getting fresh air into the cabin is the essential first step, but you also need to ventilate all the closed compartments inside the cabin. It is for ventilation that locker doors on boats are often louvered or given cane inserts. Solid locker doors should, at the very least, have a pattern of slits or holes or a fancy cut-out. It is usually a good idea also to cut vent holes at the back between lockers to give each locker that "two window" configuration that fosters good airflow.

When you are off the boat, leaving cabin and locker doors partially open is the easiest and arguably the best way to ventilate enclosed compartments. Of course, unlatched doors are at risk of swinging and banging with the motion of the boat, but the solution to that is easy and inexpensive. Simply install a latch hook on the inside of the door frame and screw the eye to the inside of the door. This lets you latch the door slightly ajar. If a standard latch hook doesn't hold the door open enough to satisfy you, it is a simple matter to make longer latch hooks with a couple of screw eyes and a length of stiff wire.

Latching all the lockers ajar when you leave the boat takes only a minute, and it admits not only air, but mildew-slaying light into the locker. Try it. You'll like it.

Keeping Cool

When you are aboard you will often need more air flow than a few deck vents can provide. Opening portlights help, but the big gun is the deck hatch. The more deck hatches, and the bigger they are, the cooler it will be below decks.

When the wind is light, a fabric wind scoop rigged in the hatch can send a delightful breeze through the boat's interior. When the anemometer cups stop spinning altogether, a few strategically placed fans may be all that stands between you and discomfort. Of course, you could also install air conditioning, but that is another subject.

When the wind is light, a fabric wind scoop rigged in the hatch can send a delightful breeze through the boat's interior. When the anemometer cups stop spinning altogether, a few strategically placed fans may be all that stands between you and discomfort. Of course, you could also install air conditioning, but that is another subject.


Don Casey

Contributor, BoatUS Magazine

Don Casey has been one of the most consulted experts on boat care and upgrades for 30 years, and is one of the BoatUS Magazine's panel of experts. He and his wife cruise aboard their 30-footer part of the year in the eastern Caribbean. His books include Don Casey's Complete Illustrated Sailboat Maintenance Manual, and the recently updated This Old Boat, the bible for do-it-yourself boaters.