Re: full tank vs. empty tank for storage
Now things are getting to silly season. Five gallons? We are talking about 20, 30, 40 or more sitting for long periods of time, like 4 or 5 months over the winter. One month is not going to make much of a difference. Five gallons at 10 % ethanol is 1/2 of a gallon of ethanol which, if it separates is going to attract maybe a cup of water? Of course your water separator will catch it. You have nothing to worry about. If your boat is still on the water or on a trailer, take it out for a run and burn up the five gallons. Or siphon it off and put it in your car. It will be gone a couple of days.
And on the other subject of vents. Fuel tanks are required by Federal regulations and ABYC standards and every other marine fuel system standard you can think of, to be vented. If you want to close it off when it's in storage fine, but don't forget to open the vent when you are using the boat. There is a very good reason for this. If a fuel system on a boat is closed (like on cars now) and there is any positive pressure in the system, and you get a leak, it will dump the entire contents of the fuel system into your boat. This will not make for a pleasant day when you try to start your engine.
On the problem of vents spitting fuel: Most boats are designed to have the vent at the high end of the tank when the boat is sitting on its waterline in the water. Frequently when boats are on a trailer or blocked up on the hard the fuel tank is not in its correct attitude. Especially when you see trailers sitting with the tongue lowered to the ground. if the tank is nearly full or completely full, as the temperature increases during the day, the pressure will force the fuel up the vent because the vent opening in the tank is below the top of the fuel level. Most tanks today (and made for the last 10 years or so) have a 5% ullage, that is, a space at the top of the tank for expansion. Usually this is accomplished simply by extending the vent a short way below the top of the tank. Some tanks actually have a bump or or space built into the top of the tank, but that is the exception. So you need to be careful if storing a boat with tanks full to make sure that the waterline is level. if it is, the tank vent opening should be above the level of the fuel.
Ps: this is also why many tanks cannot be filled all the way to the top. You're not actually supposed to fill them to the top. This is also why the auto shut off on gas pumps doesn't work with boats. ABYC is currently working on developing a method for the auto shutoff to work.
One added note: I am talking about permanently installed fuel tanks. Portable fuel tanks are a different matter.
