Engine room ventilation may keep the space at negative pressure when passenger areas are adjacent to the room or at positive pressure, which has the advantage of preventing dust from entering. Engine rooms may need ventilation air both for cooling and for combustion.

In the case of passenger areas, ventilation is needed to provide fresh and clean-smelling accommodation and prevent carbon monoxide poisoning. Any kind of petrol use or stove, especially where there are fuel leaks, can potentially introduce carbon monoxide into the boat. Ventilation also helps to control mould, mildew and corrosion. A combination of cabin fans and passive vents can make the trip safer for the crew and passengers.

To comply with these requirements, a minimum of two ventilator ducts need to be installed. Larger boats will require more. While passive or natural ventilation can be used while the boat is running, ventilation systems will still need to be powered before the engine is started. For marine ventilation, the possibility of taking in water through vents also needs to be taken into account, especially for engine air inlets. Louvered vents made of stainless steel or plastic may be used for the latter application. Cowl vents are a popular and reliable option, especially Dorade boxes (sometimes called simply “dorades”), which are designed to permit air to enter while keeping water out.