Fuel Tank incorrectly labelled?

[Joshto]

My boat has an under deck aluminum fuel tank. The label on the tanks indicates a capacity of 98 liters (25g), but I noticed that when the boat runs out of fuel, I can only fill it back with ~60 liters (16g).

Is there any chance the label is incorrect, or is it the fuel pickup line too short?

It's difficult to access the tank as the previous owner enclosed the tank under the floor permanently. I could only take the picture attached .

I also think the fuel sending unit is not set up correctly as the lowest the gauge goes is 1/4. Need to adjust it but the access is crazy tight (I'll cut an opening and install a marine hatch).

 

[Pmt133]

It is possible the pickup has been cut short. Are you running out of fuel like engine won't run then refilling or something else? Under that condition you could pull the level sender and see how much fuel is in the tank when "empty". I suppose it also is possible that you aren't able to fill all the way either due to an improper vent line routing, incorrect tank pitch or something of that sort.

 

[mike_i]

I once had a boat that had the fuel tank mounted in the rear of the boat. I had to lower the bow at the gas station to be able to fill the tank completely. If I didn't the pump would shut off 5 or 6 gallons short of actually being full.

 

[tpenfield]

I'd be thinking the fuel sender is not right (as in too short, if it is a tube type sender)

 

[airshot]

That is a lot of difference in the fuel load. Most likely the tank is properly labeled. Follow the suggestions mentioned to see for yourself where the issue really is. Could also be the float in the tank is catching on something. Check the vent line from the tank....there should be an upward loop before the line reaches the exterior vent so water does not back up into the tank.
It also create a sound when filling so you know when the tank is getting full before it shoots fuel out the vent. Let us know your findings......

 

[Chris1956]

Gee, the tank is likely labeled right, IMO. A lot of things come into the amount of fuel that can be drawn out of a fuel tank. Most replacement fuel gauges are universal. They can be real inaccurate in judging fuel level.

Fuel pickups can be the wrong length. Too long and they suck water. too short and you run out of fuel.

As an example, My speedboat has an 8" tall fuel tank of 18Gal. The fuel gauge shows empty when the tank has 1/3 the fuel or 6 gal. I have had 3 or more fuel gauges over the years, and they all show the same thing. The key is to run her until the fuel gauge doesn't move much. At that point you need to head in as she is near empty.

 

[nola mike]

You could also have a crack/hole in the pickup such that it won't draw when the fuel is below the level of the crack. Easiest would be to pull the sender and have a look.

 

[DeepCMark58A]

Never trust a gas level based on gauges old boating lesson

 

[airshot]

When I get a new to me boat and every once in a while I do a test on my tank and gage. I use two gallon portable fuel cans as they are easy to handle. I use my pump to get as much fuel out as possible ( usually at end of season for winter storage). Because it is a plastic tank, there is very little chance of moisture from condensation like there is in a metal tank. In the spring, I add two gallons at a time and watch the fuel gage and make note where it reads. I have a little gage I made to show how my boat sits in the water as far as how level it sits. I adjust my trailer so my boat sits at that same angle. I keep adding two gallons and noting the gage reading, until the tank is full. I now know where every two gallons is marked on my gage and also have a total usable number of gallons availiable. I can run my boat down to the last two gallons if needed. I usually fill it up when it reads about 3/4 empty. At this level, I know I have enough fuel to get back from wherever I normally go. During the busy fishing season, I just fillup after every three fishing trips, about half a tank so I don't forget. But, if you do this little test each spring, you will know if anything changes and needs addressed.

 

[alldodge]

Looks like @Joshto was last seen at 3:47PM today, so not that long ago. Why he has not tried to help fill in some questions is anyone's guess.

 

[Joshto]

Ok...thanks all for your help and comments.
I was on the lake pulling the tube. Lol.
With lots on patience I managed to get the sending unit out. I'm not an expert but I think is shot lol...pictures. I'll get it replaced.

Second, from the sending unit hole I can see the pickup line and I actually measured the distance from the bottom of the v shaped tank to the tip of the line. it's exactly 1/2 inch.

So the pickup line is fine, which leaves only two explanations. One, the tank label is wrong and two, I'm losing my head.

To elaborate on the option two. When I first got the boat in 2019, the gauge showed 1/4, so I stupidly assumed it had 1/4 of fuel...I went off the dock, fooled around for 5 minutes, and the boat died on me. Tried to start it multiple times; it would start and die again. Someone helped me get to the docks again.

So I went straight up to the gas station and filled it up. Here is where I think I'm losing my memory. I recall I filled 60 liters (16g)...or maybe it was 60 dollars lol the gas was pretty cheap those days so that could explain that I actually put some 85 liters of gas.

I ordered a new sending unit but I'll drain all the gas and measure it before installing the new unit. Then I will head to the gas station to top it up and get the absolute true capacity.

Funny is that all this time I have been using the boat thinking I only had 60 liters and aware the sending unit wasn't reliable...I've been pretty cautious about not running out of fuel somewhere remote.

I only noticed the label yesterday when trying to get the sending unit out for adjustment. I always assumed that 16 gallons was the tank's capacity. Based on todays use of the boat and the gas left in the tank, I'm almost certain the label is right. (Tank is 10 inch deep and today I used about 2 inches of gas, which I estimate is 20 liters based on the time I've pulled the tube before)

 

[Pmt133]

I just realized its labeled as a fuel thank not tank. And here I was talking about something completely different.

Kidding aside, that looks about normal for draw down. If you get in 22 gallons to fill or so with what's in there I'd say thats correct. My tank I had built is 39 gallons total volume but will only take 37 gallons to fill from bone dry.

 

[Joshto]

I just realized its labeled as a fuel thank not tank. And here I was talking about something completely different.

Kidding aside, that looks about normal for draw down. If you get in 22 gallons to fill or so with what's in there I'd say thats correct. My tank I had built is 39 gallons total volume but will only take 37 gallons to fill from bone dry.

Lol. Quebec labelling.
Yes, if the tank takes 23 gallons or so after emptying it out, I'm happy with that.

 

[Alumarine]

Is it possible for you to measure the height, width and depth?
If so, you can determine the capacity.

 

[Joshto]

Is it possible for you to measure the height, width and depth?
If so, you can determine the capacity.

Not from outside...tank is literally burried. I could if I stick the measure tape through the sending unit hole. But the tank is v shaped at the bottom so not sure how to work that out.

 

[Joshto]

Check the vent line from the tank....there should be an upward loop before the line reaches the exterior vent so water does not back up into the tank.
It also create a sound when filling so you know when the tank is getting full before it shoots fuel out the vent. Let us know your findings......

Yes I have the upward loop and the vent. When it fills up, if I keep pumping, gas comes out of the vent. In fact, when filling up at the gas station, I always put paper towel directly below the vent..in case it spills.

 

[airshot]

Odd shaped tanks are very difficult to measure for volume. In all my years of boating, have never saw a tank mislabeled for capacity. Not saying it never has happened, just very rare. With all the testing fuel tanks are made to go thru, hard to see the capacity rating being off more than a gallon.

 

[Joshto]

You could also have a crack/hole in the pickup such that it won't draw when the fuel is below the level of the crack. Easiest would be to pull the sender and have a look.

Thank you. This idea pushed me to get that sender out...got all scratched up.

 

[airshot]

Yes I have the upward loop and the vent. When it fills up, if I keep pumping, gas comes out of the vent. In fact, when filling up at the gas station, I always put paper towel directly below the vent..in case it spills.

Good....if you can get your ear close to that vent, you can hear when the air stops coming out and the fuel begins to gurgle, so you know to stop filling.
I have shot lots of fuel out until I figured out the vent noise

 

[Joshto]

When I get a new to me boat and every once in a while I do a test on my tank and gage. I use two gallon portable fuel cans as they are easy to handle. I use my pump to get as much fuel out as possible ( usually at end of season for winter storage). Because it is a plastic tank, there is very little chance of moisture from condensation like there is in a metal tank. In the spring, I add two gallons at a time and watch the fuel gage and make note where it reads. I have a little gage I made to show how my boat sits in the water as far as how level it sits. I adjust my trailer so my boat sits at that same angle. I keep adding two gallons and noting the gage reading, until the tank is full. I now know where every two gallons is marked on my gage and also have a total usable number of gallons availiable. I can run my boat down to the last two gallons if needed. I usually fill it up when it reads about 3/4 empty. At this level, I know I have enough fuel to get back from wherever I normally go. During the busy fishing season, I just fillup after every three fishing trips, about half a tank so I don't forget. But, if you do this little test each spring, you will know if anything changes and needs addressed.

That's quite methodical approach. I always fuel before every trip, no matter if I know it will only take a few gallons.