Rudy,

You don't find water in your tanks because you are in in a tropical area,
where the nighttime temperature does not usually drop below the dew point
for the absolute moisture content in the air. In the Pacific Northwest, in
particular in southeast Alaska, nighttime water deposits into the fuel is a
big problem, mainly in the winter.

During the daytime,  when temperatures are relatively high, moist air
enters the tank thru the vent. At sundown the temperature drops about 20
degrees,  the moist air in the tank falls below the dew point and water
condenses. If the tank air could be continuously exchanged with the outside
air this would not happen.

One year I left my Willard 40, Lilliana, in SE Alaska during the winter
with the boat not properly vented and no dehumidifier chemicals. When I
returned nine months later, in June, I discovered every drawer with
standing watering it!! A terrible mess with a loss of much of the drawer's
contents.  Another time I left a sailboat in that area with the cabin not
vented. When I returned nine months later there was several inches of water
in the bilges that are ordinarily completely dry. Also all the wooden
surface were covered with black mold. Fortunately  the diesel tank was
sealed.

To keep the condensing water out of my tanks I keep them full during the
winter and , more importantly, the vents are sealed. I also keep fans
blowing air through the boat .

Richard P.

But getting conditions in a partially full fuel tank to the point that
condensation actually occurs is a lot different than what happens to the
interior of the boat in the conditions you describe.

Rich Gano
Frolic (2005 Mainship 30 Pilot II)
Panama City area

You don't find water in your tanks because you are in in a tropical area,
where the nighttime temperature does not usually drop below the dew point
for the absolute moisture content in the air. In the Pacific Northwest, in
particular in southeast Alaska, nighttime water deposits into the fuel is a
big problem, mainly in the winter.

During the daytime,  when temperatures are relatively high, moist air
enters the tank thru the vent. At sundown the temperature drops about 20
degrees,  the moist air in the tank falls below the dew point and water
condenses. If the tank air could be continuously exchanged with the outside
air this would not happen.

One year I left my Willard 40, Lilliana, in SE Alaska during the winter
with the boat not properly vented and no dehumidifier chemicals. When I
returned nine months later, in June, I discovered every drawer with
standing watering it!! A terrible mess with a loss of much of the drawer's
contents.  Another time I left a sailboat in that area with the cabin not
vented. When I returned nine months later there was several inches of water
in the bilges that are ordinarily completely dry. Also all the wooden
surface were covered with black mold. Fortunately  the diesel tank was
sealed.

To keep the condensing water out of my tanks I keep them full during the
winter and , more importantly, the vents are sealed. I also keep fans
blowing air through the boat .