Ventilation 101

Posted: August 1, 2013

Nothing enhances the quality of life belowdecks during the heat of summer or when cruising hotter latitudes like plenty of cool, fresh air.

By: Capt. Frank Lanier

Nothing enhances the quality of life belowdecks during the heat of summer or when cruising hotter latitudes like plenty of cool, fresh air. Good ventilation provides a multitude of benefits, from prevention of odors, condensation and mold growth to the elimination of minor carbon monoxide buildup and its negative effects on crew health (such as headaches and seasickness).

Passive or Active

There are two types of ventilation systems: passive and active. Passive systems rely on the wind blowing over them to move air belowdecks. They either direct air inside or exhaust it (depending on their type and orientation to the wind). Examples of passive vents include traditional cowl vents, clam-shell or scoop vents, louvered hatch boards and low-profile discs (aka mushrooms). Passive vents work best when installed in opposing pairs, and while they can move a surprising amount of air on a breezy day, they don’t perform well on days with little or no wind.

Active systems are typically mushroom vents outfitted with a small electric fan installed in the vent body. Some are powered by your boat’s 12v DC system; however, most are solar powered. Many of the solar-powered units contain a rechargeable battery (to facilitate nighttime operation) as well as interchangeable fan blades, allowing them to be used as either an intake or an exhaust.

Ventilation Basics

Efficient air flow requires not only an intake but an exhaust as well. A single intake can’t force air into a boat against pressure any better than a single exhaust can remove it against a vacuum.

Assuming a boat is oriented head into the wind (as is typical while at anchor or on a mooring), it’s pretty intuitive that opening a forward-facing hatch channels air belowdecks, much like an air ram. Conversely, an open companionway door or an aft-facing hatch acts as an extractor, pulling air from belowdecks as the wind passes over it.

Adding a wind chute to your hatch provides even greater funneling ability. The same goes for portlight scoops, which help deflect those cooling breezes down below through vertical portlights.

Boats at the dock face additional ventilation challenges, as they can’t orient themselves to the wind. Wind chutes can be repositioned to the side or rear of a hatch to help funnel air belowdecks, while some traditional hatches can be opened on all four sides (a boon in this situation).

Other challenges occur during rainy or stormy weather, which can prevent boats from leaving hatches and portlights open while in port or under way. A tarp or a canvas cover rigged over a partially open hatch keeps rain out and provides ventilation, but the air flow will be less than it would with a fully opened hatch or one using a wind chute (which typically can’t be used in conjunction with a tarp). One innovative solution to the tarp or wind-chute dilemma is the Hatch Hoodie Wind-Scoop Awning from Banner Bay Marine (, which combines the protection of a hatch awning and the functionality of a low-profile wind scoop.

How Much Is Enough?

A well-designed ventilation system should exchange the air belowdecks roughly once every hour. Let’s say you have a midsized boat with an interior volume of about 1,400 cubic feet. Airflow ratings for passive vents range from 350 to 600 cfm (cubic feet per minute). As intake volume should equal output, a 1,400-cubic-foot boat would need a minimum of four similar-sized vents (two intake, two exhaust) to provide adequate ventilation.

In a four-vent scenario, two active vents should be matched with two passive vents, but four passive vents are OK, because they will automatically adapt to intake or exhaust mode as needed (with the exception of improperly aligned cowl vents).

Vents should be arranged to provide as much cross flow as possible inside the cabin. Passive vents should be mounted in pairs at opposite ends of the boat (to the extent possible) with one facing forward and the other facing aft, which provides an intake and an exhaust, regardless of wind direction). A combination of passive and active vents provides the same effect with varying wind directions (or in the case of no wind at all).

While getting fresh air belowdecks is important, it’s only half of the ventilation battle. In our 1,400-cubic-foot example, four vents may technically provide enough fresh air, but the space itself will be broken into separate cabins or compartments that can restrict airflow throughout the boat. As such, simply installing the correct size and number of vents may not be enough to get the ventilation job done. Equally important is moving that fresh air into all the low-ventilation spaces.

Drawers, hanging lockers and other such closed storage compartments will benefit from the use of louvered doors or vent grilles — just remember, these spaces require a way for air to both enter and exit in order to provide good cross ventilation.

Cabins and heads should also have some kind of ventilation installed, particularly if they can be closed off from the rest of the boat. For these and other such living areas, nothing improves ventilation like a few well-placed electric fans. They make a hot bunk bearable, remove heat from the galley, and help reduce odor and moisture in the head/shower area. Unlike the noisy, power-hungry units of yesteryear, today’s fans are both quiet and efficient.

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