Portable Above-Deck Fuel Tanks

[Ixolib]

I recently purchased a brand new 18-gallon Moeller fuel tank from a retailer in Madeira Beach, FL. On my first fill-up, I could only "squeeze in" about 16.5 gallons of gas (plus oil) and, because it subsequently overflowed onto my deck, I had to siphon off about a gallon or so for safety sake.

On my second fill-up (at a different gas station and different brand), I had the same situation, but this time I stopped at 15 gallons of gas plus 2.5 pints of oil. At this capacity, the fuel/oil level was just below the filler port and the guage read full. BTW, the tank is level on the deck, so that is not an issue.

My question is this... Should I expect to be able to pump 18 gallons of gas into an 18 gallon tank? And, given that these tanks are constructed & used mainly for outboard applications, shouldn't I also be able to add 3 pints (+/-) of oil without overflow being an issue?

The retailer says there's nothing he can do and I'm just "stuck". I'd like to hear y'alls experiences with your portable tank capacities (i.e., does a 6 gal tank hold 6 gals, and a 12 gal tank hold 12 gals, etc). Maybe I'm just expecting too much.

Thanks...
George

 

[Silvertip]

Re: Portable Above-Deck Fuel Tanks

Large tanks (18 gallons is not considered portable) so they do not take into account that you are mixing oil with fuel. It is an 18 gallon tank period. And because the tank may be level with the floor of the boat, does not mean the boat sits level in the water. It doesn't take much slant to reduce the capacity of the tank by a gallon or two. Since it is an above deck type, remove it, drain it completely, then set is on a perfectly level surface and fill it to get an exact volume. If its not an oddly shaped tank, you can multiply LxWxH in inches to get cubic inches. You can then convert that number to quarts, gallons, or liters. One gallon = 231 cubic inches. 231 x 18 = 4158. So if you come up with 4158 cubic inches (or close to it) in your measurement you have an 18 gallon tank. (Ironically, the GM 3.8L engine is actually 231 Cubic Inches so it could be considered a "One Gallon" engine. See -- you learn something new every day eh? 8)

 

[Ixolib]

Re: Portable Above-Deck Fuel Tanks

Moeller markets and sells this particular tank -- model 31818 -- as a "portable, above-deck" tank. It has none of the characteristics of a "permanent" tank such as vent/overflow, fixed filler, and/or sender unit.

I can't imagine that this major marine supply company would sell a tank used primarily for outboard applications and NOT take into account that oil will or may be mixed in. But even if that's a bonafide fact, the amount of oil (50:1) mixed with 18 gallons of gas is only 0.37500 gallons, or a little more than a third of a gallon.

The tank measures 7.87X15X43, giving an overall volume of 21.97 gallons. Of course, there are structural indentations for strength that would reduce that number somewhat, but how is it that two different gas stations and two different brands can both end up with the same volume of fuel at fill-up. On the second fill-up, by the way, I did ensure the tank was truly level.

I'm stumped!!!

 

[tashasdaddy]

Re: Portable Above-Deck Fuel Tanks

well with no vent it's going to trap air. it might be level in the boat, but is the boat level on the trailer, is the trailer level. don't expect 18 gals on that tank. when my boats in the water (25 gal.) i get several people to stand in the bow when i fill up. makes about a 3 gallon difference. i also put some of my estimated oil in first. i'd rather be rich than lean on oil.

 

[Ixolib]

Re: Portable Above-Deck Fuel Tanks

well with no vent it's going to trap air. it might be level in the boat, but is the boat level on the trailer, is the trailer level. don't expect 18 gals on that tank. when my boats in the water (25 gal.) i get several people to stand in the bow when i fill up. makes about a 3 gallon difference. i also put some of my estimated oil in first. i'd rather be rich than lean on oil.

Yeah, I checked the tank with a level based on advice from the retailer who sold me the tank. It was level on the 2nd fill-up. Also, the filler port is plenty large enough to expell any air as the tank fills. Guess I'm just stuck. However, it still doesn't seem right to represent and sell something that it's not.

I mean, if I were to buy an 18 carat diamond (yeah, right!!) and then it turned out to be only 16.5 carats, would -- or should -- I be satisfied??!!

Your advice on the rich vs. lean concept is right on!! Thanks...

 

[Silvertip]

Re: Portable Above-Deck Fuel Tanks

You calculated 21 gallons based on measurements which verifies you do have room for 18 gallons. I feel its just a "tank attitude" issue. If the picture is what your tank looks like, and the filler is in the middle and off to one side, with even a modest rise either in the stern or the bow, or worse yet, lower on the fill side, it will be nearly impossible to fill without spilling over. When you buy a fuel container for your lawn/garden equipment they don't assume you are adding oil yet there are lots of two stroke motors that require it. Capacity is capacity.

 

[eli_lilly]

Re: Portable Above-Deck Fuel Tanks


I have two tempo "portable" 12g, a tempo 6 and an attwood 6. The 6gs will only hold about 5 and the 12g holds 10-11g. I think the gallon rating is how much liquid the tank could hold if it was filled with not a square inch of air in it.

-E

 

[shaffer]

Re: Portable Above-Deck Fuel Tanks

I have 4 6 gallon metal tanks that each hold exactly 6.25 gallons.They are 2 johnson an 2 after market tanks.You can still buy these tank on ebay and some retailers.I would not have a plastic or fiberglass tank for a portable tank .To dangerous an I never seen one that didnt leak around the filler or gauge. Just my humble opinion.

 

[studlymandingo]

Re: Portable Above-Deck Fuel Tanks

Maybe you're just getting more gas than you pay for at the pump! Yea right!!!

The roto-molded products are built within pretty close tolerances, so you probably are "stuck" with what you have. With the poly tanks you have to take into account expansion and contraction. Also, it is recommended you fill up early in the morning as the fuel is denser, so supposedly you get a little more fuel for the money while it is cooler. Here in Florida we can get pretty warm in the daytime and we definitely deal with some expansion during the day. Sometimes I will have a 2 gallon tank "full" of gas in the back of my truck in the afternoon, the next morning, it is sucked in quite a bit, I'm sure I could squeeze in several more ounces at that point, but then it would be severely bloated by the afternoon.​

 

[Frank Acampora]

Re: Portable Above-Deck Fuel Tanks

The tank holds 18 gallons of air. With the indentations, you can only fill to the level of the indentations. The rest traps air and you can not displace it unless you physically tilt and shake the tank. So: with the tank level, you can only fit 15 gallons. And, by the way, if you did get a different amount into the tank at two different stations, report the station showing the lower amount to your local authorities. There are laws: when the meter reads 1 gallon, the pump must deliver 1 gallon. Over is acceptable under is not. So: since no station wants to give away gas, I would expect the same result from two different stations.

 

[supermicr]

Re: Portable Above-Deck Fuel Tanks

I purchased the Portable Moeller Fuel Tanks, 6 gal model with gauge. Added my 16 oz of oil, filled the Tank with 6 gal gas, and their is still some room in the tank.

Supermicr

 

[coolguy147]

Re: Portable Above-Deck Fuel Tanks

Maybe you're just getting more gas than you pay for at the pump! Yea right!!!

The roto-molded products are built within pretty close tolerances, so you probably are "stuck" with what you have. With the poly tanks you have to take into account expansion and contraction. Also, it is recommended you fill up early in the morning as the fuel is denser, so supposedly you get a little more fuel for the money while it is cooler. Here in Florida we can get pretty warm in the daytime and we definitely deal with some expansion during the day. Sometimes I will have a 2 gallon tank "full" of gas in the back of my truck in the afternoon, the next morning, it is sucked in quite a bit, I'm sure I could squeeze in several more ounces at that point, but then it would be severely bloated by the afternoon.​

yes thats true and scary a bit too cause when u take the cap off the two gallon tank gives u some air