Ventilation - A Survival Guide Good condensation appears on the outside of cold beverages. Bad condensation leads to soggy potato chips, wet bunks, musty air and with time, mildew, mold and corrosion. That’s why every boat owner should know how to improve onboard ventilation. 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 it “rain” too. The solution would appear to be simple. Either you keep the air in your boat’s cabin from being moist or you keep the moist cabin air from being cooled. The devil is in the details. Just consider how air inside a boat becomes moist (humid). First, there’s ambient humidity outside the boat, which can vary from 30% to 100%. Then there’s the water vapor we all exhale as a function of respiration and the moisture produced when cooking or showering. Unvented (no chimney) fuel-burning appliances, such as galley stoves and cabin heaters, put a gallon or more of moisture into the air for every gallon of fuel they burn. Last, but not least, there may be standing water in the bilge. Fortunately, humidity belowdecks can be minimized by ventilation (bringing in less humid air from outside) and by using electrical appliances or fuel-burning appliances with vented combustion chambers. If shorepower is available, you can even use a dehumidifier. 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 (wet) 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 be motivated to track down its source. Then you can eliminate the leak 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.
Ventilation is Divine When cabin humidity levels rise, use passive and active (mechanical or powered) ventilation as needed to bring in fresh air and to exhaust stale air. Ventilation is also 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 (cabin or compartment) and the exhaust at the other. Generally speaking, if a hatch is positioned so the airflow outside the boat is deflected downward, it will bring air inside. 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. Open 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, serve as an intake or exhaust as needed. In my work as a marine surveyor, however, I’ve 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 air conditioning (or heating) system. 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, 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. At anchor is easiest because the airflow (wind) is predictably from ahead. In fair weather, the foredeck hatch can be left open to act as an intake (if hinged on its aft edge) or exhaust (if hinged on its forward edge). An aft door or hatch is then used for air intake or exhaust as needed. Opening ports and windows are typically of greatest use when your boat’s tied up in port, where the wind might come from any direction. Remember, when there’s a gasoline- powered engine or generator running onboard or on a boat alongside, exercise special caution to ensure that deadly carbon monoxide doesn’t enter your boat.
Passive Ventilation When underway, forward facing (aft hinged) hatches may be closed to avoid taking on water. In heavy seas or rain, most, if not all hatches, ports and windows are closed. At these times, passive cowl vents provide intake air and are most effective when mounted on top of a Dorade box or water trap. Other types of passive vents, such as mushroom or clamshell, are less effective than “baffled” 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, though this restricts the airflow. This avoids the need to go on deck to remove the cowl and replace it with a threaded plate when unusually heavy seas or weather threaten. 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 the flue. Non-baffled cowl or clamshell vents are best used where water intake is unlikely; for example, where the vent opening faces downward or away from the expected airflow. Clamshell vents are often used to cover a cable pass through, an application where airflow normally isn’t an issue. Active Ventilation The airflow of cowl or mushroom vents is limited by the amount of air rushing over them. Powered mushroom vents, which operate day and night in all kinds of weather, are many boaters’ first choice. 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. When there’s little air moving over the deck, a powered mushroom vent provides more airflow than a baffled cowl vent. 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 (even marginal brightness). 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 (solar powered). 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. Upgrading Your Ventilation
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 hatches, ports or vents, 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, incense or (worst case, if you’re smoking it) cigarette or cigar 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. If you use incense, you’re also likely to meet some interesting folks from neighboring boats. 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 (e.g., 3M 4200) under the vent flange. 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 its air conditioning and heating systems too. I installed a 12-volt exhaust fan in the head and one above the vented propane heater. Later, I used a windscoop (for use when anchored) for my boat’s double-hinged forward hatch. Lockers, Bilges and Bunks Providing adequate ventilation through a boat means storage lockers and bilges must be ventilated as well. Solid locker doors should be louvered or have cane inserts, grilles or decorative cutouts that allow air movement. If the locker doors on your boat are not ventilated, they can be easily modified or replaced. While woven cane panels (real or synthetic) are not typically available at marine stores, they are sold by many boatbuilding supply shops or available from canoe and kayak builders who use them in chair seats. (Or search the Internet for “woven cane insert or panel.”) Cutouts with screens or grilles can also be used in the backs and/or bottoms of drawers and lockers to provide additional ventilation. Condensation that collects on locker floors and under mattresses can be a particularly annoying problem. Here too, there are several solutions. Dri-Deck selfdraining 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 (www.ventairusa.com), a 1/2"-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 its inherent resilience provides additional cushioning.
Flow Adjustments 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. Cover Up Shielding your boat’s cabin from a hot summer's sun with a well-fitting awning also helps to keep temperatures at least 10°F (5°C) cooler belowdecks in summer. For a relatively small amount of money you can make a dramatic difference in the comfort of your boat. U.S. prices for vents (including deck plates) start at $25 for a stainless-steel clamshell; up to $130 for stainless-steel low-profile or mushroom vents or $30 for a plastic one; $80 for a 4" (10cm) screw-in PVC cowl vent and about $350 for the same size in stainless-steel with a mosquito screen and threaded deck plate plus an extra $161 for a Vetus water trap; and $200 or less for a day-night solar vent. 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. Just imagine having no more
cooking odors, musty air or mildew in the
cabin. Good ventilation (and insulation) will
Forward and aft facing hatches and large cowl vents on Dorade boxes
offer optimal intake and exhaust ventilation. Underway and in foul
weather, hatches may have to be closed to avoid taking on water.
When a boat is left unattended, open hatches can also invite thieves.
Consequently, it’s best to
ensure that your boat has sufficient vents to provide adequate airflow when
all hatches are closed. Two or more solar vents would prevent cabin air from
becoming stagnant when hatches are closed.
help to keep you, your crew and boat happy
and healthy. Let your motto be: “The only
good condensation belowdecks is on the
outside of a cold beverage!” This article is a condensed version of the original in DIY 2005-#1 issue, which is available on the DIY Hands-On Boater 1995- 2006 CD-ROM. To order one, call 888-658- BOAT. |
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