We can use the suction on the lee side of a sail to pull the boat forward. In the same fashion, we can use pressure differentials surrounding the cabin to ventilate the cabin. Thus, understanding the flow over the cabin and the pressure differential it creates is vital to the success of any passive ventilation scheme. It also explains why some passive ventilation methods did so poorly in our testing.
The cabin top has a number of pressure zones, and these change, depending on the wind direction. At anchor, the front of the cabin is high pressure, the top slightly reduced, but this depends on the location; it is lowest just behind the front edge, slowly rising to neutral depending on the extent of turbulence. The companionway area has the lowest pressure; open a hole anywhere forward and air will be sucked out the companionway, through gaps in the weather stripping if it’s closed.
Air flow through vents can be interrupted not just by insufficient pressure differential, but also turbulence. Cabin edges, masts, changes in wind direction, and even other cowls can dramatically reduce the wind speed striking the cowl.
The most complicated case is at the dock, with everything buttoned up to go home. The wind can blow from any direction, and depending on the extent of protection afforded by the harbor and neighboring boats, it will be light most of the time. Passive vents are often rated at 10 knots, but the true wind, at deck level, in most marinas is less than 5 knots 90 percent of the time.
- Cowl vents on top of dorade vents reach up above disturbed air flow to capture air in what would otherwise be a ‘dead’ zone with very little flow. Notice the need for powered vents astern to ensure any significant ventilation.
- In the same location disturbed by turbulence in a bow-on wind, a low-profile Nicro Mini-Vent 500 may produce very little ventilation flow. However, the same vent exposed to wind-flow across the bow will be effective.
