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keep fuel tank full or not?

Old 09-18-2020, 02:03 PM
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Default keep fuel tank full or not?

Just bought my first boat with an enclosed fuel tank. Always had a skiff with two portable tanks, for 30+ years. This boat has a 70 gallon tank. I would like to keep things light and only have a limited amount of fuel in the tank, but a friend tells me I should always top it off after running to keep condensation down, and to keep the tank from rusting or oxidizing. Advice?
Old 09-18-2020, 02:04 PM
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Keep tank full.
Old 09-18-2020, 02:08 PM
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I’ve always filled it up and added stabil. Sitting for 2-3 months and never a issue
Old 09-18-2020, 02:15 PM
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I Fill up after each trip
plus ring plus and stabilizer although inconsistent mostly when fill 5 gallons and over
Old 09-18-2020, 02:19 PM
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Unless I'm going on a long trip I keep mine at about half tank. I have a 300 gallon tank and 150 gallons of fuel is a lot of weight so it helps the mpg some.
Old 09-18-2020, 02:23 PM
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Condensation will happen regardless of how much fuel is in there. If it's topped off - only the very upper portion can corrode. This goes for the mechanical and electrical equipment inside the tank as well. The less fuel that's in it opens up the area exposed above the fuel to corrosion. The only way to prevent condensation is to store it in a climate controlled building.
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Old 09-18-2020, 02:34 PM
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We are keeping ours full thanks to lower prices now, but we have 120 gallons and it is a lot of weight. 7.3 pounds per gallon of fuel means, 60 gallons is 438 lbs. if we only wanted to keep it half full. We also don't add anything to the Regular gas since we use the boat regularly and only buy marine dock gas (which is supposed to have the good stuff in it). I think it would be OK to only keep in it what you'll use, factoring in safety.
Old 09-18-2020, 02:39 PM
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my boat is 33 yrs old never filled it for storage i live in the north east i always used MDR stor and start about 12oz for the winter and have never had an issue, im not saying its wrong to fill the tank, just saying i dont do that i have a 120 gal tank and never have more than 60 gal in it.
Old 09-18-2020, 02:39 PM
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I don’t pack mine tight unless a long run day is happening. I’m only at 40 gallons or so. Keep it at under 3/4 tank on a fill up with a calibrated fuel sender .
Meaning i have pulled and checked it and a history of where it reads on the guage.

The most important thing rather than keeping it full ,,,,,,, is Stabil added at a maintenance ratio every fill with non ethanol fuel everytime !!

IMO
Old 09-18-2020, 02:41 PM
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When you go out for the last time for the season, Use the stabil at the storage rate before the run .
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Old 09-18-2020, 02:41 PM
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90% - 95% full, allow for expansion but reduce the ullage area.
Old 09-18-2020, 02:47 PM
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During storage or low use I’d keep it full. If using boat, let it run down and refill.
Old 09-18-2020, 02:48 PM
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I'm somewhere between 1/4 to 1/2 just to save weight unless I need more.
Old 09-18-2020, 03:05 PM
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depends on usage
Old 09-18-2020, 03:11 PM
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My boat holds 130 gallons. Thats a fair amount of weight on trailer. I fill it up, slash it , and run it. Store it between 1/4 to 3/4 depending on last trip. Add marine stabil and watch my fuel separator bowl. No issues last 5 years.
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Old 09-18-2020, 03:11 PM
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Originally Posted by Absolute View Post
90% - 95% full, allow for expansion but reduce the ullage area.
Word of the Day
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Old 09-18-2020, 03:24 PM
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What is tank ullage?
Ullage is the void space in the tank measured from the top of the tank to the upper surface of the fluid. Ullage is measured when the content of a tank is highly viscous and if the tank is filled to the maximum. Sounding pipe is constructed in the tank.


Hell yeah!.....Always glad to learn something new!
Old 09-18-2020, 03:31 PM
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Originally Posted by Absolute View Post
90% - 95% full, allow for expansion but reduce the ullage area.
Agreed. I believe condensation comes from air in the tank in the ullage area. So reducing the amount of air in the tank reduces condensation. But can't completely eliminate condensation, so use fuel conditioner. That's my plan.
Old 09-18-2020, 03:31 PM
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It depends on the tank size. At 340 gallons I'm not keeping mine full. Too much weight. Only if needed for long runs.
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Old 09-18-2020, 03:34 PM
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Best thing you can do for a boat besides maintaining and cleaning is to use your boat. If you use it weekly in summer does not matter if tank is full or empty. Stabilize fuel in winter
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