Originally Posted by kellysails
Hmmm, I have always understood that you want the tanks full which reduces in-tank condensation. This is interesting.
Think about this .... do empty tanks magically fill with water from condensation? of course not.
Its the large thermal mass of fuel that leads and lags the 'thermal swing' hence accelerating the 'condensation' ... but this is on a minimal scale. Fuels are made at high temperatures thus are delivered in a 'dehydrated' state ... the water enters (equilibrates) from the atmosphere (through tank vents, etc.) and the 'drive' that causes fuels to eventually saturate with water is simple chemical/physical 'equilibrium' ... the larger amount of fuel mass the greater the 'drive' towards 'equilibrium'.
Once a fuel starts to 'drop out' water, or radically begins to 'condense' on the tank wall surfaces above the liquid ..... its already near to or is fully saturated
with water from the atmosphere. If you want less water, simply have less stored fuel.
Tank vent desiccant traps (often changed or often 'regenerated') is the simplest way to keep water out of a boat tank with 'fresh' fuel; after of course, preventing leaks at the 'deck fill'.
BTW 'boat fuel tanks', cannot withstand the 'vacuum' generated if you happen to think that putting a valve on the vent line will stop water vapor uptake ... because you risk IMPLODING such a tank when the temp. goes way down.
NO valves on boat tank vent lines !!!!!!!