Just to be clear...........

When I say "empty", You still need to add some stabilizer. Just thought that was a no-brainer.

Reason is...
 

You're filling the tank so there's less room for AIR,
which expands in the sun out the vent during the day.

Moisture laden air enters the tank at night
when things cool down.

Less air space = less moisture come spring.

Really, ya should fill up AFTER a hard days run, too.

Same reason.

Just to further confuse things... Here in Chicagoland gas is 10% ethanol. Mercury says to avoid it but if you can't never let it sit in the tank. So now what?

Fill it is what Merc says (but allow for expansion or it will overflow and leave stains on the boats around you or out of your vent)

This hits on some of the subjects.
I coppied this from this web site:
http://www.yachtsurvey.com/winter_lay_up.htm

There is plenty of controversy about whether gas fuel tanks should be empty or full at layup and you can find reasonable arguments for either condition. However, the argument for leaving them full overlooks the fact that modern gasolines go bad much too fast and six months is FAR too long. Old gas will definitely harm fuel injected MPI systems. Moreover, diesel oil is subject to bacterial blooms and oxidation as well.


Condensation developing in the tank is a much over-rated problem and here is the reason why: An empty tank contains a limited number of cubic feet of air, and therefore water vapor that could possibly condense. Air does not flow in and out through those tiny vents, plus you can always tape the vents over if that bothers you. The amount of water vapor in the tank comes to several ounces. However, if you have good Racor, Dahl or similar filters (which you should) these will easily be able to handle that amount of water. Smaller canister filters will be strained to get that much water out, but the large types like Fram cartridge filters can handle it.


What about fuel stabilizers, biocides etc.? While I'm not a petroleum chemist, I do know that these potions have a less than spectacular record of success. Personally, I'm not a believer in miracles sold in a can.


Therefore for both gas and diesel boats, it is best to arrange your fuel situation that the tanks be as empty as possible before you take the boat to the yard. Old fuel is a greater problem than a little water in the tanks.


Should tanks be completely drained for the winter? No, that isn't necessary. Besides, if you do, you'll have to refill them from 5 gallon cans in the spring which is messy, hard work and dangerous with gasoline. Leave just enough to get the engines started in spring and get to a fuel dock.


The old fuel left in the fuel lines is one of the reasons we have so much trouble getting engines started in the spring. In the spring you can bring a gallon of fresh fuel and prime the filters with it for easier starting.

*************

Sorry, they didnt even touch the issue of aluminum tank corrosion and Ive never had a problem with carburated motors after fuel sat for a lot longer than 6 months. The debate goes on.

MY engines won't even run with the bad gas. And they are only 600 HP 502's. The carbed engines will have the worst effects since an EFI engine will just detune for the bad gas. I pump it into my wife's suv and even it looses milage as the gas gets older.

While reading this topic this thought came to my mind. There are a lot of boat dealer businesses that are in the colder regions that would have to winterize their boats. I’m talking about the new ones that were taken out for demo rides and all the ones that have been taken in on trade. I can see them adding some stabil to the gas that is in the tanks but I would find it hard to believe that they are filling up all of the boat tanks during the winter months.
Just something to think about,
Cordell

What about keeping your tank full in the summer months when the moisture in the air is much higher than in the winter.

I try to keep my boat fairly full in the summer but probably like a lot of people I usually fill it before I go boating and then it seats all week with whatever was left in the tank. Since the air is holding much more moisture in the summer than in the winter this could be just as big as a problem.

Since I can never get a clear answer I just put stabilizer in with whatever I have left in the tank at the time I winterize. Also, be sure to run the engines to get the stabilizer in the fuel lines.

Someone mentioned what dealers do about boats on their lots and I'm sure their decision is based more on economics then what's good for the boat. I bought my boat in March which I know was on the dealers lot for over a year, I had a problem with water in the gas tank and had to run on one engine in August for 3 hours to get back to the ramp, and it was stinking hot.

Why did I have one engine? Formulas port side engines have the lower pick up in the gas tank and it was picking up a majority of the water, so it crapped out. So if you have a Formula you want to keep an eye on the port side fuel separtor to determine if you have a water problem. Although you should probably check both.

Oh yea, if you think it's expensive to put gas in your boat it's nothing compared to getting it pumped out.

CJC,
your just cheap
LOL
anything not to spend a buck.
funny how a credible argument can be made for either side.
I would rather leave 3/4 full and stabilize. Add some on the way to the first fire up. never had a problem, but then again...either way most have not had a problem according to this thread.

Can someone say when they had a problem????

I did 1995...454 carb. Left with about 3- 5 gallons in the tank for winter. Only time I ever had problems...last time I left her dry