89 mid cabin fuel question

[mrrudely]

looking for opinions

we have a total fuel capacity of 240 gallons
we have gone through about 250 gallons per year over the last 5 years
we are wondering if it is not a better idea to fill the tanks only to 1/2 or 2/3 rather than full
this way a larger percentage of new fuel would be mixing with the old more often
I always add Stabil each time

rr

[prowlersfish]

Good question. The only issue I can see is condensation. With that said my tanks rarely stay full. If I fill them it's a good chance they will be at 1/2 to 3/4 the next day. And will not be filled until I get to 1/4 tank, so my boat normally stays at 1/4 to 3/4 when sitting with no issues. So I don't see any issues and keeping the fuel fresher is a good idea.


Oh course using the boat more and burning more fuel would be best and more fun.

[Scott67]

mrrudely,
I'm of the opinion that this whole "condensation in the tank because you don't keep it full" is a myth. Condensation only happens when there's a temperature and "dew point" difference between the two spaces. I've done a lot of research and have personally witnessed a couple of tests with absolutely no condensation inside or outside of a partially filled large gas tank. All things being equal, the space in the engine room and the inside of our tanks, condensation just cannot happen.

The instances I've seen of water in tanks is self-induced, taking on bad gas from a supplier, loose fill caps, bad o-rings on the fill caps and overzealous spraying of water into the vent openings during washing. I'm sure there are others, but not one that I've seen from "condensation".

So, leave your tanks 1/4 or 1/2 or whatever suits you. I think there's more risk of running stale gas through the system then there would be trying to eliminate any chance of condensation by keeping them full. Besides, if there's ever a fear of water in your tanks, put a few gallons of 10%-15% ethanol gas in there, the ethanol is a good moisture evaporator.