Boat condensation: one sailor’s solution to keep your boat dry

Boats keep water out by design, but unfortunately that makes them very good at keeping moisture in.

On our boat Ti-Gitu in cooler weather when we have a heater on, the windows run with water, the lockers become damp and in the corners of lockers, black mould will appear.

This often happens aboard boats stored ashore for the winter but is especially true in a boat that is being lived aboard for extended periods of time.

The average human loses approximately 800ml of water through breathing and the skin each day. Working or exercising hard an average person sheds between 0.8lt to 1.4lt per hour.

Evaporation from cooking and places such as the bilges, or cleaning work surfaces, washing and showering all adds to the moisture in the atmosphere.

Obviously this moisture has to go somewhere and when it is cooler outside it will condense inside onto windows and then run down to find its way behind panelling and into lockers.

Banishing boat condensation: window experimentation

Our yacht is built from steel and despite having plenty of insulation and ventilation by dorade vents and opening hatches to keep a good air flow through the vessel with two of us permanently living aboard the moisture can be considerable.

I experimented on our windows with a form of plastic film double glazing sold in some builders merchants and found it to be successful.

This is stuck around the window with double sided tape, then the thin clear plastic is trimmed before being heated with a hair dryer which shrinks it and makes it go tight.

After a couple of years we replaced this with the thicker clear plastic used in sprayhoods, attached with Velcro.

This was cut to shape with the Velcro sewn around the edge and sticky backed Velcro stuck around the window frame.

When it begins to get cold this is put on and although the Velcro is not totally airtight it does stop most of the condensation.

The small amount of water that does collect in the corners of the windows between the Perspex and the plastic just needed wiping every few weeks.

Positive input ventilation to solve boat condensation

We also had a dehumidifier which we’d run when there was an electric shore supply, and alternate it between the aft and forward cabins.

Using that helped a lot but they are very expensive to run and actually they’re not the total solution.

In recent years various manufacturers have started producing Positive Input Ventilation (PIV) for houses.

This is basically a large, slow running fan that takes air from the loft of a house and blows it in through a vent, usually in the ceiling in a hall or on the landing in a multi story building.

By very slightly raising the air pressure inside the building and providing a constant flow of air through the building it forces all the excess moisture out through gaps around doors, windows and through air bricks.

The reports from house owners and building surveyors are really good.

It seems that many landlords, especially of places like student accommodation where windows are seldom opened, are fitting these PIV units.

This reduces the condensation and the moisture damage caused to decoration and furniture.

These mains powered PIV units also have air filters fitted to stop dust and are so effective that they also help people with hay fever and allied allergies by keeping the allergens (pollen etc) out of the house.

Unfortunately these domestic PIV units are quite large and unsuitable for use on a yacht. Also as they only run on mains electricity they would be difficult to use when there’s no mains supply.

Idea adaption

However, I thought the PIV idea was worth experimenting with and started thinking about how we might do it on our yacht.

On Ti-Gitu at the stern under the cockpit we have on one side a cabin and on the other side is a general storage area.

This storage area was never panelled out and the frames and stringers are visible with insulation between them.

In winter a lot of condensation forms on the edges of these metal strips. Also the window in this area accumulated condensation.

There is a dorade vent into this storage area, so I thought I would try fitting a computer fan on the inside to gently blow some air in.

We already use some computer fans as extractors from the shower and the engine box so I had some knowledge about them, and there are some important things you’ll need to consider if thinking about fitting them to your own boat.

• If you want to fit one of these fans under the deck it will be laying down rather than standing on edge as it would be when fitted into the case of a computer. When laying down these fans you have to have a ball bearing type. Any other type is not designed to work on the flat and will quickly fail.
• Computer fans are designed for different voltages but 12V types are easily available.
• There are many different speeds. The faster ones which can move a lot of air are also the noisiest – for our purpose, slower, quieter versions suit us well.

Fan efficiency

I fitted a 120mm slow, (2,400rpm) quiet, 12V, 80 cubic feet/minute fan under the dorade vent into the storage area and set it going.

It was extremely quiet and I forgot about it for a fortnight. When I remembered and looked in the storage area, I was amazed to see it totally dry, despite the weather being very cold.

I then looked under the cabin sole and found that the aft third of the boat was also totally dry.

If one fan could perform so efficiently by drying such a large area, it seemed sensible that by fitting another fan at the bow that would also help to reduce the condensation.

The next winter I made a temporary fan to blow into the anchor chain locker at the bow through an anchor rope access hatch and quickly found that this area and the forward cabin also dried out.

The air was being blown into these lockers and then passing behind the panelling and lining of the hull, through the bilge at the bow and stern and was finding its way from there into the lockers and wardrobes.

Now that part of the boat is condensation free. Ti-Gitu is 40ft long and I found that the PIV fans at the bow and stern left an area amidships where there was still some condensation.

A boat can be described as a long box or tube and the longer the vessel the less effect one input fan will have.

Continues below…

Positive results

After a lot of thought about where to place an amidships fan I decided to make a small deck box with a mushroom vent inside and a fan below it to blow down through a sealed section between the shower room and the hull.

The deck box is rather close to the edge of the deck and despite Ti-Gitu being flush decked sometimes a wave does come aboard, so I made sure it is easy to close the mushroom vent by fitting a screw down hatch on the top of the box.

Closing it simply becomes one of the preparation jobs we do before going sailing.

If it did get forgotten only a small amount of water would find its way aboard, but then that would sort of defeat the exercise.

We are now in the fourth year of using this system and it is a great success. The whole boat is remarkably dry.

Lockers and wardrobes that became very damp and grew black mould every winter are now dry and clean.

Even the windows with the Velcroed on clear plastic do not need wiping dry very often.

In a house the air flow needed is between 0.7 and 1.25 of the total volume of the property each hour depending on how many people live there and the amount of moisture produced by showering or drying clothes indoors etc.

After trying different air flow volumes I found that we needed 2 to 2.5 times the volume each hour.

This apparent higher volume and pressure required for a boat as opposed to a domestic property is caused because the air is entering into sealed spaces and then having to pass through limber holes and any other gaps behind panelling which restricts the flow.

If the air is entering directly into the cabin rather than from one of these hidden spaces then probably a considerably less amount would be needed – but that may not dry the bilge and lockers so well. We have three fans.

These comprise 80mm 25cu.ft/min units at the bow and stern and a 120mm 50cu.ft/min fan amidships.

Most of these computer fans have a predicted life of 120,000 hours which represents nearly 14 years continuous use.

The three fans combined consume less than 1A at 12V. This is considerably less power consumption than a dehumidifier and they’re much more efficient at dealing with condensation.

A boat on a mooring without a shorepower supply could easily run some of these fans from solar panels.

Minor drawback

The only drawback is when the weather becomes really cold.

In a house the air is being drawn in from the roof space which is generally slightly warmer than outside and the domestic units can also have a very low power heater fitted.

Finding that in really cold weather some parts of the boat, and especially the foot of the forward berth, were difficult to keep warm I started looking at ways to take the chill out of the air.

I fitted a positive temperature coefficient (PTC) heater element to the fan passing air into the anchor locker.

These modern heating elements are very efficient and very safe. These heaters are self limiting, which means that as they approach their designed operating temperature, electrical consumption is automatically decreased and they’re incapable of overheating.

A key benefit afforded by PTC technology is efficiency.

The PTC heating element draws full rated power upon startup to quickly heat the element, then reduces the power consumption as the operating temperature is reached.

The heater I fitted is a 12V 50W unit and although it appears small and only covers less than half of the area of the 80mm fan, it very gently heats the air coming in.

This is drawing 4A at 12V which relates to 0.22A at 230V. This is about 1.25kW which costs very little.

Therefore when the boat is connected to mains electricity over a winter it is easy to run the fans and put the PTC heaters on when it becomes really cold.

There are 12V ball bearing fans available which run as slow as 1,200rpm and move between 25 and 80 cubic feet/minute and there are fans and heaters combined available for egg incubators which could be utilised and fitted for use on a boat in cold weather. The cost?

The fans cost between £7 and £20 depending on quality and where you buy them from. I fitted small boxes with a connector, fuse and switch totalling £20.

The wire for the fans alone does not need to be heavy and does not cost a great deal.

I also fitted air filters costing £2 to £3 each. I made one dorade type box for our amidships fan which cost just a few pounds.

If heaters are fitted then the wiring and switches need to be heavy enough to carry the amperage.

Wiring for the 50W heater I fitted cost under £4.

Twelve watts at 12V equals 1A, therefore the 50W heater I fitted in our forward PIV unit draws just over 4A plus the fan so I consider it to be drawing 5A and have fused accordingly.

If a really powerful version was to be fitted, say a 200W heater, then this will be drawing 16.6A plus the fan and that is moving into the area of heavy cabling.

I have found that the filters and blades of the fans need cleaning once a year.

The filters will also keep any midges or mosquitoes out.

To help stop dust sticking to the fan blades give them a quick spray of furniture polish when cleaning.

For under £100 and a little work we now have a dry boat, making us a very happy pair of sailors!

---

Enjoyed reading Boat condensation: one sailor’s solution to keep your boat dry?

A subscription to Practical Boat Owner magazine costs around 40% less than the cover price.

Print and digital editions are available through Magazines Direct – where you can also find the latest deals.

PBO is packed with information to help you get the most from boat ownership – whether sail or power.

• • • • Take your DIY skills to the next level with trusted advice on boat maintenance and repairs
      • Impartial in-depth gear reviews
      • Practical cruising tips for making the most of your time afloat

Follow us on Facebook, Instagram and Twitter