If the weather permits and it's not raining cats and dogs, the hatches open on board and fresh air flows into the cabin. Ventilation is important in every living space - firstly because of the higher oxygen content, and secondly because fresh air has a demonstrably positive effect on the immune system and well-being. The indoor climate is also crucial, especially on board: moisture should be kept out and dry air in.
This is why every YACHT test also looks at whether enough hatches can be opened on the test boats and whether cross-ventilation is possible. When it comes to the indoor climate, it's all about a balanced ratio of air quality (the CO2 content is significantly higher in exhaled air), temperature and humidity. The last point in particular is decisive for the formation of mould, which must be avoided. The easiest way to influence these factors is ventilation. This is supposedly uncomplicated, but still requires consideration of the weather conditions and is only possible in its simplest form (hatch open!) when the crew is on board.
Passive fans and active fans
Most of the time, however, boats lie alone on the jetty. Special fans are then required for air exchange. These can be divided into two groups: The first are passive fans, which are just an opening in the hull or superstructure through which air exchange ideally takes place automatically due to temperature differences or wind. The second are fans that actively transport air into or out of the boat electrically using a fan, thus ensuring a continuous flow of air. What both variants have in common is that they are protected against water ingress. Otherwise their purpose of removing moisture would be defeated.
In the group of active fans, there are some special cases with systems that go beyond simple ventilation. These include automated ventilation systems, devices that change the air temperature such as heaters or air conditioning systems, and finally dehumidifiers.
The first group includes mushroom vents, scoops, gill plates, rosettes and ventilation sails that channel fresh air through the hatch. Such a solution can be found on almost every boat, and forecastle boxes in particular are often ventilated using gill plates or mushroom ventilators.
Electric fans and ventilators from the second group are also common and are often installed in the superstructure roof (solar ventilator) or in corners of the cabin that cannot be cross-ventilated (ventilator). The electric fan from Vetus, which is specially designed for retrofitting existing passive mushroom or scoop ventilators, is interesting.
Important: How is water transported in the air?
Heating alone cannot remove moisture; this is only possible in combination with ventilation. Air conditioning systems extract water from the room air through condensation. Although the units are getting smaller and smaller, they are usually only found on larger yachts. In 2021, for example, Dometic presented a particularly compact model with low power consumption.
But with all systems, the decisive factor is how the room is ventilated. Just because air is exchanged does not mean that this is also beneficial to the indoor climate. To understand this, it is helpful to look at how water is transported in the air. This depends on the temperature of the air: the warmer it is, the more water vapour it can absorb. This is why we talk about relative humidity, because it depends on the temperature. It is of course possible to specify an absolute value, i.e. grams of water per cubic metre of air. However, this is not useful because the same amount of water leads to a humid indoor climate at low temperatures and a dry one at higher temperatures.
This much condensation is possible
As an example: At an air temperature of 30 degrees Celsius in the boat (the summer sun shines on the hull and superstructure in this example), one cubic metre of air can absorb almost 30 grams of water vapour until saturation occurs. If the air then cools down to 10 degrees Celsius (sun goes down, cool night), one cubic metre of air can only bind just under 10 grams of water; around 20 grams of water therefore precipitate out of the air as condensate, preferably in particularly cold places such as the hull below the waterline or the windows. On a ship whose hull and superstructure enclose 30 cubic metres of air, 600 grams of condensation would collect in the boat in this example, and the humidity would still be 100 percent.
The scenario is somewhat exaggerated, but it is conceivable on a regatta boat with lots of wet sails on board on a hot afternoon with significant cooling towards the evening. The moisture from the spinnaker and co. absorbs the warming air, but can no longer bind it when it cools down. Before this happens, it is the right time to air the room: Let the moist, warm air out through the companionway hatch and let some cooler air in through the open fore hatch. The temperature at which condensation emerges from the water vapour-saturated air is called the dew point. Even if the fresh air that enters the boat during ventilation contains some moisture, the relative humidity drops as it warms up in the boat and the air can absorb water again, for example from the damp sails.
Hygrometer displays the relative humidity
The simplest way to track this process is to use a hygrometer - it shows the relative humidity. If the value is lower after airing than before, the airing was successful. Values below 60 per cent are considered good, anything above this provides ideal conditions for mould and should therefore be avoided at all costs. The measuring device can also help to avoid mistakes, as the conditions are not always as favourable as in the example given. And if it is warm inside and colder outside, the procedure is no different from ventilating at home.
"On board, however, the conditions are usually much more difficult, because the water temperature is another factor that has to be taken into account," says Dennis Borchert from Lindemann in Hamburg. The company is a wholesaler of many of the fans on display. The water is still cold, especially in spring, and there are two temperature zones on the side of the boat: Below the waterline it is very cold at 10 degrees and above it is already quite warm at 20 degrees. This means that the relative humidity at bunk cushion level can be significantly higher than above on the saloon ceiling. Condensation and mould problems are inevitable here.
The design of the boat can also make a big difference: Classic lines with an S-frame have more lateral surface area and therefore a larger cold side. Insulating the hull can be helpful here. The problem is less pronounced with more modern U-shaped frames with less wetted surface under water. The ideal solution here would be a combination of heating and shock ventilation. This allows the heated air to absorb more moisture; if the hatch is opened, it rises and cold air enters the ship. The fact that heating energy is lost is accepted.
Less moisture without crew
However, if there is no one on board who can coordinate heating and ventilation, the ventilation has to work alone. If no one is breathing, cooking or showering on board, there will naturally not be as much moisture. Nevertheless, condensation should be prevented. This can be done with two solar fans, for example, where one removes the air and the other blows air into the ship. This creates an air flow from the foredeck to the saloon. The hope is that by circulating the air, the cold air from the footwell will be mixed with the warm air under the superstructure roof.
Ventilation panels, on the other hand, cannot be regulated at all. This could therefore mean that warmer, moist air flows through them into a cold baking box and causes plenty of condensation - the worst case scenario, in which case the fan would even be counterproductive. Normally, the cold air from the baking box would not escape upwards and the warmer air would not flow downwards into the baking box. However, this can happen if an active fan on the superstructure extracts the air and there are connections up to the baking box through which the air then flows in. The air flow generated by the active fan should therefore be carefully considered, as the air has to flow somewhere. The premise behind the ventilation panel on the forecastle is usually that damp mooring lines and fenders are stored there and, in case of doubt, it is always more humid there than in the ambient air anyway. According to this logic, ventilation should therefore always be effective.
Ventilation systems with sensors adapt to the conditions
Ventilation systems that use special sensors to find out what the water content of the air is outside and inside and then ventilate in a targeted manner to make it drier inside are a better solution. Such systems are nothing new for houses, but are not yet widespread on yachts. The Nordship 570 DS is the only exception. However, the shipyard only installed the Pluggit system at the request of the owner, who comes from the property sector. Automated ventilation is on the rise in houses due to increasingly energy-saving construction methods. To avoid wasting heating energy, the house must be airtight, but then air is only exchanged during ventilation. A lot can go wrong with this, and incorrect ventilation not only makes the home more humid, but also results in a lot of energy being lost. In this case, money is literally thrown out the window. Ventilation systems avoid this by using a heat exchanger. The heat energy from the exhaust air is transferred to the fresh supply air, which saves heating costs. In addition, all rooms, especially the bathroom and kitchen, are always optimally ventilated and the humidity is in the ideal range. However, you will not have the problem of dry air on board. All other points can be transferred from the house to the boat.
Even simpler systems are designed for drying garages and cellars and are called dew point ventilation systems. The centrepiece consists of two sensors, a control unit and a fan. The sensors determine the humidity inside and outside, and only when the ratio is favourable are fans activated to exchange the air. The advantage over air dryers is the low energy consumption: depending on the fan, the ventilation system requires less than 50 watts, while the dehumidifier requires 650 watts or more.
Special case of engine compartment ventilation
Engine compartment vents are a special case. Their purpose is not to remove moisture, but to supply air that is needed for the combustion process in the cylinder during engine operation. The air should not be drawn from the living space below deck. If the engine room is very cramped, exhaust air may also be useful to prevent overheating.
A poor indoor climate can easily be recognised by the smell. If the sliding hatch is opened after a few days' absence, the saloon is entered and it smells musty, then it is most likely too humid. Hygrometers are available for little more than ten euros and can be used to measure how humid the air is when you enter the boat. A data logger is even better - it provides an overview of the entire period of absence. To prevent moisture, there should be no wet sails on board and the bilge should be as dry as possible. In this case, active ventilation that generates an air flow through the boat makes sense. Solar fans are sufficient for this. This way, the cruise does not start with thick, but healthy and pleasant air below deck.
Automatic ventilation systems
Particularly in energy-efficient homes, air exchange cannot take place without ventilation. As it is easy to forget to open windows regularly and a lot of heating energy is lost in the process, automated ventilation systems are a good way of ensuring a good indoor climate, mould prevention and low heating costs. The air in damp rooms such as bathrooms, kitchens and bedrooms is extracted and fresh air is supplied to the living area. A so-called cross-counterflow heat exchanger transfers the heat energy from the exhaust air to the colder supply air. Until now, electronically controlled ventilation systems have not been used on yachts. The Nordship 570 is an exception, but it is also a very large boat. A ventilation system from Pluggit was installed. The ASPV1.0 living area ventilation unit measures 60 x 100 x 48 centimetres and weighs 34 kilograms. The system can circulate between 80 and 160 cubic metres of air per hour. That's quite a lot for a boat. At the same time, it only consumes 0.25 watt hours per cubic metre of air circulated - i.e. 20 watts per hour at the lowest setting. That is very little and is roughly equivalent to the consumption of a classic light bulb in the position light.
How to measure the humidity in the boat
Measuring is knowledge, and a little theory can't hurt when it comes to dealing with moisture in the boat
The warmer the air, the more water it can absorb. As a result, the relative humidity changes with the temperature. The value is given as a percentage; at 100 per cent, the air is completely saturated. If the air is heated, the relative humidity decreases. If, on the other hand, the temperature drops when the air is completely saturated, water condenses and can no longer be bound in the air.
This effect causes moisture on the inside of the side wall, for example. However, it can be used to remove moisture from the air, such as in a condensation dryer or with the air conditioning system. Especially in cold outside temperatures, there is a risk of getting wet inside the boat.
In order to take targeted action against humidity, it is crucial to know how much water is contained in the air on board. This is measured using a hygrometer. As the value only provides information in conjunction with the temperature, a thermometer is integrated into the measuring device. To prevent mould growth, aim for values below 60 percent relative humidity.
