How to Improve Cabin Ventilation with Solar Vents, Dorades and Hatches

Look for active leaks: stains under portlights and hatches, wet lockers, drips at chainplates and stanchion bases, clogged cockpit scuppers. Fix those first (see the hatch, portlight and rebedding guides). Then note where you already smell must or see mildew: the head, hanging lockers, under cushions, the bilge. Those are the places that need air.

Roughly multiply length x width x average headroom of each cabin space and add them. Compare that number with what your current vents can move in an hour. If the boat is unattended most of the time, count only the vents that work with no wind and no shore power.

Air has to have a way in and a way out. Practical Sailor stresses that planning for exhaust is just as important as planning intakes, and BoatUS says larger boats typically need two powered vents, one in and one out. A common layout:

• One vent set to exhaust where the air is worst: the head, galley or aft cabin.
• Intake at the other end of the boat: a dorade or cowl facing forward, or a second solar vent set to intake.
• Interior doors, locker doors and floorboard openings left open so air can travel between them.

Practical Sailor found that the exact spot on the cabin top changed everything: a passive vent in a low-pressure area exhausts, and moved a few feet into a high-pressure area it becomes an intake. In turbulent spots, flow can stop entirely. Boats on a mooring usually point into the wind, so forward-facing cowls work well there; boats in a slip may face any direction. That's one reason a powered solar vent, which doesn't depend on the wind, is so popular.

• Size: larger vents move more air. One Day/Night 4-inch model is advertised at about 1,000 cubic feet per hour; smaller 3-inch models move less. Check the rating on the box.
• Battery: a Day/Night style vent with a rechargeable battery keeps working at night.
• Replaceable from above: some newer vents thread into a deck plate and can be removed from outside and the hole capped with a solid plate for storms or winter.
• Fits an existing hole: many replacement vents fit the cutout of older snap-in vents. Measure yours before you cut anything new. In SAIL's example, an old 3-inch clamshell cutout had to be enlarged slightly to fit a solar vent.

• On a flat, unobstructed area of the cabin top or deck that gets sun for most of the day (solar vents) and stays clear of sheets, lines and feet.
• Away from where green water or cockpit spray can sit on it.
• Clear underneath: no wires, hoses, handrail bolts, headliner panels or bulkhead framing in the way. Check from inside with a flashlight before you mark.
• Vents need a reasonably flat surface. If your cabin top has a lot of curve, the vent maker's instructions will say when a shim is needed.

Mark the center, tape the area to protect the gelcoat, and drill a small pilot hole. Check inside that the pilot came through where you expected. Then cut with the hole saw from outside, letting the saw do the work. Clean up any dust inside right away.

Most decks have a core of balsa or foam between two fiberglass skins. If water gets into it, the core rots and the deck goes soft. In SAIL's install, the author coated the cut edge of the hole with thickened epoxy, and drilled the screw holes oversize, filled them with thickened epoxy and redrilled them after it cured. That way water can never touch the core. Let the epoxy cure fully before you mount the vent.

Dry-fit first. Apply the marine sealant the vent maker recommends around the flange and screw holes, set the vent, and tighten the screws evenly until sealant just squeezes out. Don't overtighten plastic flanges. Clean the excess sealant with a plastic putty knife and the solvent the sealant maker recommends. Install the interior trim ring and insect screen.

Charge the vent in sunlight as the instructions say. Set the switch to intake or exhaust as planned. On a still day, close the boat and hold a tissue or a light ribbon near each vent to check air is moving in or out. In SAIL's example, the owner set both solar vents to exhaust and let two cowl vents feed fresh air in.

Dorade boxes work by catching water behind a baffle and letting it drain out through holes at deck level. Good Old Boat points out that water trapped in the corners can cause rot or corrosion, so keep the drain holes clear of dirt, leaves and wax. Check the box's bedding to the deck like any other fitting. In heavy weather or offshore, turn cowls away from the seas, or remove them and fit the deck plate. Practical Sailor measured that a dorade box itself costs some airflow, but often adds flow overall by lifting the cowl above the turbulent air at deck level.

• Wipe the solar cell clean regularly; dirt, salt and bird droppings cut the charge.
• Batteries wear out. If a Day/Night vent stops running at night, the battery may need replacing; check the maker's instructions.
• Listen for a noisy or stopped fan and clean or replace it.
• Recheck the sealant around the flange each season and reseal if it's cracked.

Prop open lockers, lift cushions on edge, and open floorboards over the bilge when you leave (and put a note on the companionway so nobody steps into an open bilge in the dark). BoatUS also suggests taking some gear out of lockers so air can circulate, and using mesh liners under stored gear. Under a winter cover or shrink wrap, make sure the cover has vents too, so the boat's vents have somewhere to exchange air.