The answer based on lab and field testing is that it requiress VERY specific conditions to accumulate water from the air, but these conditions do occationally occur.
No, an ampty tank is not similar to a 1/2 full tank. In an empty tank, condensation
evaporates during the day yielding no net accumulation. In a 1/2 full tank the water drops to the bottom and does not re-evaporate if--and this is the important part--the fuel cannot simply absorb the water. Virgin fuel has a very small but signifigant ability to disolve water.
The thing is it depends on:
* How dry the fuel was.
* How full the tank was.
* Size of the tank (smaller tanks are more vulnerable).
* Temperature swings (Spring and Fall are most troubling, mostly Spring).
* Humidity and water temperatures (50% is low for boat in the water, 100% at night is more common).
In some tanks I could create haze and a few drops in the bottom, and in some not. Never enough to be important and it ALWAYS
redisolved when the temperature rose. Thus, you would be very unlikely to ever see it.
rates and bio-growth both increased even without free water, as the disolved water content rose, so keeping out disolved water is also important.
What to do (in order of importance, IMHO).
* Check your filler cap and vent. Most common problem, by far.
* Install a good filter separator.
* Buy from a high volume outlet.
* Burn enough to fuel to change the tank 2x per year. You change your oil
, don't you? I understand this is a real challenge for some large tank boats. See my last suggestion. And cruise
* Use an additive that prevents corrosion
(steel or alluminum tanks).
* Use an additive that prevents bio-growth.
* keep the tank full.
* Consider a fuel tank
vent filler. Cheap insurance
and they work.
What to do with gasoline tanks is a whole nuther issue. However, I will start with the fact that many yards (the one I use and several other is town) prohibit emptying gasoline tanks in dry storage
; the explosion risk is greater and the insurance
company says NO.
For gasoline the questions are different, but without getting long winded, empty for very small tanks and full with a vent filter for larger tanks. 1/2 tank is pure trouble.
I'm assuming Cotmar understands that water in the crankcase is from blowby (burning fuel makes CO2 and H2O), not the atmosphere.