As ventilation experts explored ways to make indoor spaces safer during the COVID-19 pandemic, we became curious about ventilation in our boats. What products and practices ensure that we maximize the volume of air exchange belowdecks, as measured in cubic feet per hour (CFH)?
While this exchange of fresh air provides direct benefit to our health, it also provides an indirect benefit by preventing the growth of mold and mildew. Generally, to successfully combat mold in any given space you need at least one air change per hour, and for boats from 25-40 feet that is about 700-1800 CFH. Good ventilation also helps remove warm moist air created during cooking that can condense on the inside of the cabin—creating the moist environment that mold loves.
We’ve looked at ventilation dozens of time over the past 10 years, but most of those previous studies focused on a particular product group—fans, hatches, vents, opening portlights, or air-conditioning systems. In the September 2020 issue we narrow the focus to passive ventilation—cowl vents, dorades, hatches, “wind scoops,” and other ways to boost the exchange of air on board without any mechanical assistance.
Boats are relatively small spaces, and you’d think that it would be easy to keep interior air fresh with a few ports and hatches. As it turns out, where you install your inlet and exhaust vents is as important as the kind of vent (hatch, port, or vent) you use.
Most sailors know that wind blowing across a uniformly smooth surface such as the leeward side of a well-trimmed sail creates a region of reduced pressure. Just as we can use the resulting suction on the leeward side of a sail to pull the boat forward, we can use pressure differentials in the air surrounding the cabin to maximize the ventilation belowdeck.
Understanding the pressure differentials created by the flow of wind over our boat’s deck is vital to the success of any passive ventilation scheme. Mapping this flow (see the below image), helps explain why some areas of the boat seem stuffier than others. It also explains why passive ventilation methods did so poorly in our testing.
Your boat’s cabin trunk has a number of pressure zones, and these zones change depending on the wind direction. At anchor, the front of the cabin trunk is an area of high pressure, and this pressure reduces slightly as you move aft. The pressure differential between locations along the cabin top on depends many factors, including the shape of the cabin and any items on deck that might interfere with flow. In general, the pressure is lowest just behind the front edge of the cabin trunk, slowly rising to neutral pressure as you move aft—depending on the extent of turbulence caused at the front of the cabin trunk or by other interruptions in the smooth surface that would disrupt the flow of wind.
The companionway area has the lowest pressure. Open a hole anywhere forward and air will be sucked out the companionway. In a breeze, the pressure differential is usually so great that air will push out through gaps in the weatherstripping, escaping even if the companionway hatch closed.
Maximizing air-flow through cowl vents is trickier than it might seem. Air-flow can be interrupted not only by insufficient pressure differential, but also by turbulence. The vertical cabin-sides, dinghies stored on deck, masts, changes in wind direction, and even other cowls can dramatically reduce the amount of wind that reaches the cowl.
The most complicated case is when you keep the boat at a dock with everything buttoned up. The wind can blow from any direction, and depending on the amount of protection afforded by the harbor and neighboring boats, the wind will be light most of the time.
Bottom line: Although many passive vents are rated by CFH, these ratings don’t tell the whole story. Flow will vary greatly depending on the location of the vent, and since the rated CFH is typically based on a 10 knot breeze, the rating won’t give a clear picture of air flow when your boat is at the dock. At many marinas around the U.S., the true wind at deck level is less than 5 knots most of the time.
For more on ventilation, see the September 2020 issue of Practical Sailor. If defeating mildew is your main objective, our eBook The Mildew-free Boat will cover everything you need. If boosting breeze belowdecks is your primary aim, the following related Practical Sailor tests will help you improve the flow of air down below.