The problem with boats and moisture is that there is a "ratcheting" effect. That is, warm/moist air enters the boat where it cools down and condenses. If The humidity in the boat is allowed to remain high, this condensed water does not evaporate during the day when it warms up again. So, each warming and cooling cycle deposits just a little more moisture on the surface of things.
Ironically, heating the boat does not necessarily help and, uneven heating, can actually exacerbate localized issues (like up in the V-berth).
Similarly, bringing in fresh air does not necessarily help either. It can just create a situation whereby you are just introducing new moist air at a faster rate. When the conditions are hovering around dew point, that is exactly what happens. You bring in air that is near the dew point and that air comes into contact with surfaces that are slightly cooler (below dew point). The faster you bring in that air, the more efficiently you create condensation.
Since you cannot economically heat all interior surfaces to prevent this, the next best approach is to manage both temperature and humidity simultaneously.
I think it might help to lay out why condensation happens aboard and why solutions like fans or heaters often give mixed results - especially if used alone.
Every boat, especially in cool and damp climates, goes through a daily condensation cycle:
- Warm, moist air enters the cabin
- At night, interior surfaces - especially uninsulated hull or deck panels - cool rapidly.
- When those surfaces are below dew point of the interior air, water condenses out and clings to them.
- This repeats daily. Moisture checks in, but it doesn't check out - kind of a ratchet effect.
- When conditions are hovering right around dew point, this can happen continuously. If you are bring fresh air in and some surfaces are continuously cooling the air below the dew point. So, pumping in air is just accelerating your condensation problem, in that case.
Why just using a fan or heater often doesn't work very well:
- Fans move air, but if you’re just circulating humid air, or bringing in more damp outside air, they don’t reduce moisture — they just spread it around.
- Heaters, especially passive ones like small electric cabin heaters, can raise air temperature without removing moisture. This increases the dew point and can even encourage more water vapor to be absorbed into the air, which then condenses on colder surfaces.
- Localized heating or airflow can actually concentrate the problem elsewhere — you warm the cabin but leave a cold locker untouched, and now that’s your mildew zone.
So, what actually helps?
To break the cycle, you have to control both temperature AND humidity. That means:
- Removing moisture, not just moving it. Dehumidifiers, desiccants, or timed air exchange with drier outside air can all help — but only if the outside air is actually less humid (watch the dew point).
- Interrupting cold surfaces. Insulation (even reflectix or foam padding in lockers) can theoretically prevent temperature dips below the dew point. In all honesty, I am not convinced the pros outweigh the cons, when discussing cabin insulation - but it is a strategy.
- Timed heat/vent cycles work best: a short heating period to bring air and surfaces just above the dew point, followed by ventilation to remove the moist air before it cools and condenses again.
- The key to effective ventilation is to control the flow instead of just turning on a fan and leaving it to chance. You need to plan the route for an effective exchange of air throughout all interior spaces.
If you just put an intake fan on a crib board, what will happen is you will create a high pressure area just inside the companionway. The air will take the path of least resistance. Most of the intake air will go right back out the cracks/vents around the companionway.
At minimum, you should put an intake fan at one end of the boat and an exhaust fan (or unobstructed vent) at the other. That will at least ensure that air passes all the way through the main cabin. Again, this is only part of a good anti-condensation plan, but it is better than nothing.
