Activities The Great Outdoors How to Keep Your Boat Dry and Prevent Mildew Share PINTEREST Email Print W.carter/Wikimedia Commons/CC BY 4.0 The Great Outdoors Sailing Gear Navigation & Seamanship Types of Sailboats Hiking Climbing Skiing Snowboarding Surfing Paddling Fishing Scuba Diving & Snorkeling Learn More By Tom Lochhaas Tom Lochhaas is an experienced sailor who has developed several boating safety books with the American Red Cross and the U.S. Coast Guard Auxiliary. our editorial process Tom Lochhaas Updated January 11, 2019 Boats live in a damp environment, and moisture inside the boat causes trouble when there is inadequate ventilation. Fiberglass boats are particularly a problem, as moisture in the warm daytime air may condense on the cooler hull inside at night. The problem is generally worse when boats are covered during the offseason, or not used on the water for long a time. Moisture allows mold and mildew to grow, producing unpleasant odors and black mildew spots, and ultimately causing fabrics and other interior boat materials to disintegrate. Ventilation Is the Best Solution Adequate ventilation through the boat's interior spaces is the ideal solution to prevent moisture buildup, thus preventing the growth of mold and mildew and the associated problems. A boat that is opened up and used frequently seldom has a problem, except in very humid environments or when leaks allow rainwater and spray to enter the cabin. Mechanical ventilation helps provide some relief. Dorade boxes allow wind-driven air to enter the cabin. For a boat sitting unattended, dorade boxes do not result in enough air exchange by themselves to prevent moisture buildup. Another option is to install passive (nonelectric) vents on hatches or on the hull. As the wind blows over the vent outside the boat, interior air is exhausted. Like dorade boxes, such vents can help but alone are seldom an ideal solution for a boat not used often. Also, they don't work on a covered boat in the offseason. Solar-powered vents are increasingly popular and a better solution, although it is not cheap to install several to maintain a good air exchange. Solar vents have solar cells on the outer surface, which charge a small battery that powers an exhaust fan. Manufacturers claim an exhaust capacity up to 25 cubic meters per hour in full sunlight. Successful ventilation depends, in part, on the positioning of such vents so that the interior as a whole is ventilated, rather than air pulled in at one location being immediately evacuated at a point a short distance away. This leaves the rest of the cabin air to stagnate. More powerful electric vents are also available, using either the boat's battery or external power at the dock or during the winter when covered. This can be a great solution when available but is simply not practical for many boaters. Using Calcium Chloride Calcium chloride is a chemical salt that attracts water vapor from the air. It won't drop the humidity to zero, but it does help lower humidity considerably in the absence of continuous ventilation. It significantly prevents the growth of mold and mildew for a covered boat. No matter how tightly covered, moist air still finds its way inside boats. The cheapest way to use calcium chloride in the offseason is to purchase it in bulk as a sidewalk ice-melting product. When purchasing, be sure to read the label to ensure it is calcium chloride and not a different melt product. Pour several pounds into a large container, like a drywall compound bucket — or better yet, two or more — and leave the buckets out in different parts of the boat before covering for the winter. In the spring, you'll find the dry crystals fused into a mass of puffed-up little white balls, possibly with liquid at the bottom. How to Remove Water From a Stored Boat You don't want to have open buckets of calcium chloride sitting around on the boat during the active season, however. When the boat is in motion, and also for winter use by those who prefer a "cleaner" alternative, try using moisture-removal products made for houses, basements, boats, and so on. These products are available at many hardware stores in large tubs that contain calcium chloride, but also have a top barrier cover that prevents spills. A gauge on the side lets you monitor how "full" the tub gets, and then you simply throw it away and start another one. The product is available also in refillable tubs and smaller hanging units for lockers and smaller spaces.