I prefer to keep my tank near empty for winter storage. Ethanol blended gas absorbs water. My thought is less gas=less absorption or less contaminated gas. Before ethanol it made sense to keep the tank full. The gas could be preserved with a stabilizer helping to eliminate water from getting into the gas. Today many say that "no product exists that will prevent the absorption of water by alcohol (ie: Ethanol). Read this: http://www.fuel-testers.com/expiration_of_ethanol_gas.html

So if what they say is true and no product exists that will prevent the ethanol from absorbing the water. I think less gas with a stabilizer is the way to go. I like the idea that my tank is filled in the spring with 80-90% fresh gas to dilute the possibly contaminated phase separated gas. Also keep in mind that during the cold months we store our boats the air (cold air) holds less moisture (humidity) and all this worrying may be for nothing.

We store with the tank full and use Star-Tron the last trip of the year and the fill-up afterward.

This looks like a pretty fair analysis: Seaworthy on Winter Fuel Storage. I like the advice to use your boat regularly!

The volume of the tank walls would need to be so much greater to actually accumulate condensation worthy to notice.

Here's a good reading with actual reasearch done.

Myth of Condensation in Fuel Tanks

Thanks phat, I've been looking for SOME evidence, one way or the other, about fuel tank condensation all my life...

On top of this you add our fuel/water separator filter it really becomes a moot point.

Whatever is left in my tank is always Top Tier gas. Because of this, dare I say it, I never use any gas stabilizer over the winter. Obviously I don't recommend this for marina gas. Unless you know which retailer they purchase their last batch from.

While Pascoe is a well known and published surveyor, his analysis is full of holes. It totally ignores the accumulation of liquid water over a long period of time. This water is below the gas, so does not have a chance to evaporate.

I agree that there are other mechanisms that are more likely to allow water to enter the tank, but to claim that condensation is not significant is foolish without some experimental data to back up his rather skimpy calculations.

His analysis that it often gets cold quickly, but rarely gets warm quickly completely amazes me. Ever been outside on a cloudy day and have the sun come out? Ever been inside a tent (think covered boat) and been roasted when the sun comes out?

One thing that I suspect is that Pascoe mostly surveys yachts, which are typically kept in the water year round, and his analysis probably makes more sense for them. Pull the boat out onto a trailer and put a cover over it and the situation is quite different.

One point that he makes that I buy into is that the risk of condensation is higher during the summer than the winter. The risk here is that water may still be in the tank during the winter and you may end up with phase separated gasoline. Once it separates, you MUST pump it out, because you cannot add something to get the fuel to combine again. Running your engine on fuel that has separated will most likely damage it, so avoidance is prudent.

I just topped off today at $2.87 a gallon and I just know that's gonna feel good when I next use it in May when gas is around $4.00.

Don't be surprised if you fill up for $2.50 (or LESS) in May of '09.

I'd be very happy too.

Six year old boat with 5.7 Gi Volvo. Use Stabil and a fuel conditioner(not sure which one, comes in a black container). Starts in three seconds or less (except when the kill switch has been bumped--oh, well)always. I suppose there is nothing wrong with keeping fuel tanks full if they are small, but I am not going to tow with an extra 500 lbs of gas if I am going very far.
Previous two boats were carbureted outboards and did nothing and they always started--not very fast.

Mike

I added about 20 gallons to my 35 gallon tank. I was at a quarter tank, so I didn't fill it all the way up, but I wanted to add some ethanol free gas for winter storage. Unfortunately I paid 4.39 for it. This station by the lake sells ethanol free premium gas. I did add stabil.

I need about 50 gallons and haven't winterized yet.
How long does it usually take for oil price drops to trickle down to gas price drops? I figure I've already saved $50 by not topping off after our last outing, and saving another $50 would be cool.
Yep, I store it full.