As I was doing research, I was looking at potentially installing a household Honeywell dehumidifier in a 58-foot wooden boat (vented from outside) while connected to shore power, for use in New England where temps this weekend dropped into the 20s F. The water outside obviously is colder than the inside air and the outside air is freezing. However, the boat is well designed with good ventilation and two stoves to keep the cabin warm. The builder, a wise sea captain, told me he would not install a dehumidifier in a wooden boat. In a fiberglass boat, yes, because they sweat and start to smell of unpleasant things. This is a wooden boat, but Im not sure ventilation alone will be effective. My concern that the dehumidifier would dry the oak planks out and possibly open up the seams. If the boat was in San Francisco Bay, I would not be so concerned, but the cold New England temperatures are a worry.
We passed your question two experts, our winter sailing guru, PS contributor Drew Frye and wood boat maven Lin Pardey, whose new book, Taleisins Tales: Sailing toward the Southern Cross offers an ringside seat on South Pacific adventures aboard a wooden boat.
Both Frye and Pardey downplayed the risk of an ordinary household dehumidifier your boat out, but it would be easy enough to monitor relative humidity while you are aboard, and some large dehumidifiers have automatic controls. He said 65 percent relative humidity was probably dry enough for your heated boat, about 50 percent is normal for fiberglass boats in cool climates. If the dehumidifier doesnt have controls and you are leaving your boat, you can set the dehumidifier on a simple light timer to run only about four hours per day. Frye runs his at night, since that is when humidity is higher.
If humidity is too high, condensation will appear at the colder surfaces, bronze hatches, single-glazed ports (storm windows help here), any metal through-deck fittings. The February 2015 issue of Practical Sailor online Eva Dry Adds 12-volt Cord has links to several articles we have run on dehumidifiers. Lin seemed to think a dehumidifier was overkill, especially if you are living aboard. She describes here experience living aboard in Risor, Norway:
With the air temperature plummeting well below freezing, the water temperature just above freezing and the cabin kept warm with a well vented oil burner, I wasnt surprised to see condensation forming inside the hull of Taleisin. I was really glad we had ceiling at the back of each clothes locker to keep sweaters and gear from being mildewed by soaking up this moisture. But I did worry about rust forming on all the cans stored in other lockers which had no ceiling.
Our neighbor in Risor, lived on board his 40-foot oak planked, 70-year-old Colin Archer Rescue Boat (Redningsselkapet). After several seasons and several trials his solution to this problem was creating good pathways throughout the boat to allow for airflow then using fans combined with well-vented heaters, with one small electric heater as low as possible in the boat (but well away from the planking).
We found this did work, adding a small fan at each end of the boat and leaving as many lockers open as possible to ensure airflow and adding a small electric heater which was positioned right on the cabin sole to back up our vented oil heater. The reason for the extra heater, to get the drying heat as low as possible in the boat and assist with circulation.
A dehumidifier, which we tried one season, was not as successful as it is too centralized and does not assist in moving the air from the ends of the boat.