(Ventilation Cont)

Active vents

These are usually mushroom type vents, with a powered fan installed inside the vent body. These are usually solar powered, but there are a couple of units available that are powered from the ship's 12 volt system. There are even units available that offer both solar and 12 volt power sources. Nicro™ makes a Day and Night™ solar vent that uses the sun’s energy to run the fan during the day and charge a Nicad battery, which will run the fan at night. Most of the active vents have interchangeable fan blades that allow them to be used either as exhaust or intake vents.

A proper ventilation system will exchange the air within the cabin about once an hour. An average 35' cruising sailboat will have an interior volume of about 1,200 cubic feet. The airflow ratings of the various passive vents range from 350 to 600 cubic feet per minute (cfm). The airflow ratings of a powered vent range from 700 to 1,200 cfm. This suggests that a minimum of two vents for exhaust and two vents for intake are required to properly ventilate a boat of this size. Smaller boats will need fewer vents, and larger boats will need more.

The vents should be arranged to allow for as much cross flow inside the cabin as possible. If using passive cowl vents, try to mount them in pairs. Align one of the cowls so it is facing forward, and the other facing aft. Then, no matter which way the wind should blow, one vent faces upwind, forcing air in, and one faces downwind, creating a draft and pulling air out. A combination of cowl vents and mushroom vents will provide similar results. An even better system might include two passive vents in the middle of

the boat with a solar powered vent at each end. The powered vents pull air from the interior, and the passive vents allow air to flow in. This creates a very complete circulation inside the boat. In some cases, you could consider using hose cuffs and plumbing bilge vent hose directly into the bilge, which would make sure fresh air traveled all the way into and through the bilge area. Careful thought is required to be sure the optimum locations are chosen for vent locations, and that the best combination of available vents is used.

I am currently planning an improvement to the ventilation system on our boat. The boat is a classic sailboat, with dorade boxes on the cabin top, and cowl vents at each end. I am planning to install two of the Nicro 12 volt fan kits into a couple of the traditional looking cowl vents on top of the dorade boxes. This will retain the traditional look of the boat, but improve the circulation substantially. These little fans only draw about 0.10 amps, so battery drain will be minimal, and will be easily replenished by the onboard battery charger. A modification to this idea might be to mount a solar battery charger and wire it direct to the fan in the dorade box.