Ventilation
by Don Casey
When I walk the docks on a weekday, I see far too many closed-up boats
without any signs of ventilation. If yours is one of them, you should
know that stagnant air in the cabin is not doing your boat any good.
Because hotter air can hold more moisture, the air inside a sealed boat
on a summer day will be about three times as wet as the air outside-air
that in the summertime at the shoreline is likely plenty humid to begin
with. This steamy air permeates everything inside the cabin, fostering
rot, corrosion, and mildew that are damaging to boat, gear, and locker
contents. Moisture trapped inside the boat even contributes to saturation
of the fiberglass-the cause of hull blisters.
The most obvious source of interior moisture is water in the bilge,
but even when the bilge is bone dry, the air inside an inadequately
ventilated boat will still be wet. The daily heating and cooling cycle
acts like a heat pump. The warming air sucks in moisture from the outside,
which condenses out when the cabin cools at night. A few days of this
cycle and the interior of your boat is as soggy as a rain forest. This
is doing damage to your boat.
In
AND Out
Every boat with a cabin should have at least two ventilators to let
the cabin "breathe" when hatches are closed and portlights dogged. One
ventilator isn't enough. Like opening two windows in a house to get
cross ventilation, you need at least two ventilators so wet air can
flow out as dry air flows in.
The simplest of vents is the louver. Overlapping slats admit air but
exclude rain. You can create a louvered ventilator in a companionway
door or drop board with a series of parallel horizontal saw cuts made
with the blade set at about a 30¡ angle. Or you can cut a square or
rectangular hole in the same door or board and screw an inexpensive
metal louvered vent over this opening to deflect rain.
Another simple vent is the so-called clamshell. In this case, the vent
opening is a round hole cut with a hole saw. The clamshell vent fastened
over the hole serves as a hood to keep out rain. Clamshells are often
used to shield the outlet opening for a bilge blower. Clamshell vents
are generally less satisfactory on horizontal surfaces than on vertical
ones.
The traditional boat ventilator is the cowl vent, a vertical pipe with
a bell-like horizontal opening. A large-mouth cowl vent standing proud
above the deck and facing into a breeze can funnel a great deal of air
below. Unfortunately, cowl vents also admit rain. The solution for a
boat on a mooring is as simple as facing the vent aft. While it no longer
"catches" the breeze, an aft facing vent becomes quite effective at
extracting air. In concert with a second opening-a louvered companionway,
for example-an aft-facing cowl does an admirable job of exchanging cabin
air.
For a boat in a slip, facing the cowl aft will not exclude rain since
the wind is just as likely to blow from that direction. In this case,
the cowl vent should be mounted on a water trap or a Dorade box, either
of which prevents rain from coming below.
A trouble-free method of providing excellent closed-cabin ventilation
on any boat is a pair of water-trap-mounted cowl ventilators, one facing
forward and one facing aft. In nearly all conditions, this configuration
sets up a beneficial flow of air in one vent and out the other.
For continuous closed-cabin ventilation, the ideal ventilator may be
one with a solar-powered fan. Powered vents use an integral solar panel
to run the fan during daylight and, in some units, to recharge internal
batteries that keep the fan running after dark. Paired with a cowl ventilator
or a sizable louvered vent, a single solar-powered exhaust vent can
extract a volume of air every hour equivalent to the entire interior
volume of a 30' boat. Not only is this good for your boat, but it makes
going aboard on a hot day noticeably more pleasant.
All vent installations are essentially the same, whether louvered, clamshell,
cowl, or solar. You determine the appropriate location, cut the appropriate
opening, carefully seal the edge of any core material your cutout exposes,
and mount the vent over the hole. Where the vent opening is exposed
below, a trim ring completes the installation. More detailed instructions
can be found in How-To #32 (Item 911032) titled "Installing Hatches
and Deck Plates." For the most effective ventilation, be sure there
is plenty of separation between intake and exhaust vents.
Ventilating
lockers
Getting fresh air into the cabin is the essential first step, but you
also need to ventilate all the closed compartments inside the cabin.
It is for ventilation that locker doors on boats are often louvered
or given cane inserts. Solid locker doors should, at the very least,
have a pattern of slits or holes or a fancy cut-out. It is usually a
good idea also to cut vent holes at the back between lockers to give
each locker that "two window" configuration that fosters good air flow.
When you are off the boat, leaving cabin and locker doors partially
open is the easiest and arguably the best way to ventilate enclosed
compartments. Of course, unlatched doors are at risk of swinging and
banging with the motion of the boat, but the solution to that is easy
and inexpensive. Simply install a latch hook on the inside of the door
frame and screw the eye to the inside of the door. This lets you latch
the door slightly ajar. If a standard latch hook doesn't hold the door
open enough to satisfy you, it is a simple matter to make longer latch
hooks with a couple of screw eyes and a length of stiff wire.
Latching all the lockers ajar when you leave the boat takes only a minute,
and it admits not only air, but mildew-slaying light into the locker.
Try it. You'll like it.
Keeping
Cool
When you are aboard you will often need more air flow than a few deck
vents can provide. Opening portlights help, but the big gun is the deck
hatch. The more deck hatches, and the bigger they are, the cooler it
will be below decks.
When the wind is light, a fabric wind scoop rigged in the hatch can
send a delightful breeze through the boat's interior. When the anemometer
cups stop spinning altogether, a few strategically placed fans may be
all that stands between you and discomfort. Of course, you could also
install air conditioning, but that is another subject.
For more information about improving onboard comfort, consult Dragged
Aboard: A Cruising Guide for the Reluctant Mate by Don Casey.
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