don’t hesitate to renovate | by Capt. Frank Lanier
Shooting the breeze Practicing ventilation basics on board.
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othing enhances the quality of life below decks during the heat of summer like plenty of cool, fresh air. Good ventilation provides a multitude of benefits, from prevention of odors, condensation and mold growth to elimination of 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.
Active ventilation options include solar powered “mushroom” vents as well as wind scoops.
Passive vs. active There are two types of ventilation systems: Passive and active. Passive systems rely on the wind blowing over them to move air below decks. 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 (also knows as “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 wind. Active systems, on the other hand, are typically mushroom vents outfitted with a small electric fan installed in the vent body. Some are powered by your boat’s 12-volt 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 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 bow 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 below decks, much like an air ram. Conversely, an open companionway door or aft-facing hatch acts as an extractor, pulling air from below decks as the wind passes over it. Adding a wind chute to your hatch provides even greater
A wind scoop can direct a huge amount of fresh air below decks.
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funneling ability. The same goes for portlight scoops, which help deflect those cooling breezes down below. 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 below decks, 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 underway. A tarp or canvas cover rigged over a partially open hatch keeps rain out and provides ventilation, although the air flow 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). One innovative solution to the tarp or wind chute dilemma is the Hatch Hoodie Wind-Scoop Awning from Banner Bay Marine (baNNerbaymariNe.com), 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 below decks 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 cfm (cubic feet per minute). As intake volume should equal output, 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 below decks is important, it’s only half of the ventilation battle. In our above example, four vents may technically provide enough fresh air, but the
photos by capt . frank lanier