Us boaters have more to worry about than keeping our boats watertight: we have to consider laying up and winterisation, we have to clean and antifoul the hull, and we also have to think about keeping naturally occurring atmospheric moisture at bay – and in the UK, that’s a particularly difficult issue because conditions are relatively damp all year round. In fact, according to Meaco, the relative humidity in the UK is above the mould growth point for no less than 80 per cent of the year. In addition to the issues generated by our climate, commonplace acts like breathing, showering and cooking can all add to your damp issues.

Simple desiccant sachets are useful for cupboards

Simple desiccant sachets are useful for cupboards.

Now you might question whether airborne moisture matters that much, but the problems associated with high humidity are more profound than you might think. In addition to mould and condensation, moist air means higher bills (because moist air is more difficult to heat) and the aggravation of various health issues from asthma to arthritis. And while moist air can accelerate the corrosion of electrical connections and the decay of internal furnishings, relatively dry air will actively encourage the evaporation of moisture from the various surfaces within your boat - so in the long run, it pays to keep the air dry.

What does humidity actually mean?

Humidity describes the amount of moisture in the air - but it is always relative to the temperature of the air being measured. Warm air is capable of containing more moisture than cold air, so the humidity (measured as a percentage) will be higher for a given amount of moisture in 15-degree air than in 20-degree air. That is why you see water from warm moist air condensing so readily on cold surfaces.

How can you prevent damp on your boat?

If you want to decrease the humidity in your boat, there are various options open to you:

  1. Produce less moisture in the first place, but of course that is not always possible.

  2. Move and refresh the air via open windows or extractor fans, but that can chill your boat down, especially in winter.

  3. Heat up the air to reduce the relative humidity, but that is obviously unpleasant in the summer and difficult in the winter.

  4. Buy yourself a dehumidifier.

There are various types of dehumidifier, from simple sachets of desiccant that absorb airborne moisture to small powered versions with rechargeable batteries that can be dried out and re-used. These can be excellent (and very affordable) solutions for low-volume regions like cupboards and storage spaces, but for larger living areas, a more sophisticated mains-powered solution is often required.

Compressor dehumidifier

The traditional form of dehumidifier (and one that has now been around for several decades) uses a compressor to create a cold surface inside a box. The warm moist air is drawn over this surface and as it makes contact, the moisture condenses into liquid water - in exactly the same way as it might on a cold beer glass on a summer’s day or on a cold mirror in a warm shower room. This moisture is then collected in a reservoir and the dry air is pushed back out into the room.

Compressor dehumidifiers

Compressor dehumidifiers can be large, heavy and ineffective in cold conditions.

Desiccant dehumidifier

A more modern method is known as the desiccant dehumidifier. Instead of using a cold surface to extract the moisture from the air, it uses a slowly rotating wheel of desiccant (usually Zeolite or Silica) to absorb the water in much the same way as a sachet. The dry air is then free to exit the machine. Meanwhile, at the other side of the rotating wheel, the desiccant is regenerated by a small heater, which draws the moisture out of the material and ejects it either into a reservoir or via a pipe to an alternative outlet.

A modern dessicant dehumindifier

A modern dessicant model may even come with ionisers and anti-bacterial filters to help clean the air.

Which dehumidifier is right for your boat?

A dehumidifier is most commonly defined by the number of litres of water it is capable of extracting from the air per day - and naturally, you should pick a product on the basis of the size and type of space you are attempting to keep dry. But the first decision to make is the big one: compressor or desiccant?

The fact that compressor humidifiers need to run relatively warm air over a cold element means they work best at higher temperatures. In fact, if the air is too cold, they can actually ice up, so in chilly conditions, such as during winter lay-up or when the air temperature routinely drops below 15 or 16 degrees Centigrade, a desiccant model is by far the more effective solution. Otherwise, your compressor dehumidifier will waste a lot of energy simply attempting to defrost itself.

Desiccant dehumidifiers are also smaller, lighter and quieter than compressor models, making them better for portable use - and if you plan to run your dehumidifier unattended for long periods, the continuous drain-off and auto-restart facilities of the modern desiccant models are vital. This enables the device to pipe the extracted moisture somewhere other than the internal reservoir, and to restart automatically in the event of an interruption to the power supply.

Meaco DD8L dehumidifier

The Meaco DD8L is a very highly regarded boat solution.

As regards the ambient temperature, while both methods tend to warm the air, desiccant models increase the temperature more radically than compressor models - by up to ten degrees compared to about two. So if you want to dehumidify a routinely warm living environment without making things too hot, the compressor model has the edge here. But as damp is a problem that is particularly pronounced when the air is cold (and able to contain less moisture), this is unlikely to figure too highly in the thought processes of the boat owner.

One issue that will be at the forefront of the canny skipper’s mind, however, is energy. Desiccant dehumidifiers tend to have a higher wattage rating, but because they extract water more quickly, they can be programmed to run for shorter periods, making their overall power demands much more competitive. As an example, Meaco’s very highly regarded DD8L desiccant dehumidifier (rated to remove up to eight litres of water per day) will draw up to 650 Watts on full power. For those with shore power, that is a very minimal demand indeed.

Dehumidifiers on boats: a summary

Keeping the inside of your boat warm and well ventilated with the use of heaters, extractors and open windows will certainly help limit atmospheric humidity - and this in turn will help reduce condensation, mould, mildew and the various problems associated with that. But if you want to actively remove the moisture from the air - and you have either a substantial 12v domestic battery bank or the luxury of regular mains power - a desiccant dehumidifier is a very sound investment indeed.

For more maintenance tips and tricks, see boat maintenance: how to look after your boat.

Written by: Alex Smith
Alex Smith is a journalist, copywriter and magazine editor with a long history in boating and a happy addiction to the water. He’s worked on boats, lived on boats, bought boats, sold boats and – when he’s not actually on board a boat – he can generally be found in his Folkestone office, tapping away at the computer and gazing out to sea.