A dehumidifier or your boat's AC lowers the humidity below deck, and AIRLOCK One eliminates the musty odor in the whole boat's air the AC moves. AIRLOCK One is not a dehumidifier and doesn't remove moisture. It installs at the AC air handler and eliminates odors whenever the fan runs, cooling or not. Most boaters install it in 15 to 30 minutes. For damp lockers, closets, drawers and compartments, AIRLOCK MoistureGuard is a silica desiccant that needs no power and is reusable for up to a year. AIRLOCK does not sell an electric dehumidifier, so this guide covers how to choose one.
A boat needs an electric dehumidifier when the whole cabin stays damp: condensation on hatches and ports, a musty smell below deck that keeps coming back, or a hygrometer reading of 60% relative humidity or higher, which is the ceiling in the EPA's guidance (it recommends 30 to 50% where possible). Choose the type by temperature. Compressor units suit warm weather, but ENERGY STAR notes their coils can frost below 65°F, so for cold months and winter storage (on the hard or in the slip), marine guidance points to desiccant-wheel units. For size, ENERGY STAR's chart starts at 20 to 30 pints a day for a space that feels damp. Passive absorbers handle the lockers and storage spaces the airflow can't reach, with no power needed. AIRLOCK MoistureGuard is made for those spaces, and it works only inside the space it sits in.
When a boat needs a dehumidifier
A hygrometer settles the question. The EPA describes it as a small, inexpensive instrument sold at many hardware stores. Hang one in the salon for a few days and check it in the morning, after cooking and after showers in the head.
ENERGY STAR describes three levels of damp:
- Slightly to moderately damp: the space feels damp and has a musty odor that comes and goes. Relative humidity sits around 50 to 75%.
- Very damp: the space always feels and smells damp, and damp spots show up on surfaces. Relative humidity runs around 75 to 90%.
- Wet: surfaces sweat, at around 90 to 100% relative humidity.
If the cabin reads above 60% most of the time, or it matches any of those descriptions, an electric dehumidifier is the tool that brings the number down. If the cabin stays under 60% and the trouble is a hanging locker or the storage under a berth, passive absorbers in those spaces are usually enough.
Katharine S., a boat owner, put the everyday version into words: "We hang the towels to dry but with the moist boat environment they always seemed to have a musty smell after two or so days."
Compressor vs desiccant boat dehumidifiers
Compressor (condenser) dehumidifiers use a compressor and refrigerant to create cold coils. A fan draws cabin air over them, and the moisture condenses and drips into a bucket, as Yachting Monthly explains in its boat dehumidifier test. ENERGY STAR notes that frost can form on the coils when the air drops below 65°F, which can make the compressor cycle on and off without removing moisture. Practical Boat Owner describes compressor units as performing best in warmer climates or within sheltered storage.
Desiccant-wheel dehumidifiers pass air over a rotating wheel coated with an absorbent material, usually zeolite. Heated air dries the wheel, and the moisture is condensed and collected. Yachting Monthly says desiccant units use more power, weigh less, warm the air around them and work better at low temperatures. It cites Meaco's rule of thumb to switch to desiccant once the temperature drops below 50°F (10°C), and says compressors can work well at 59°F (15°C) and above.
Passive absorbers use no power while they work. Crystal tubs like DampRid absorb moisture and dissolve into a brine you empty and refill. Rechargeable options include the Eva-Dry E-333, which holds up to 13 oz of moisture in silica gel, covers up to 333 cubic feet and recharges when you plug it in for about 10 to 12 hours, and AIRLOCK MoistureGuard, a 500 g silica desiccant bag that recharges in a microwave and is reusable for up to a year. These belong in lockers and storage spaces. None of them is sold to dry the air of a whole cabin.
Cold weather, winter storage and fire safety
At a UK marina, Practical Boat Owner reported, an investigation into a boat fire found a household compressor dehumidifier set up to drain permanently into the bilge; its coils froze and then overheated. The marina group, Yacht Havens, calls a frozen coil a known cause of compressor overheating, and Yachting Monthly reports that compressor dehumidifiers used for long periods at low temperatures have been known to overheat. Yacht Havens now requires boat owners at its marinas to use only well-maintained desiccant dehumidifiers designed to run unattended and at low temperatures, and Yachting Monthly names Parkstone Bay as another UK marina that allows desiccant models only.
Before you leave any dehumidifier aboard, the same sources give practical checks:
- Read your marina's rules and check your insurer's terms.
- Stand the unit on a level surface with room to breathe, and consider fixing it in place, because a boat rocks and heels and a unit that tips over can block its own ventilation (Yacht Havens).
- Check that it's upright before you plug into shore power, especially after a haul-out or a rough day (Yacht Havens and Practical Boat Owner).
- Clean the filters regularly. Meaco's founder told Practical Boat Owner that no dehumidifier can be left aboard for the whole winter without maintenance.
What size dehumidifier for a boat
Capacity is measured in pints of water removed per 24 hours. ENERGY STAR says the size you need depends on the size of the space and how damp it is without a dehumidifier. A boat's cabin falls in the smallest bracket of ENERGY STAR's chart (a "small-medium room" under 2,000 square feet), so the choice comes down to how damp it is:
| How the cabin feels without a dehumidifier | Relative humidity | Recommended minimum capacity |
|---|---|---|
| Feels damp, musty odor comes and goes | 50 to 75% | 20 to 30 pints per day |
| Always feels and smells damp, damp spots on surfaces | 75 to 90% | 25 to 40 pints per day |
| Surfaces sweat | 90 to 100% | 30 to 50 pints per day |
ENERGY STAR adds that it's better to oversize than undersize, and that the water you empty each day is a good guide if you're replacing a unit. The chart was written for rooms ashore, so treat it as a starting point for a cabin.
What humidity to aim for below deck
The EPA says to keep indoor humidity below 60%, ideally between 30 and 50%. ENERGY STAR adds that during the heating season, 30 to 40% helps prevent condensation on windows, which aboard means hatches and ports. A hygrometer in the salon and one in the forward cabin show whether the whole boat is in range or only part of it.
How to stop condensation in a boat cabin
The EPA explains that condensation forms when warm, humid air touches a cold surface. Below deck, that means hatches, ports, the hull behind lockers and the underside of berth cushions.
The EPA's four ways to prevent it all work aboard:
- Reduce the humidity. Run a dehumidifier sized from the chart above, and vent moisture where it starts. The EPA notes that stoves and other combustion appliances add water vapor unless vented outside, so open a hatch or run the galley fan while you cook. Some marine AC systems have a built-in routine for this, too: Dometic's Cruisair manual describes an SMX humidity control routine that turns the air conditioning on at timed intervals to remove moisture from the air. Check your AC's manual to see whether yours has one.
- Increase air movement. Open hatches and ports when it's practical, and use fans.
- Cover cold surfaces with insulation.
- Raise the air temperature.
When condensation appears, the EPA says to act quickly, dry the wet surface and reduce the moisture source.
Where MoistureGuard fits
AIRLOCK MoistureGuard is a silica desiccant for lockers, closets, drawers and compartments. Each 500 g bag soaks up damp in the space it sits in, with no power needed and no water to spill, and it's reusable for up to a year. When the indicator turns pink, microwave it on high for 5 minutes (based on a 1000W microwave), let it cool for 5 minutes and put it back. Blue means it's ready to use.
MoistureGuard works only inside the space it sits in. It won't lower the humidity of the cabin, which is the job of a dehumidifier or the AC.
Where AIRLOCK One fits
AIRLOCK One is for the musty odor in the whole boat's air. It installs at the AC air handler, in the intake airflow, and eliminates odors in the air the AC moves whenever the fan runs, cooling or not. You'll need one per AC unit. AIRLOCK One is not a dehumidifier and does not remove moisture. The EPA lists air conditioners alongside dehumidifiers as a way to reduce humidity, and One adds nothing to that.
Tammy M., a boat owner, wrote: "Boat always had a lingering mildew smell no matter how much we cleaned and used every option - nosguard, deep cleaning, air purifiers. Put this in and did not think it would work. After two days onboard, running the air conditioning and there was no smell at all." The moisture behind that smell still needs a dehumidifier, ventilation or absorbers in the lockers.
AIRLOCK One runs on AC power at the air handler (shore power or a generator), and most boaters install it in 15 to 30 minutes. For one spot that needs extra help, like the galley, the head or a berth, add AIRLOCK Voyage alongside it. Voyage plugs into a 12V socket or a wall outlet and eliminates odors up to 4 ft around it. Neither one is a dehumidifier.
Boat dehumidifier options compared
| Compressor dehumidifier | Desiccant-wheel dehumidifier | Eva-Dry E-333 | DampRid | AIRLOCK MoistureGuard | |
|---|---|---|---|---|---|
| How it removes moisture | Fan draws air over refrigerant-cooled coils; moisture condenses and drips into a bucket | Air passes over a rotating wheel coated with zeolite; heated air dries the wheel and the moisture is collected | Renewable silica gel absorbs moisture from the air; beads change color when full | Crystals absorb moisture and dissolve into a liquid brine | Silica desiccant soaks up damp in the space it sits in; indicator turns pink when full; reusable up to a year |
| Power | Plugs into a grounded outlet | Plugs in; uses more power than a compressor unit | Cordless in use; plugs into a 110-120V outlet to recharge | Crystals work once exposed to air | No power needed; recharges in a microwave |
| Where the maker or tester places it | Portable units for a single room or space | Boats in cold conditions | Closets, cabinets and boats; up to 333 cu ft | Closets | Lockers, closets, drawers and compartments |
| What you empty or recharge | Bucket (auto shutoff when full) or a drain hose | Collected water | Plug in 10 to 12 hours until the beads change back | Pour out the brine and refill, or discard a disposable unit | Microwave on high 5 minutes, cool 5 minutes |
AIRLOCK One isn't in this table because it's not a dehumidifier. It eliminates the musty odor in the whole boat's air the AC moves, and the humidity itself is the job of a dehumidifier or the AC.
Boat dehumidifier FAQ
What's the best dehumidifier for a boat in storage?
For a boat in cold-weather winter storage with shore power, marine sources point to a desiccant-wheel dehumidifier, since compressor coils can frost and overheat in the cold. Check your marina's rules and insurance, and plan to check the unit during winter storage. In lockers, closets, drawers and compartments, passive absorbers such as MoistureGuard need no power, and MoistureGuard is reusable for up to a year.
Is there a 12V boat dehumidifier?
Every dehumidifier in the marine sources we cite here plugs into shore power. If you look at a unit sold as 12V, compare its pints-per-day rating with the ENERGY STAR chart above and check what it draws from your batteries. With no power at all, passive absorbers are the option.
What is a marine dehumidifier?
In marine guidance, the term usually means a unit built and rated for boats. Yacht Havens requires well-maintained desiccant units designed to run unattended at low temperatures, and some insurers ask for marine-rated equipment, so check your insurer's terms too.
Can I get a boat dehumidifier with a drain hose?
Yes. ENERGY STAR notes that some portable dehumidifiers take a hose fitting so there's no bucket to empty. Practical Boat Owner's fire report involved a compressor unit set up to drain permanently into the bilge, so check where the hose runs and look at it whenever you're aboard.
What size dehumidifier for a boat cabin?
Size by pints per day. ENERGY STAR's chart recommends at least 20 to 30 pints per day for a space that feels damp, 25 to 40 for one that always smells damp and 30 to 50 when surfaces sweat. ENERGY STAR says it's better to oversize than undersize.
How do I stop condensation in my boat cabin?
Reduce humidity, keep air moving, insulate cold surfaces and raise the temperature, which are the EPA's four steps. Aboard, that means venting the galley and head, opening hatches when practical, running a correctly sized dehumidifier and wiping condensation as soon as it forms.
Does AIRLOCK One work as a dehumidifier?
No. AIRLOCK One is not a dehumidifier and does not remove moisture. It eliminates the musty odor in the whole boat's air the AC moves whenever the fan runs, while a dehumidifier or the AC lowers the humidity. For damp lockers, closets, drawers and compartments, AIRLOCK makes MoistureGuard. AIRLOCK does not sell an electric dehumidifier.