When the winter lay-up season begins, thoughts invariably turn to how best preserve your boat when she is out of the water. Mould is the major enemy, and it loves the moist, stale air found below decks in a yacht shut up for winter. Good ventilation is key to prevent air going stale, but moisture can need some extra help to keep it at bay. An example of this is that on a damp day, the air entering the boat through the ventilators will be moist and this will condense on cool surfaces.
The best solution to this is a mains powered dehumidifier as it keeps the interior dry and prevents mould growth. There are two types of dehumidifier available; compressor models and those that use a desiccant to dry the air.
You will need to chose a machine that has the extraction capacity to cope with the air volume within the boat, but the most important feature is that the unit be able to work at low temperatures.
We recommend using desiccant based units such as the
Meaco DD8L at £179 including VAT and delivery or the
Eco Air DD122FW at £165 including VAT and delivery as, unlike compressor driven units, they will work effectively at low temperature. In addition they are small, light, quiet, have the option of continuous drain off (by-passing the holding tank and draining out of a small pipe into the sink or other point) as well as auto restart after a power cut.
Compressor Dehumidifiers
These work on the same principle as domestic refrigerators. A compressor raises the pressure of a refrigerant when it is in gas form. The gas is then led to a heat exchanger (condenser) which cools the gas, changing its state to liquid. The liquid is then forced through a small valve into a low pressure region where it evaporates back into gas. It is the evaporation process which cools the gas.
A fan draws the surrounding air over the evaporator unit, the temperature is lowered and the water held in the air reaches its dew point and condenses onto the evaporator surface. This water then runs off to be collected in a reservoir or through a pipe. The cold dry air is then directed over the condenser coil where it assists in the condensing process. This warms the air, which then exits the dehumidifier.
Desiccant Dehumidifiers
These use a desiccant material, typically Zeolite, which absorbs water vapour in much the same way as silica gel (a packet of which you would often find in a pair of new shoes). A fan draws air into the dehumidifier and passes it through a section of a slowly rotating wheel which holds the desiccant. The material draws the moisture out and dries the air. The moisture is then extracted from the desiccant, this is done by heating the portion of the wheel not being used to dry the air. The water then evaporates from the desiccant, is passed over a cold plate where it condenses, runs off and is collected in a reseviour or a permanent feed.
Desiccant vs Compressor
Compressor dehumidifiers work best at higher temperatures and humidities. They are rated at a temperature of 30°C and a relative humidity of 80%. Desiccant dehumidifiers work better at lower temperatures, this suits the conditions found in UK boatyards during the winter. In fact, if the temperature drops too low then parts of a compressor dehumidifer will ice up. Different models have different ways of dealing with this: they either switch off, or they can have a reverse hot gas cycle which quickly heats up the frozen evaporator plates and melts the ice. Compressor dehumidifiers do warm up the air slightly but desiccant models greatly warm it. This is obviously advantageous for a boat in winter helping reduce the chill and the likelihood of parts freezing up.
Key features to look out for on a dehumidifier that is being left to run unattended in a remote location for long periods of time are auto restart and continuous drain off facility.
Auto restart means that after a power failure or power cut the unit will start back up automatically and continue to run at the same setting it was running at prior to the cut in power rather than going into standby and having to be manually reset. It also means that the unit can be left running on a plug timer if required which can be useful as it provides even greater flexibility of use than just the inbuilt timer and variable humidity level that can be adjusted by the built in humidistat.
Continuous drain off means that rather than just collecting the extracted moisture in a holding tank there is option to drain off the moisture through a small hose which can be fed into a sink, drain point or seperate holding tank.
Our best selling boat dehumidifier is the
Meaco DD8L at £179 including VAT and delivery
Motor Boat Monthly Magazine November 2011 said
"....a perfect on board dehumidifier. The DD8L would be preferable on a boat that suffers from damp due to its higher performance."
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