Autumn's mists are beautiful on an early morning sail down the river, but damp air can cause mould, mildew, nasty niffs, rust and corrosion - eating away at boat fixtures and fittings.

Whether you lay-up for the winter season or continue to use your boat for racing and cruising, keeping the interior dry is vital to avoid problems with damp.

Damp = Condensation

The key to avoiding problems with damp is to reduce moisture inside the boat. Wet oilies, boiling the kettle, using the shower, even breathing adds to the moisture level below decks. Even without the steamy inputs of daily use, condensation will increase with damp air and changes in temperature between the outside and in.

Top 5 Tips - avoiding damp when winter cruising:

  1. Fit a cockpit tent to create a 'porch' as a place to hang wet jackets
  2. Use the showers ashore when possible
  3. Keep surfaces and the cabin sole dry
  4. Boil pans with lids on and don't let kettles overboil
  5. Keep the interior warm

Top 5 Tips - a dry boat when laying up:

  1. Run a low output electric bar heater
  2. Use a dehumidifier or a desiccant pack
  3. Remove bedding, soft furnishings and books
  4. Lift mattresses and seat cushions
  5. Allow air movement into seat back lockers

Long term cruisers and liveaboards often insulate the hull and hatches, reducing the cold bridge between cool air and water and a warm interior that causes condensation.

Beat the Damp - Heaters

If the boat is being left for any period whether afloat or ashore, running a low output electric tube heater (greenhouse heater) will keep the air temperature stable, reducing condensation and avoiding freezing. This type of heater is safer to leave unattended and uses less electricity than larger domestic heaters. Save the plug-in fan heaters or oil filled radiators for when you're onboard and want toasty toes.

Low output tube heater with thermostat

Put the heaters in the main living area and in any low areas of the boat (cold air falls). Motor boats often have sleeping cabins at a lower level than the main saloon, and a couple of heaters in the lower level cabins will keep warm air circulating. Motor cruisers may want to place a small heater in the engine room.

Top Tip

Fix the heater down to a plank or plate to give it a wide base so that it can't tip over in stormy weather.

Beat the Damp - Dehumidifiers

The job of a dehumidifier is to remove moisture from the air. Dehumidifiers can be either electric or passive (using moister absorbing crystals). The water collected from the air is usually stored in a reservoir or bucket which will need to be emptied, or dispersed via a hose into the galley sink or bilge.

Electric dehumidifiers are generally of two types - condenser or desiccant. Condenser dehumidifiers work like a fridge, chilling coils with a compressor and refrigerant drawing in the air which condenses on the coil and drips into a bucket. Electric desiccant dehumidifiers have a fan to pull air over absorbent material which again drips into a bucket.

Non-electric dehumidifiers also use a desiccant material, usually laid over a plastic grid above a water reservoir. These simple products are easy to use, ideal if you're not connected to mains and are zero risk.

Top Tip

Electric dehumidifiers are not usually made to be used continuously over a period of weeks or months. Boat fires have been caused by overheating condenser (compressor) dehumidifiers. Read the manufacturer's instructions and switch off and check the machine regularly. Check the small print on your boat insurance.