Ventilation

By Susan Canfield

When moist warm air is cooled, it drops its moisture. Outdoors, this moisture is called rain. Inside a boat, the moisture that collects on hull and cabin surfaces is called condensation or sweat. However, when condensation gets so heavy it begins to drip (typically above your bunk), you can call that “rain,” too.
The solution would appear to be simple. Either you keep the air in your boat's cabin from becoming moist or you keep the moist cabin air from being cooled. But the devil is in the details.
Just consider how air inside a boat becomes moist or humid. First, there's ambient humidity outside the boat, which can vary from say 30 percent to 100 percent. Then there's the moisture produced when cooking or showering. Unvented (no chimney) fuel-burning appliances, such as galley stoves and cabin heaters, can put a gallon or more of moisture into the air for every gallon of fuel they burn. There may also be standing water in the bilge. And, of course, there's our own breathing.
High humidity and condensation – in the absence adequate ventilation – will damage electronics and cause fabrics to deteriorate, metal to corrode, paint to peel and wood to rot. Fortunately, humidity belowdecks can be minimized by managing ventilation in a number of ways – most of which are cost efficient. The key is to have a strategy.

BATTLING HUMIDITY
One of the best ways to battle humidity belowdecks is to keep the bilges dry – and I mean completely dry! If your boat's bilge pumps don't discharge all the water, use a bucket and sponge or a Shop Vac to remove what's left behind. After doing this for a few weeks, you will have a much better idea just how much water is leaking in, and perhaps be motivated to track down its source.
Then you can eliminate the leaks by rebedding deck hardware and thru-hulls, recaulking ports and windows or whatever else may be required. Your reward will be a dry, sweet-smelling boat, which is no small achievement.
You can also use passive and active (mechanical or powered) ventilation as needed to bring in fresh air and to exhaust stale air. Proper ventilation is essential in hot weather to moderate cabin temperatures and, in cold weather, to circulate heat throughout the boat.
Effective ventilation requires at least two openings to the outside: an intake (add to) and an exhaust (remove). In the absence of either, you'll never get proper airflow. Ideally, the intake should be at one end of the boat and the exhaust at the other.
Suggested placement of passive and active vents: (top) Underway, even in foul weather, clamshell vents circulate air through the engine compartment while active intake and exhaust solar vents move air throughout the cabin. (bottom) Passive cowl and solar vents move air throughout the entire cabin. Both layouts benefit from opening deck hatches and interior fans.
Generally speaking, if a hatch is positioned so the airflow outside the boat is deflected downward, it will bring air inside (intake). If positioned to deflect the airflow outside the boat upward, the hatch will create a partial vacuum and air will be sucked from the boat (exhaust). Opening ports, windows and doors function as intakes or exhausts, depending on their location relative to the airflow outside the boat. Cowl vents, which can be turned to face in any direction, can also serve as intakes or exhausts as needed.
PROPER VENTILATION
My 37-foot sailboat was designed for offshore use. It has three hatches (including a large butterfly hatch and louvered companionway doors), 11 opening ports and four Dorade vents. Consequently, there are many ventilation options available. I lived aboard comfortably, winter and summer, for more than 10 years.
In my work as a marine surveyor, however, I have seen many boats that have little or no provision for passive or active ventilation. When hatches, ports, windows and doors are closed, there is no air movement absent an air conditioning (or heating) system.
Fortunately, there are a number of options for improving a boat's ventilation. The trick is to use your boat's hatches, ports, windows and vents to provide adequate ventilation in fair weather and foul. There are no set rules governing how many vents you should have and where they should be located. A boat's natural ventilation can travel in any direction, fore and aft, port or starboard.
Before you buy and install vents – let alone hatches or ports – experiment with what's already available on your boat. When underway, at anchor and in port, adjust your hatches, ports and vents to optimize ventilation. Use a candle or incense to help you see air movement and locate dead spots. With experience, you'll find out what works best and where ventilation needs to be improved (and if you use incense, you're also likely to meet some interesting folks from neighboring boats). You can then address those problem areas with new additions.
UPGRADING YOUR VENTILATION
There are many options to alter the airflow on your boat. For example, in heavy seas or rain, hatches, ports and windows are apt to be closed. At these times, however, passive cowl vents can be effective, particularly when mounted on top of a Dorade box. Typically made of fiberglass or wood, a Dorade box covers a duct into the cabin, which is offset from the ventilator; the box has holes so that any water that enters can drain out.
Other types of passive vents, such as mushroom vents or clamshells, are less effective than cowls for cabin ventilation, but they work well in other applications. A mushroom vent, operable from inside the boat, can also be used to close off the air duct inside a Dorade box in heavy weather. Mushroom vents can be installed vertically too, on the side of a trunk cabin or over an exhaust fan duct from the galley or head. I use a low-profile mushroom vent as a deck cap for my boat's cabin heater flue pipe. In heavy weather, it can be closed to keep water from entering. Clamshell vents are best used where water intake is unlikely; for example, where the vent opening faces downward or away from the expected airflow.
Taller and wider is best when is comes to cowl vents for optimum airflow. A cowl vent with an opening that is 4 inches in diameter will allow roughly twice the airflow of a 3-inch vent of similar design.The airflow generated by passive vents is limited by the amount of air rushing over them. Powered vents, which operate day and night in all kinds of weather, provide a strong alternative. When there's little air moving over the deck, a powered vent provides airflow. These vents may be powered, either alone or in combination, by solar cells, an integral rechargeable Nicad battery, and/or 12-volt ship's power. They can be set to function as an intake or exhaust vent as needed.
Solar vents are a godsend for boats that are left unattended for even a few days. A solar-powered fan operates without any drain on the battery, provided there is sunlight. At night, when there is a slight breeze, it acts as a mushroom ventilator to exhaust or intake air. Combination solar vents provide 24-hour ventilation. Sunlight, when available, powers either an intake or exhaust fan and, when overcast and at nighttime, the fan runs off an internal battery.
When using a powered vent, you'll find that some are quieter than others. Consequently, it may be better to install a powered vent in your boat's head than directly above the V-berth. The fact that powered vents can fail (most often electrically) should be considered when upgrading your boat's ventilation system. If you spend many nights onboard, it's best to carry a spare.
Marine stores sell clamshell, cowl, mushroom and solar vents in styles to fit any boat. Vents can be installed in a hatch or through the deck, cabin top or sides. Before cutting holes in your deck make certain you won't accidentally be cutting through wiring or structural supports. Measure twice, cut once. A saber saw can be used, but a hole cutter of the proper size will give a more accurate cut. Be sure to seal any exposed deck core against water intrusion before installing the new vent and use an appropriate sealant such as 3M 4200 under the vent flange.
ADDITIONAL CONCERNS
There are many places on a boat that require ventilation, and not all are obvious. For example, storage lockers and bilges often require attention. Solid locker doors should be louvered or have cane inserts, grilles or decorative cutouts that allow air to move. If the locker doors on your boat are not ventilated, they can be easily modified or replaced. Cutouts, with screens or grilles if appropriate, can also be used in the backs or bottoms of drawers and lockers to provide additional ventilation.
How a Dorade vent works: baffles prevent any water from entering the interior and water exits via drain holes while air is funneled below.Condensation that collects on locker floors and under mattresses can be a particularly annoying problem. Here too there are several solutions. Dri-Deck self-draining ventilated vinyl panels are readily cut and custom fit to any locker or bunk. These interlocking “waffle” panels facilitate air circulation and the evaporation of moisture. I've used them successfully on locker floors for many years.
Ventair, a ½-inch thick, three-dimensional nylon mesh with polyester cover, can also be used under mattresses. It's lighter and easier to move about (when accessing storage lockers under berths) than Dri-Dek, and it provides additional cushioning.
Cabin fans, including hard wired 12-volt fixed or oscillating fans and battery-powered portable fans, can be used to augment a boat's ventilation system and to increase the effectiveness of air conditioning and heating systems. I installed a 12-volt exhaust fan in the head and one above the vented propane heater. Later, I used a windscoop on my boat's double-hinged forward hatch when anchoring.
Shielding your boat's cabin from a hot summer's sun with a well-fitting awning can also keep temperatures several degrees cooler below deck. When I ran my boat's air conditioning on hot, muggy days in Maryland, I appreciated the cool air, but disliked the noise and sense of being “canned.” With a couple of simple awnings (no battens, no frame), I didn't need air-conditioning during my six years in southern California and Hawaii.
As you work to improve ventilation onboard, don't forget that you may want to adjust airflow in late fall, winter or early spring. Most of the ventilation gear available in marine stores is intended for summer use, so you may need to get creative. When I lived aboard during winters in Maryland, I fabricated sliding acrylic panels to close off the louvered portion of the anchor locker and companionway doors. I adjusted the airflow from Dorade vents using acrylic discs (since the discs were transparent, I left them in place throughout the year).
Solar vent mounted on hatch provides intake air to aft cabin, resulting in a sweet smelling boat.ADDED INSULATION
Insulation is another concern. Without adequate insulation, the temperature inside a fiberglass or metal boat will quickly respond to the temperature outside. When it's hot outside, it will get hotter inside. And, when the temperature outside falls below the temperature inside, the hull, cabin and any metal-framed hatches or portholes will sweat. Wood is a better insulator than fiberglass or metal. Consequently, wood boats and wood-framed hatches have fewer condensation problems. Fortunately, the hatches on my boat are all framed with wood. I do get condensation on the cast bronze opening ports, but there are teak rails underneath that will catch and hold drips until they evaporate or are sponged away.
Cored (balsa or foam) fiberglass construction, typically used in fiberglass decks and some hulls, has both thermal and acoustic insulating qualities. If your boat's hull and deck are cored, you may find you do not need additional insulation. However, if the hull is solid fiberglass and you use your boat in cold waters or cold weather, you'll very likely want to install insulation. There many different insulating materials to choose from, so do some research and pick what will work best for your boat, geographic location, and the type of boating you do.
Whatever solutions you embrace, the bottom line is that, for a relatively small amount of money, you can make a dramatic difference in the comfort of your boat. Vents start at about $25 and run to a few hundred dollars, and can be installed yourself. Using a system of both intake and exhaust, passive and active vents, you can achieve a comfortable level of cross-ventilation that will keep mold, mildew and corrosion at bay and help regulate the temperature in the cabin. Just imagine having no more cooking odors, musty air or mildew. Proper ventilation will help to keep you, your crew and your boat happy and healthy.

Susan Canfield is a marine surveyor in Annapolis, Maryland. A version of this story was previously published in DIY Boat Owner magazine.