How to be warm, dry and comfortable living aboard a small boat in cold weather.
After thirty years in Hawaii we had a lot to learn about being comfortable in cooler
weather. In the tropics it is easy. No condensation to worry about. Just protect the boat from the sun and rain
with an awning. Open all the hatches and let the trade winds do the rest. As we settle in for our second winter in the
sub-arctic rain forest of Southeast Alaska the challenges are somewhat
different and we are applying lessons gleaned from many sources along the
way. The learning curve was steep that
first winter with snow on the decks in Friday Harbor, WA.
Here is what we have learned about keeping the boat warm and dry through
the winter or while cruising the high latitudes:
the obvious priority but it is imperative to heat intelligently and make every
effort to limit the amount of water vapor released by combustion. Yes.
Fire makes water, or more accurately, releases the water in the fuel. Some
fuels contain more water than others. Unfortunately,
the most popular fuel in modern boats is the worst culprit. Propane releases more water vapor during
combustion than any other popularly available fuel. Propane, from a moisture
content standpoint, is the worst choice for heating or cooking on a boat. It is, however the cleanest and easiest to
use, especially for the cook.
power is available, electric heat is probably best as no water vapor is
generated. We use a 5200 BTU electric heater with variable speed fan that works
very well to both heat and circulate the air. We also have a 5000 BTU Origo
alcohol heater for use when shore power is not available. Either will keep us
comfortably warm with outside temperatures down to about 20f. We are also
looking into solid fuel and diesel heaters small enough for the Vega. Heater
design, as well as fuel type, is worth mentioning. Heaters that bring in fresh air for
combustion and send the water vapor produced by that combustion out through a
chimney are best, especially when combined with a low moisture content fuel. Emerson White reminded me in an email: “Heating
with an open propane or alcohol burner is alright, but it releases lots of
water vapor and other exhaust gasses into the cabin. A closed and vented stove
like a Dickinson (very popular up here in AK) can heat your cabin without
consuming O2, releasing CO2, CO or H2O into the air that you intend to breath.”,
an excellent point.
can hold more water vapor than cold air can.
That is why your breath does not condense in the now 70f warm cabin. But two adults, just through respiration,
produce an astonishing amount of water vapor (2 to 2.8 pints of water per day
per adult). Add the steam from making
tea and cooking plus the water vapor released by the combustion of your cooking
and heating fuel and pretty soon it is condensing on every hard surface and
dripping from the overhead. Your bunk is soaked in the morning from
condensation under the mattress and the clothes in the lockers are wet through. You insulate where possible and use products
like Dri Deck to keep stuff in the lockers away from the hull but it is still
wet in the boat; so wet that you notice the automatic bilge pump is running
more often. What is required is to
evacuate all the warm, moist air before the water vapor condenses out of it and
drips from that bolt head over your bunk into your ear. When it gets cold outside we must resist the
urge to light the propane burners and seal up the boat “To keep the cold
out”. This tactic just keeps the water
in and results in a wet, miserable cabin.
warm and dry we have to heat the air and get rid of the water. What to do?
following are notes I took before I gave away my copy of "A Warm, Dry
Boat", now out of print. I suggest you get a copy of if you can find it.
Also four-inch and one-inch hole saws.
The ideal ventilation system would
bring cold fresh air in at one end of the boat, through the heater into the
living spaces and lockers, then take the warm, moisture laden air out through
the opposite end.
Place intake vents as far as
possible from exhaust vents
Active exhaust. Passive intake.
Minimum flow = 15 CFM(900 CF per
hour) per crewman
Typical 30 footer = 1000 cubic feet
of cabin space
4" Nicro Day/Night vent flows
1000 CF per hour (when the fan is running)
Use a low water vapor heat
Insulate, where possible.
note from Emerson on insulation: “With regards to insulation, you want to be
sure to control vapor flow through that insulation. Water is drawn out of the
air as it cools, and if the air can pass through the insulation the temperature
falls as it migrated out towards the hull, drawing water out into the
insulation. This leads to biological issues inside of the insulation, and
reduces its effectiveness.”
Use Dri Deck and Hyper Vent to
create air spaces against hard surfaces where possible, especially under
mattresses and cushions.
ventilate. A good rule of thumb is one four inch active exhaust vent and one
four inch passive intake vent per crew member.
A four inch vent will flow nearly twice as much air as a three inch vent
so use the larger one where possible.
The solar vents by Nicro seem a good idea but after 7 years we have to
say that they do not work well in the high latitudes. There is just not enough sun when you need it
most. 12 volt active vents are what you
need but there are none commercially available from the usual marine
chandleries at this writing. We have adapted 12V computer type fans for use
with Nicro, Beckson and Vetus vents with excellent results.
in your lockers. These can be three or four inch holes with a chrome louver or
other trim piece or just one inch round holes or half inch slots between
lockers. Just make sure you have
adequate air flow throughout the boat. You will probably need to add both
active and passive intake and exhaust vents as well as circulating fans to any
boat if you plan to live aboard in the high latitudes. Computer fans are cheap,
quiet, use little amperage and are designed to run constantly. Two or three for your active exhausts and a
couple more for internal circulation should be sufficient for most boats.
non-absorbent insulation (Lealea has mylar/bubble wrap type insulation)
insulated and ventilated boat should not need a dehumidifier (But it can't
hurt.) I think that the best use of a
dehumidifier is when the boat is unoccupied.
A small electric heater with a low speed fan - no collection bucket,
would be my choice. When the boat is
occupied it should not be necessary.
the passive intake vents are three-inch Vetus mushroom vents on the after deck
behind the coaming and in the cockpit well.
The active exhaust vents are located in the forepeak. They consist of a four-inch Vetus mushroom
with a 12 volt computer fan right forward in the chain locker, a four-inch
Nicro solar Day-Night vent in the forward hatch and a three-inch Beckson “Bowl
vent” over the head compartment with a Vetus 12 volt exhaust fan.
is at the bottom of the companionway ladder.
This arrangement brings cool air into the aftermost part of the boat,
through the cockpit lockers and through vents into the main cabin. There it is picked up by the electric heater
at ankle level near the bottom of the companion way and drawn out through the
active vents forward, in the sleeping cabin.
tweaking the system but the principles clearly work. We still have work to do
but the boat is comfortable and dry. The
Nicro Solar vent, while it worked fine all summer simply does not work at all
after the equinox at this latitude, nor did it work well at 48 North. We hope solar and battery technology improves
because this is a great idea and works well in the tropics and during the long
days of the Alaska summer. While we have an additional
three-inch vent, the total flow through the two vents is less than we would
like. We will probably add another
four-inch exhaust vent in the form of a Vetus Mushroom with a computer fan and
another passive intake mushroom aft.
Another thing we need to do is improve ventilation in some of the
lockers. Just a matter of drilling