Keep your boat fungus-free in winter

Keep your boat fungus-free in winter

By Dave Osmolski

You’ve winterized your boat, perhaps wrapped and sealed it against the elements so it will be clean and ready to sail come spring. Or will it?

Most boaters pull and winterize their boats before the onset of cold weather. If your boat has been shrink-wrapped or has closed-off areas with little or no ventilation, the trapped air contains much more moisture than the cold winter air will hold.

If you’ve taken the USPS Weather course, you understand relative humidity. For example, air at a temperature of 70 degrees Fahrenheit will hold more water vapor than the same volume of air at 36 degrees Fahrenheit. This means that when you seal up your boat at 70 degrees in the fall and the temperature drops to 36 degrees in January, the excess water vapor condenses as it does on the outside of a glass of iced tea.

Moisture inside your boat combined with still air almost always results in mildew. In addition, condensation can collect inside electronic devices and on things that can rust and cause stains.

To a small extent, ventilation can prevent both mildew and condensation, but an open hatch or port will allow dust, insects and larger critters into the cabin.

A quick, safe and odorless way to dehumidify an enclosed area on your boat is to make a simple desiccator using a hygroscopic chemical. Chemists use this techniques for preserving things in small, enclosed spaces. Desiccating chemicals such as silica gel remove moisture from small areas like electronic device shipping containers and medicine containers. Silica gel is the sandy material enclosed in little packets stamped “Do Not Eat.”

I have used calcium chloride to remove excess humidity from an enclosed cabin or closet. Calcium chloride becomes a puddle of salty water, unlike silica gel, which traps the water and doesn’t become wet itself. To overcome this problem with calcium chloride, I cut a gallon jug in half transversely and invert the top into the bottom so I have a funnel in a tub. Don’t open the neck of the funnel (leave the cap on the jug). Poke numerous holes in the funnel. Then put a cup or two of calcium chloride into the funnel. As the calcium chloride absorbs moisture from the air, salty water drips into the tub. The salty water won’t evaporate unless the temperature gets really high.

Check your chemical dehumidifiers a week or two after setting them out. If no calcium chloride remains in the funnel and the tub is full of salty water, dump the water and refill the funnel with calcium chloride flakes. You can safely pour out the water in salt water or down a household sink drain. Keep checking the dehumidifier until calcium chloride remains in the funnel and little or no water sits in the tub. Then leave it until you’re ready to launch again in the spring.

You can buy calcium chloride, an ice-melting salt, at most hardware stores. Be aware that not all ice-melting salts are calcium chloride and hygroscopic. Read the label before buying, and use only calcium chloride.


D/Lt/C David H. Osmolski, SN, of Charlotte Power Squadron, has been repairing boats since high school when his first boat, a canvas-covered canoe with cedar ribs, leaked in gallons per minute and required constant repair.

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