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Our 10/17/2012 post “Putting Your Boat to Bed” generated many conversations regarding fuel in the tank. I asked Columnist and freelance writer Captain Fred Davis to review the subject and here is what Captain Davis reported.

I’ve spent quite a bit of time researching and reviewing the topic regarding fuel in the tank your bloggers inquired about. I’ve talked to chemists, fuel company reps and other experts.

Oil company reps suggest your fuel tank should be filled to around 90% after treating with a marine stabilizer. It’s best to replace the water separator filter just prior to stabilizing the fuel. When adding the stabilizer, (Sta-Bil or StarTron additives work well), be sure you use marine grade which is twice as strong as the automotive mixture. Do not skimp on the stabilizer; it will do no harm if you use extra. Be sure to run the treated fuel completely through the entire system.

Now the controversial questions: Why not drain the tank, leaving it near empty? Or, as some suggest, why not “leave it half-full” so you can add a half tank of fresh fuel next season?

Ethanol is an alcohol; it absorbs moisture during the season. The water, mixed with the ethanol will pass through the system and burn with the mixture. While in storage, the fuel starts to break down in approximately 30 days. Once the alcohol absorbs enough moisture the breakdown separates leaving a thick content at the bottom of the tank. Absorption takes place as the moisture in the tank, having entered through the vent, exceeds a certain point causing phase separation to occur.

If the fuel tank has more air space it can circulate more moisture, which expedites phase separation. Less air space means less moisture and therefore, if the tank is 90% full there is still enough room for expansion as the tank warms from the sun shining on the cover or as warmer air temperatures circulate around it.

There are opinions put forth that emptying the fuel completely is the best action. Actions, such as using a shop vac may prove dangerous. In many states, Fire Marshals prohibit storing a boat with an empty fuel tank. An empty tank could be considered a bomb because of the danger of fumes.

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29 Responses to Fuel in the Tank

  1. Mark Bolduc says:

    Sea Foam is my additive of choice. Has been for years.

  2. Gary says:

    Yes there are additives for diesel as well and do the same job or better. I wont be worring anymore since I lost my 43′ Egg Harbor in Huricane Sandy. It sank with twinn 6-71 Detroit deisels,

  3. John Borum says:

    Is Marvel mystery oil considered a good fuel stabilizer? will this product prevent phase separation?

  4. Tim Shealy says:

    Great advice. I use Sierra and Lucas ethanol additives but they are very expensive. Don’t be afraid to add a little extra. 1 oz treats 10 gallons.

  5. gary rose sr says:

    full tank is best in anything you have car. mower. mc. boat. marine stabilizer is the best have done this fore years ben a mach for 30 years

  6. Rod Stewart says:

    This article talks about storing a gas powered boat. What is the recommendation for diesel?

  7. Clifford Peterson says:

    My boat stays in the water year a round (Galveston, TX area). The boats sits for long periods of time between uses. There are additives on the market called BioGuard or BioBoard. Are these any different than a Stabil additive?

  8. Jim Lundie says:

    Another good reason NOT to use Ethanol added fuel in any boat. How about advice for those of us who don’t?

  9. Bud Magaldi says:

    What about boat motors that run marine (non Ethanol)fuel?

  10. Mike Baquet says:

    What if you use what is sold as “ethanol free” gas from marinas? I live in S. Louisians & all marinas in my area advertise ethanol free gas. If there still truely ethanol free gas, or is it that they don’t have to state it has ethanol if the percentage is less than 10%????

  11. Mark Bolduc says:

    What if you use ethanol free, or pure gas? That topic seems to be missing form this conversation. Thank you.

  12. Neil Iden says:

    1-I assume you are talking gasoline and not diesel. You never identified which you were talking about.

  13. Joseph Krukosky says:

    Good advise, but does the same apply to diesel?

  14. Scott Versluis says:

    Thank you for the info this has been a question of mine for along time . What do you recommend staybil or seafoam for the fuel tank . I have a 90 hp Honda on my boat with a 12 gallon tank . Thanks , Scott ( Michigan)

  15. Doug Penland says:

    Great Information! How about a story on Diesel fueled boats?

  16. Robert Mayer says:

    Why not install a quick drain in the bottom of the fuel tank and drain out what ever seperation there maybe after the winter season is over. Water is heavier than fuel. General Aviation aircraft do this all the time prior to each flight.

  17. Isidro Salas says:

    Nitrogen is cheap. Take N2 purge the empty tanks. Leave an N2 blanket in the tank. N2 not only keeps a tank safe from an explosion or fire, but it also dries the moisture in the space. N2 is an amazing gas to use. Just my opinion.

  18. Eileen says:

    Please help me. I have a boat that has been in storage with a full 2 tanks for the last 6 years, and I’m afraid to run the engines with this fuel in it. What should I do? If I need to drain the tanks and refuel, who does that? My boat is at Great Oak Landing in MD. Please help as my husband passed away and left me with this situation and I don’t know what to do. I don’t want to sell my 3807 Carver cause I love boating. But, I know something needs to be done. Maybe I should just run the bad fuel out??? HELP!! Thanks, Eileen

    • Do NOT run the old fuel thru the engine. There are many safe ways to get the fuel from the tank. In an open, not pressure tanks after 6 months gasoline starts to loose much of its octane. Contact the storage facility and ask if they know of someone who can remove and clean the system.

    • Capt. Fred says:

      sorry to hear of your loss
      i suggest you have your marina
      take care of your tanks. dont
      just run them out

  19. Alan says:

    How about Diesel?

    Additives or not, full or not?

  20. George says:

    I take it this applies to gasoline engines/ tanks, not diesel. There is no alcohol additive in diesel.

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