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Ventilation: move that air

All boats need ventilation, but most don’t get it. Proper ventilation is important for both the maintenance of your boat and your personal comfort. Most of us with boats that have enclosed living spaces control the comfort side by using fans and air conditioning, but when it comes to the maintenance aspect things can get a bit sketchy.

The last thing we do before leaving our boats is to secure and lock the hatches and doors or put the cover on. This is when problems begin. As a result of the difference in temperatures between water, air and hull surfaces, the unattended boat is constantly generating moisture inside, promoting an ideal environment for mold, mildew and musty air. These temperature differences vary as the day progresses, and the process is accelerated in humid climates.

Boats that live on trailers in the backyard should be covered and, as such, also require vents. Proper ventilation is more than moving air. It should equalize the humidity level inside and outside the boat, preventing damage to interior fabrics, electronics, wood, metal and fiberglass.

As stated above, ventilation can be broken down into two categories: maintenance and comfort. For the purpose of this article let’s deal with maintenance type, which generally means replacing the air inside the boat at least once per hour. Maintenance ventilation can be broken down further into “active” and “passive.” Active vents typically are mechanical in nature — using fan blades to move air — and are powered either through the boat’s 12-volt system, solar array, internal batteries or a combinations of these. They force outside fresh air into the boat or exhaust stale air from it. Passive vents rely on wind and air currents to circulate air, without moving parts.

The most basic passive vent is a louver. Typically made of either stainless steel or plastic, they are inexpensive, easy to install, and available in a variety of shapes and sizes. Louvers typically are installed in lockers, bilges, deck boxes, lazarettes and behind refrigeration and electronic equipment. They allow compartments to be securely closed, yet still allow air to flow. If the door to a stateroom or cabin is normally closed, it should be vented for intake and exhaust, just like cabinetry, drawers and all closed compartments. When venting compartments, I install one louver high and one low to promote air circulation through convection.

Cowl and clamshell vents are commonly used to vent bilges or as intake air supplements for active vents. They are available in several shapes, sizes and profiles. Cowl vents can be connected to 3- and 4-inch-diameter hoses so the air flow (intake or exhaust) can be directed as needed. The cowl vent’s large bell-like opening can be positioned to face the breeze, and these are capable of funneling large volumes of air into the boat. When facing downwind, they create a draft as the breeze passes by, exhausting air from the inside of the boat.

Some cowl vents can provide a path for water intrusion, which, depending on their location, may create a problem. Dorade boxes provide a mounting surface for cowl vents and incorporate either a water-sensitive valve that opens or closes automatically to let air in and water out, or have baffles and drains built in to eliminate water intrusion below deck. Cowl vents that are well-positioned and installed on dorade boxes provide great passive ventilation.

Any vent that allows air movement to or from the boat’s interior should be fitted with screens. You can find cowl vents and accessories from West Marine (, Nicro (, Plastimo (, Beckson (, Perko ( and others. Prices range from about $30 to several thousand dollars, depending on size and construction materials. They are favored by traditionalists for their salty appearance. You will always find cowl vents on large commercial ships.

Another form of passive ventilation is a round cabin/hatch vent such as those from Nicro, Lewmar (, and Beckson. They require round cutouts of 3 to 4 inches in diameter and can be installed on almost any horizontal surface. This style vent isn’t as efficient as a cowl vent, but it is much less obtrusive, projecting only an inch or two off the deck. They typically move 200 to 600 cubic feet per hour and usually have shut-off dampers. I have priced passive hatch vents at around $25 to $55. If I were considering this type of installation, I would spend the extra money and opt for active vents of similar configuration.

When it comes to active ventilation, the most common product for recreational boats is offered by Nicro. Nicro has been in the ventilation business for more than 30 years and offers a line of powered ventilators that work well and are straightforward to install. The cabin/hatch vent described above has been modified and is now outfitted with an electric fan and interchangeable blade that allows you to select either intake or exhaust. These ventilators are available with solar, battery or 12-volt DC power, and provide around 700 cubic feet per hour to 2,000 cubic feet per hour of air flow, depending on the model. An advantage of the Nicro system is most of its installations require only a 3- or 4-inch-diameter opening for the snap-in base plate. Once installed, the base plate accepts almost all of Nicro’s products and accessories, from a 3-inch PVC cowl vent to a two-speed 12-volt DC fan.

How much ventilation does your boat require? You cannot have too much, and even a little helps a lot. Nicro says a typical 30-footer contains approximately 800 cubic feet of air. To calculate your boat’s air volume, multiply interior length times beam times average height times 70 percent (0.7). Look at available product ratings as a guide, then go bigger. It is best to have vents at both ends of the boat — one for intake and one for exhaust. Where my boat is docked, the bow gets the majority of the breeze, so I use the forward ventilator as intake and the aft as exhaust.

Covered boats require ventilation, as well. Unfortunately, my Boston Whaler spends much of the week covered in the backyard. The cover is weatherproof and has several passive vents installed. These clever little devices from Airlette Manufacturing ( snap into the flexible cover and have insect screens, along with removable hoods to prevent rainwater from entering. The hoods are easily removed to roll the cover snugly without damaging the vents. I’ve also installed them on the cover for my RIB, which is on davits at the stern of my trawler. Everything stays dry, clean and well-vented. Without the vents in the cover, there was always a mold buildup inside the boat after a week or two. At less than $10 each, they save a lot of scrubbing and time.

There aren’t any mysteries about boat ventilation, except why more boaters don’t take advantage of the products on the market that can make life aboard a bit better and improve the overall condition of the boat and its equipment.

This article originally appeared in the October 2008 issue.