Why Ventilate?

Unattended boats generate humidity and moisture below because of water, air, and hull surface temperatures that are never identical and always changing. While true even in a “dry” climate, this process accelerates in a humid climate.

When you ventilate your boat, you want to do more than simply move air—you want to equalize the humidity levels inside and outside the boat. When you close off your boat's cabin to “protect” it from humidity, you actually cause the humidity level below deck to rise. And no matter where your boat is docked, humidity and temperature levels vary, and there is a differential between air temperature and water temperature that's constantly changing.

When there is a difference in temperature, moisture forms. You see this happen in a rainstorm, when a hot air mass and a cold air mass meet, reducing the humidity in the atmosphere intowater, or rain. Although not as extreme, the same phenomenon is constantly taking place on your boat, due to the temperature difference between the cabin (which is affected by water temperature and heat build-up during the day) and the air temperature outside. It is that humidity (or condensation)
in the cabin that will do the most damage.

Over a relatively short period of time—just a few days or a few weeks at most—moisture inside the boat creates an ideal atmosphere for various molds and fungi to grow. Not only are mold, mildew, and musty air unpleasant, but they can damage your boat's interior wood surfaces,
fabrics, electronics, and metal components. Over the long term, wood rot is an additional potentially costly problem as well.

Active or Passive?
Active systems, like solar and 12vDC vents, push air mechanically with fan blades. Passive systems use wind and air currents, but have no moving parts. They include cowl vents, clamshells, louvers, and vent plates.

Passive Vents are inexpensive systems that can be installed for either intake or exhaust. They're a great choice for small areas like chain lockers or lazarettes.

Louvered Vents, the most basic and least expensive vents, are available in stainless steel or ABS plastic. These are used for ventilating fuel lockers, bilges, deck boxes, lazarettes, or behind electronics. Installed in hatch boards, they can provide intake air flow from the pressure drop created by solar-powered vents.

Solar Vents provide intake or exhaust ventilation without draining your 12-volt supply by using the sun's energy to power a fan. Install two or more, and you can switch them from intake to exhaust and back simply by changing the two fan blades provided. The less expensive models have the lowest cost per cubic foot for air volume moved, and are ideal for extracting heat during the day.
Solar vents come in 3" and 4" sizes. The 4" size moves about 40% more air, and should be used on boats 40' and over.

Day/Night Solar Vents are the most versatile vents, moving more air in 24 hours than any other
solar vent. Equipped with a rechargeable NiCad battery, they store solar energy to work up to 48 hours without exposure to the sun. They are recommended for exhaust in the head, galley, lazarette, sail locker, or deck boxes, and for intake and exhaust in aft and main cabins,pilot houses, nav stations, and forepeaks.

Two-Speed, 12vDC Vents are the most powerful topside vents. They are designed for boats with multiple battery systems, a battery charging system (110-volt, solar, or wind) or shore power, and for boats where vertical installation —usually in the head or galley—is a must.

Combo 12vDC/Solar Vents are ideal for live-aboards and cruisers because they use solar power during the day to preserve battery power, and at night give a higher air flow rate. These are available through our Special Order Center at 800-568-0319.

Opening Ports with insect screens are another excellent way to increase your comfort aboard.
You can install screens in just about any exterior vent. We sell screen kits for do-it-yourselfers, and Nicro insect screens for both 3" and 4" vents.

Windscoops are an inexpensive, nonmechanical, easy-to-stow source of fresh air. Put in a hatch
screen to keep out the bugs.

Cabin Fans circulate air and equalize humidity while you’re aboard. Many models are available:
Battery-powered portable models that won't drain the ship's battery (and can be used in the cockpit or at home), 12vDC oscillating fans, compact models that fold out of the way when not in use, and convertible fans that are either 12vDC or use batteries.

Bilge Blowers are the most important ventilation system source for boats with gasoline engine
rooms. Dangerous fumes can accumulate in the bilge, creating a situation ripe for a fire, or more likely an explosion. High-capacity 12vDC bilge blowers are required to clear fumes before starting gasoline engines. Passive vents should also be installed to keep the engine room clear while under operation.

How Much Ventilation is Required for Your Boat?
Ideally, the air in the interior of a boat should be circulated once every hour. A typical 30’ boat contains approximately 800 cu. ft. of air. The general guidelines for providing maintenance ventilation are outlined in the chart below.

Boat Size              Recommended Ventilation
Up to 24’  ------------------1 Exhaust Vent*
24-40’      ------------------1 Exhaust Vent & 1 Intake Vent
40’+         -------------------2 Exhaust Vents & 1 Intake Vent
*One vent assumes there will be access for air to get into the cabin (i.e., cowl vent, clamshell vent, or louvered panel.) 

To specifically calculate your boat's air volume, use this simple formula:

A x B x C x 70%=Boat Interior Volume

A The interior length (excluding cockpit)
B The boat’s width
C The average interior height

Diagram courtesy of Nicro Ventilation Systems.