Unusable fuel?

[UnfixedPyle]

I haven't looked at how the fuel tanks are designed and plumbed in my boat and it didn't really occur to me until I got a low fuel light and watched the fuel indicator jump around between 1% and 12% coming back to the boat ramp. I have since discovered in this forum that the trip meter is much more accurate than the fuel quantity gauge so hopefully I will now know how much fuel I have burned. But it makes me wonder if there is certain amount of fuel in the tank that is unusable. In my case (21') I have a 50 gallon tank, can I actually use all of that? Assuming not what is a safe reserve?

 

[Beachbummer]

1/3 is a good reserve. I have gone as low as 7 gallons left out of 50.

(Not counting when I ran out of fuel in my old boat.)

 

[FSH 210 Sport]

I would think the boats are designed so that while under way probably 2° bow rise you will be able to use most of the fuel. I ran mine down to 1% one time and the port engine quit at the dock. I put it on the trailer with the starboard engine. The port engine started and ran once on the trailer. My owners manual shows a tank capacity of 52 gallons, I put 51 gallons of fuel in the tank that time. So that’s pretty good if you ask me.

I think maintaining at minimum of 10% reserve is prudent but I’d test that number to be sure.

 

[Jores]

I never go below 1/4 tank- same as my car, Not sure about the boat but most cars use the fuel in the tank to keep the fuel pumps cool. People that frequently run low on fuel have a much higher rate of fuel pump failure.

 

[lazergeek]

I accidently miscalculated my trip last week and luckly made it back to the dock with 3 gallons left in the tank. By the time I realized I would be cutting it close it was after all the fuel docks were closed. Luckly I reset the fuel flow meter before so was using that to caclulate route back and knew it would be a squeeker. Ended up doing 92 miles on my 50 gal that day.

 

[FSH 210 Sport]

I accidently miscalculated my trip last week and luckly made it back to the dock with 3 gallons left in the tank. By the time I realized I would be cutting it close it was after all the fuel docks were closed. Luckly I reset the fuel flow meter before so was using that to caclulate route back and knew it would be a squeeker. Ended up doing 92 miles on my 50 gal that day.

So you used 50 gallons to go 92 miles? Or did you mean you went 92 gallons and your boat has a 50 gallon tank?

 

[lazergeek]

So you used 50 gallons to go 92 miles? Or did you mean you went 92 gallons and your boat has a 50 gallon tank?

Used 47 gallons to go 92 miles. Not bad for 5 people on board and bad ocean swell for 20 of the miles.

 

[lazergeek]

@FSH 210 Sport I need to do the extra tank under the seat like you did. Would the 24 gallon fit or just the 22 gallon?

 

[FSH 210 Sport]

It‘s important to understand why running the fuel low “can” lead to accelerated wear on a rotary vane type of fuel pump.

Fuel pumps don’t get their cooling from being immersed / submersed in gasoline, the cooling and lubrication comes from the fuel itself, otherwise they‘d start running hot as the tank comes off of full, most are mounted the same way our boats fuel pumps are mounted, from the top. On a lot of modern vehicles the fuel pumps are mounted on the frame rail outside of the gas tank, obviously these pumps, which are also vane style pumps, don’t need to be immersed in the fuel to stay cool. On our boats there is no fuel return line coming from the common rail on the engine where the injectors are attached, the fuel return is within the pump itself as is the regulator. The regulator allows the excess fuel / fuel pressure to go through a return line back to the tank, this occurs to some degree even at WOT.

As the fuel level reaches a low enough point where air can get into the fuel pump pick up, the fuel that then enters the vane portion of the pump where the high pressure is generated now has air in it that in turn reduces not only the lubricity of the fuel which generates heat and can produce scarring. The normal heat then cannot be hauled away from the pumps as well as the increased heat being generated by the aerated fuel. Remember, fuel, like oil, is a liquid and cannot be compressed, the crank, rods, bearings etc in your engine literally float or are suspended on a thin layer of pressurized oil, and that is how they keep from rubbing metal on metal. The same thing is happening in your fuel pumps, now put a bunch of air / gas which can be compressed into that fluid and the metal parts are no longer suspended from one another.

Keeping in mind that our fuel pumps with their pickup screens are mounted as low as possible in the fuel tank, primarily to have as much useable fuel as possible, ergo, as much liquid as possible can be extracted from the tank. The fuel pumps in the gas tank of my boat are located as far aft as possible, our boats run with a slightly bow high attitude and this keeps the fuel bias if you will at the rear of the tank. Or put another way, ensures the greatest amount of useable fuel from our tanks. As mentioned in my post above, I ran my tank down to 1% one time mainly to find out just how low the tank could go before the engines quit. As stated above the port engine quit while I was tied up to the dock, but started right up when it was on the trailer. It never hiccuped at all while underway. According to my owners manual, my tank is 52 gallons, and I put 51 gallons in it at the service station, pretty impressive really.

The fuel pump in my motorcycle is basically the same design as our boats and I’ve run my bike out of fuel numerous times, and ran low a lot of times and I‘ve not had a fuel pump failure associated with this. In fact I’ve not had a “pump” failure, what I have had is a fuel pressure regulator failure, stuck open at maximum bypass, caused by corrosion from ethanol laced fuel.

Do I advocate running around with a near empty gas tank? Absolutely not, that’s stupid but not fuel pump preservation stand point, from a reliability standpoint. On my diesel truck half tank is fill up time, unless I’m on a road trip then I may push that down to where the light comes on and I get the 50 miles to go warning, but I’ve also got two five gallon jugs of fuel in the bed. When I’m out riding, any time fuel is available the tank gets filled, that reliability thing. In my boat, it depends, but most of the time when I get fuel it takes 30 gallons, so that’s 40% left. BUT, I have no problem running the tank down to where the alarm goes off, I think that is 12% or 6 gallons remaining.

 

[BlkGS]

I would think the boats are designed so that while under way probably 2° bow rise you will be able to use most of the fuel. I ran mine down to 1% one time and the port engine quit at the dock. I put it on the trailer with the starboard engine. The port engine started and ran once on the trailer. My owners manual shows a tank capacity of 52 gallons, I put 51 gallons of fuel in the tank that time. So that’s pretty good if you ask me.

I think maintaining at minimum of 10% reserve is prudent but I’d test that number to be sure.

I did that once, except my gauge showed like 1/8 tank left. Didn't think about it shutting off due to pump placement at the time. Needless to say, I didn't take chances with fuel levels after that and don't really trust my fuel gauge lol.

 

[FSH 210 Sport]

Used 47 gallons to go 92 miles. Not bad for 5 people on board and bad ocean swell for 20 of the miles.

Dang, 1.95 mpg…that’s some real world mpg right there!

 

[FSH 210 Sport]

I did that once, except my gauge showed like 1/8 tank left. Didn't think about it shutting off due to pump placement at the time. Needless to say, I didn't take chances with fuel levels after that and don't really trust my fuel gauge lol.

Agreed! I don’t think I was hip to the “fuel used” part of the connext at that point….I‘ve got enough experience with the fuel used gauge that I trust it to 1/10th of a gallon. I use the % gauge as a quick reference, and fuel used as a true measure.

 

[FSH 210 Sport]

@FSH 210 Sport I need to do the extra tank under the seat like you did. Would the 24 gallon fit or just the 22 gallon?

Man, I don’t think so… the Moeller 22 gallon just fits in there. I’d have to check dimensions, to be sure though, the only place there is any room is height wise I think. The depth is such that you can get the OEM ice chest behind that tank but you need to put something between the ice chest and the transom. I think the width of the tank is such that it’s sitting on the aluminum feet of the seat support, I’d have to check that as well. It’s actually a clean set up.

I have toyed with idea of putting a T fitting in either the fill line or the vent line inside the battery compartment that I could put a quick disconnect fitting on to connect the transfer pump directly to those lines, but that might be more trouble than it is worth. Was just considering that mainly to keep the fuel police from taking notice, and it would be a bit cleaner, but with the valved quick disconnects I have there might be a drop or two that ends up on a paper towel.

 

[lazergeek]

Man, I don’t think so… the Moeller 22 gallon just fits in there. I’d have to check dimensions, to be sure though, the only place there is any room is height wise I think. The depth is such that you can get the OEM ice chest behind that tank but you need to put something between the ice chest and the transom. I think the width of the tank is such that it’s sitting on the aluminum feet of the seat support, I’d have to check that as well. It’s actually a clean set up.

I have toyed with idea of putting a T fitting in either the fill line or the vent line inside the battery compartment that I could put a quick disconnect fitting on to connect the transfer pump directly to those lines, but that might be more trouble than it is worth. Was just considering that mainly to keep the fuel police from taking notice, and it would be a bit cleaner, but with the valved quick disconnects I have there might be a drop or two that ends up on a paper towel.

I was only asking because we talking about going to Dry Tortuga's this year which is 70 miles each way from Key West so I would still need gas cans in in addition to the 22 gallon tank to be safe. Definatly going to get the tank however as we also want to go to the Exumas this year too and that will help increase our rage and prevent issues if Fuel is low on any islands.

 

[FSH 210 Sport]

I was only asking because we talking about going to Dry Tortuga's this year which is 70 miles each way from Key West so I would still need gas cans in in addition to the 22 gallon tank to be safe. Definatly going to get the tank however as we also want to go to the Exumas this year too and that will help increase our rage and prevent issues if Fuel is low on any islands.

Right on…. Sounds like some fun trips! Some quick math based on your 1.95 mpg with a 10% reserve you would need 78.97 gallons of fuel for the Dry Tortuga‘s trip, so call it 80. Based on my personal experience you‘ve got 50 gallons usable in the main tank, 22 in the Moeller tank for 72 gallons useable. So perhaps three five gallon jugs to bring total fuel to 87 gallons for just a bit more insurance, that’s a 21% reserve.

So I went and checked the Moeller site…

This is the spec sheet for the 22 gallon tank which is the one I have, and is white in color.




This is the spec sheet for the 24 gallon tank. It appears that it is only 1/2” taller than the 22 gallon one, and it is red in color. So it may fit under the seat, I’ll have to go out and put my 22 gallon in and see if there is room for it, but I’m thinking there is.




This is their 15 gallon tank. It is 42.5” long and 14.75” wide. I’ll have to check the dimension behind the 22 gallon to see how much room there is, and see if that 42.5” length will fit between the storage compartment on the Starboard side and the battery compartment on the port side . I measured my OEM ice chest and it’s darn close to the 15 gallon tanks measurement but it was a tight fit. This would be a great way to carry that other 15 gallons, better than three five gallon jugs just more expensive. This could be set up with the double sided shut off quick disconnects and would be a very clean and safe way to carry and transfer fuel.



Even if you have to go with fuel jugs it will be way easier to put the gas from the jugs into the 22 gallon Moeller Tank than putting it into the main tank while on the water.

There is one other option, while I’d have to measure to be sure, it’s possible another tank could be placed where you stand at the helm, you’d just need to make a starboard box to go over it to protect the top and make it a suitable foot rest. I put my OEM ice chest there as a foot rest which, for me, is a better foot rest for long cruises. The only down side is that you cannot just step through the helm area, I either walk around or slide across the seat.

 

[lazergeek]

Right on…. Sounds like some fun trips! Some quick math based on your 1.95 mpg with a 10% reserve you would need 78.97 gallons of fuel for the Dry Tortuga‘s trip, so call it 80. Based on my personal experience you‘ve got 50 gallons usable in the main tank, 22 in the Moeller tank for 72 gallons useable. So perhaps three five gallon jugs to bring total fuel to 87 gallons for just a bit more insurance, that’s a 21% reserve.

So I went and checked the Moeller site…

This is the spec sheet for the 22 gallon tank which is the one I have, and is white in color.

View attachment 193121


This is the spec sheet for the 24 gallon tank. It appears that it is only 1/2” taller than the 22 gallon one, and it is red in color. So it may fit under the seat, I’ll have to go out and put my 22 gallon in and see if there is room for it, but I’m thinking there is.

View attachment 193120


This is their 15 gallon tank. It is 42.5” long and 14.75” wide. I’ll have to check the dimension behind the 22 gallon to see how much room there is, and see if that 42.5” length will fit between the storage compartment on the Starboard side and the battery compartment on the port side . I measured my OEM ice chest and it’s darn close to the 15 gallon tanks measurement but it was a tight fit. This would be a great way to carry that other 15 gallons, better than three five gallon jugs just more expensive. This could be set up with the double sided shut off quick disconnects and would be a very clean and safe way to carry and transfer fuel.

View attachment 193122

Even if you have to go with fuel jugs it will be way easier to put the gas from the jugs into the 22 gallon Moeller Tank than putting it into the main tank while on the water.

There is one other option, while I’d have to measure to be sure, it’s possible another tank could be placed where you stand at the helm, you’d just need to make a starboard box to go over it to protect the top and make it a suitable foot rest. I put my OEM ice chest there as a foot rest which, for me, is a better foot rest for long cruises. The only down side is that you cannot just step through the helm area, I either walk around or slide across the seat.

Thank you, I was guesstimating 25% reserve in my basic calculations. We use the ice chest as a foot rest not too.

 

[FSH 210 Sport]

Thank you, I was guesstimating 25% reserve in my basic calculations. We use the ice chest as a foot rest not too.

The only time I could see not being able to stand at the helm being a detriment is if you were in bigger water, much easier to stand at the helm with a hand on the T top upright and one on the wheel.

 

[lazergeek]

That's standard for me but I'm in the ocean 100% of the time.

 

[84rzv500r]

with my 70 gallon ski locker i have fuel for most all of my excustions i can (and have carried an extra 10 gal (2x5)) in each console under the dry storage...

 

[FSH 210 Sport]

Okay, good news and bad news. @lazergeek

Bad news first, it appears there is not enough room behind the 22 gallon tank for the 15 gallon tank. There is just over 14” of space between the 22 gallon tank and the transom.





Bad news #2. The 24 gallon tank is 1/2” wider and I think the 24 gallon tank is too wide to fit between the legs of the helm seat uprights.

.





Good news. Three 5 gallon VP jugs fit easily behind the tank, and if up against the transom the middle can will not sit on any of those three screws. Those screws would have to be avoided if one was going to lash the VP cans up against the tank using the helm supports as a place to tie it down. But, it would be pretty easy to lash the cans between the helm support and the rear cleats. If I was to use VP cans, I’d get another top / cap that would be the one fill cap to use on all three cans. The cans would all have their plugs installed in the top of the caps to increase the safety during transport. I suppose a little box could be made to put the three cans in to add stability and the ability to tie them down more securely and easier.



I also set the OEM ice chest behind the tank and you can see that the ice chest hits the fill cap while up against the transom.



In this picture you can see where the tank sits up against the radius of the foot rest. I have several hundred miles of operation and the chaffing is minimal. However with that in mind I’ll be putting a strap on the tank to hold it back from the radius in the future…there is very little loss of area by doing this, perhaps a 1/4“ from the back of the tank to the transom.



If anyone needs other measurements or pics let me know!