Get Air Down There: How To Improve Ventilation On Any Boat

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 including preventing odors, condensation, and mold growth as well as eliminating minor carbon monoxide buildup and its negative effects on crew health (such as headaches and seasickness). Here's a look at ventilation basics and how to make them work for you.

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, clamshell or scoop vents, louvered hatch boards, and low-profile discs (aka mushroom vents).

Passive vents work best when installed in opposing pairs. 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.

Passive vents work best when installed in opposing pairs. 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, which allows them to be used as either an intake or an exhaust.

Ventilation Basics

Efficient airflow requires not only an intake, but also an exhaust. 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 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.

If your boat has limited or no portlights, consider swapping out some of the “deadlights” (non-opening portlights). Portlights (which open to allow airflow) come in a wide variety of shapes, sizes, and frame materials (e.g., plastic, bronze, aluminum, stainless steel). As such, it is typically easy to find units to replace existing deadlights. When buying portlights, be sure to match the material of the new units to the remaining deadlights or existing boat hardware to prevent the nautical equivalent of an eyesore. Installing portlights, exhaust vents, or even additional hatches over cooking areas can also make a big difference in air quality belowdecks.

Boats at the dock face additional ventilation challenges as they can't orient themselves to the wind. Wind chutes can be repositioned to the sides or rear of a hatch to help funnel air belowdecks, while some traditional hatches are hinged or pinned so that they can be opened to face any of their four sides (a boon in this situation).

Other airflow challenges occur during rainy or stormy weather, which can prevent boats from leaving hatches and portlights open while in port or underway. A tarp or canvas cover rigged over a partially opened hatch keeps rain out and provides ventilation, although the airflow will be less than that of a fully opened hatch or one using a wind chute (which typically can't be used in conjunction with a tarp). The best solution here is a unit that 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 mid-sized boat with an interior volume of around 1,400 cubic feet. Airflow ratings for passive vents range from 350 to 600 cubic feet per minute (cfm). Intake volume should equal output, so based on the above we would need a minimum of four similar sized vents (two intake, two exhaust) to provide adequate ventilation.

If using two active vents, they should be matched with two passive vents. If four passive vents are installed, 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 inside the cabin as possible. 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 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).

Move It Around

While getting fresh air belowdecks is important, it's only half of the ventilation battle. In our example above, four vents may technically provide enough fresh air, however the interior will be broken up into separate cabins or compartments that can restrict airflow throughout the vessel. So 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 closed storage compartments will benefit from 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 cabin fans. They make a hot bunk bearable, remove heat from the galley, and help reduce odors and keep moisture down in the head/shower area. Unlike the noisy, power-hungry units of yesteryear, today's fans are both quiet and energy-efficient.

My recommendation is that every bunk have a dedicated fan — two if needed due to size. It's almost as if just the soft whirring sound of the fan alone makes the bunk feel cooler. Although many boats suffer from less-than-ideal ventilation, improvements can be made to most any vessel fairly easily and at minimal expense. The effort spent constructing a hatch hood or installing a few fans will pay cool dividends for years to come.