Why Ventilate your Boat?

Revised by BoatUS editors in April 2012

boat ventilationUnattended 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 and in cold water.

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, including areas between hulls, behind paneling and other enclosed areas that you don’t normally see. When you close off your boat's cabin to “protect” it from humidity, you may 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 into water, 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, as well as make you and your guests sick. 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 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, or vice versa.

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. Solar vents come in different sizes and, obviously, the larger sizes move more air.

Some solar vents also work during the night because they are equipped with rechargeable batteries. They are very helpful 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.

Vents powered by the boat’s 12 volt system are the most powerful topside vents. They are designed for boats with multiple battery systems, an adequate 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.

Opening ports with insect screens provide another excellent way to increase your comfort aboard. You can and should also install screens in just about any exterior vent.

Wind scoops are another inexpensive, non-mechanical, easy-to-stow source of fresh air. These, particularly if attached to a forward hatch, can produce a large flow of air through the cabin and make an otherwise uncomfortable night at anchor quite pleasant.

Cabin Fans circulate air and equalize humidity while you’re aboard. Many models are available ranging from battery-powered portable models to 12vDC fans and convertible fans that are either 12vDC or use batteries.

Bilge Blowers are the most important ventilation system source for boats with gasoline engine spaces or rooms. They should also be used in diesel engine rooms or spaces. Dangerous fumes can accumulate in the bilge, creating a situation ripe for a fire or explosion. High-capacity 12vDC bilge blowers are required to clear fumes before starting gasoline engines.  Many of these blowers are not designed for continuous use. They will overheat and perhaps seize up or cause a fire if used in this manner. If you want to use them the entire time you’re running your engine or generator, be sure to get units that are designed for this.  Also be sure your boat’s wiring and connections are adequate. Passive vents should also be installed to keep the engine room clear while under operation.

Many use air conditioning systems to help control humidity inside a boat. See Installing a Self-Contained Air Conditioning Unit for discussion of that type of unit. Some leave the units on all the time, including while they’re away from the boat. Some air conditioning units have special control features for this purpose.  However, marine air conditioning units typically are cooled by sea water pumped in to the unit by a relatively strong pump and then discharged back overboard. If a hose or other part of the plumbing should give way while you’re gone with the system running, that pump is going to flood your boat with a lot of water fast, causing at least a lot of damage or perhaps a sinking. There are steps you can take to somewhat lessen these risks, at least as to sinking, including extra capacity bilge pump systems and alarms and frequent checking of the entire system and plumbing. What you do will depend on your boat and systems. Many experts warn against leaving an on board pump running all the time when you’re not aboard.

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 for living areas are outlined in the chart below.

Boat SizeRecommended 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

Don’t overlook enclosed spaces mentioned at the beginning.  Unless you have a better method for ventilating these spaces, open drawers and cabinets and other access panels below deck when you leave the boat.

Always beware of the risk of and take caution concerning induction of harmful fumes, exhaust or otherwise. For example, if a boat nearby is running its engine and/or generator, odorless but lethal carbon monoxide may be present in sufficient quantity to be taken into any vents, and capable of causing death.  Also, far too many deaths have occurred caused by onboard generator or engine fumes finding their way inside living areas through vents.

Always keep in mind the likelihood of boarding seas, driving rain and other water intrusion when installing vents.  Dorade vents can be quite helpful as passive vents forward.  If properly designed and installed, water which finds its way into the vent scoop “pipe” is prevented from going below by a built in dam inside the vent box and the water drains out holes in the side of the box (unless you take on too much water). With these vents you can also rotate the vent pipe so that it sucks air out and remove it altogether, covering the hole with a plate when expecting bad weather.  Don’t install vents where people will be walking or in a manner that will allow them to become broken while docking or in other activity.

Diagram courtesy of Nicro Ventilation Systems.

BoatUS Magazine promo