I have always thought that it was better to store your boat over the winter with a full tank of fuel to reduce the formation of condensation in the tank.
Now I hear that because of the reformulated gas with ethenol, it is better to store the tank empty because ethenol actually accelerates the formation of condensation. Others tell me that is simply not true.
I'm so confused!
Can someone in the know settle this argument please.

Thanks-

I leave them as empty as possible. Condensation occurs when warm moist air hits a colder surface. In winter, there is a lack of warm moist air so there shouldn't be any condensation. If I'm off base here, somebody please correct me.

Store the boat with empty tanks. Or with as little as possible. Put some gas stabilizer in what is left. Fill up in the spring. If there is a little condensation and you are in an area where it's going to freeze, you can add a little gas line antifreeze before you lay up the boat for the winter.

Indy,,, you are correct in your theory.
Condensation builds in gas tanks when the fuel is replaced with moist air as the fuel is consumed. When the fuel pick-up in the tank is slightly higher than the bottom of the tank, moisture will condense into a puddle of water at the bottom. This is a cumulative effect and after many filling cycles and cool evenings the moisture begins to build in the tank.If fuel antifreeze is added at the end of the season or at intervals during the boating season the puddle of water is dispersed into the gas and burned. A cup of denatured alcohol in the last tank of the season will take care of any moisture that might be left in the tank. Fuel line antifreeze sold in auto parts stores in just denatured alcohol. Don't use isopropyl alcohol, that is fifty percent water already. You can buy a pint of denatured alcohol at a paint store add a little to your tank at the end of the season. It doesn't take much to mix with the moisture in the tank and send it through. About 1 cup to 50 gallons is plenty.

Thanks Reed- Have you always stored your tanks empty or have you only started doing this after the introduction of the ethenol fuels ?

I've always stored them as close to empty as possible. That way in the spring you will get a higher ratio of fresh gas to the older gas. Old gas has a little difficulty igniting. If you are going to lay up the boat for a very long time it is better to completely drain the system and and carbs.

With Ethanol fuel (E10 meaning 10% alcohol 90% gas mixture) it's not the condensation that is the threat. It's the hygroscopic properties of the alcohol/fuel mixture meaning that the ethanol will separate from the fuel (lowering the octane rating) by bonding with any water present in the fuel (called phase separation). This water is not necessarily globbed up at the tank bottom, but can be mixed (emusified) in the fuel itself which normally is how it typically will pass thru your system. So ideally you stow with as little fuel as possible and load the remaining with ample stabilizer like "Stabil" or the new products "Pri-G" and "Startron" enzyme fuel treatment. If the the tank sweats a little during the off-season that's fine and can be eliminated with a fresh full tank of "treated" fuel in the spring and harmlessly ran thru. No matter what you will need to treat what remains then run your engine so the treated fuel gets into your separators and injectors/carburators. First thing you do in the spring "prior"to running your engine for the first time, is put new filters on even if you put new ones on to winterize. Reason is you might spin off your thought to be new filters only to find them heavily rusted on the inside from the water produced by phase separation. If not, then the filter is probably good to go.

Regardless, if you're using E10 fuel change your filters more frequently than ever before, and pour it's contents into a glass jar to check it's condition.

There are some gas stations outside the metro area that do NOT sell E10. I have about 10 gallons in a 200 gallon tank right now. I'll be towing to those far off stations (actually on the way to the river) for the rest of the "summer" so I can store the boat without E10 in the tanks. Still will stabilize the fuel as always but will probably leave the tanks low.

Be carefull! From what I've heard ALL fuel pretty much is E10. It's not necessarily labeled on the pumps. They may swear it's not E10, but they probably have no idea. They usually don't even know who they're actually getting the fuel from.

PS. What's the differnence between Stabil (expensive), and the other alcohol based gas treatments (cheap)? What does Stabil do different?

Exactly