Adding Under Belly Fuel Tank

[Blank-N-Ship]

Re: Adding Under Belly Fuel Tank

We have two 25 gallon tanks opposite each other in the rear for balance. We try to use them equally as much as possible. Since our primary use is skiing, tubing, kneeboard, we are on and off the throttle most of the time. We go through 40 gallons in a weekend, probably using it two weekends a month in the summer. Fuel consumption is pretty much dependent upon your chosen use. I can certainly see if one parties, swims, fish that a tank of gas could be stretched much longer.


PS- gas does not weigh 12 lbs/gallon:

"The weight of gasoline is dependent on the temperature, but it also depends on how much alcohol has been added. Today, that's sometimes a lot. So we come up with
1 gallon of water equals 8.33 lbs.
1 gallon of gasoline equals 5.8 to 6.5 lbs.
1 gallon of ethanol equals 6.59 lbs."

The following chart is in our user's manual (assuming about 6 lbs/gallon).
6-gallon tank: 36 lbs
11-gallon tank: 73 lbs
24-gallon tank: 146 lbs
35-gallon tank: 213 lbs

I must respectfully disagree that a performance difference will be noticed. There should be less than 10 extra vertical inches to pump the fuel so I don't think any detectable performance will be realized. Practically all fuel pumps are designed to pump more fuel than is required by the engine, and a fuel regulator controls the actual amount delivered to the carbs or injectors.

Was doing some volunteer work down in Peru summer before last so I was researching the various formulas for pressure-at-a-given-height for the sake of designing a water tank. The specification that we're questionning at the moment is called Total Head and it's the rating for the pump's maximum vertical distance from the intake in the fuel tank to the pump's destination. I honestly don't know the typical Head ratings for these outboard motor pumps, tbh.* I wouldn't imagine them to be anything like what you'd find in other industries, though, given that a typical motorcraft of this size wouldn't need much vertical pull.

1 PSI will push water up a distance of about 2-1/3 feet

One could easily guesstimate that if there's about 2-1/3 feet or more of vertical difference between the intake of an above-deck fuel tank versus the intake of a below-deck intake then the fuel pressure at the outboard motor's intake will minimally be 1 PSI less.

If the pump's Total Head is rated for something huge like 90' then obviously the difference of a couple feet is insignificant. But if the pump's Total Head is rated for, say, 5' then you're dramatically changing the intake pressure.

*Okay, finally doing a little research... http://www.boatpartstore.com/fuelpump.asp

So how do I know the fuel pump is doing what it is suppose to?
As described above, the fuel pump requires an adequate pulse from the cylinder that it is attached to, so first thing is to make sure compression on that cylinder is up to snuff or the fuel pump can't do it's job. If compression checks out, then next thing is to check fuel pump pressure. All non fuel injected outboard motors operate with a fuel pump pressure of about 5 to 7 psi. If consideribly less than 5 to 7 psi exists between fuel pump and carb/s, the motor is likely to be starving for fuel causing idle and/or upper rpm operation to suffer, not to mention a dangerously lean condition.

So without painstakingly researching the particular outboard motor's make/model, its pump specs and the fuel-versus-water pressure difference, I'd say that a 1 PSI drop could, at 20% loss of intake pressure, be significant in some cases. Granted, if it's fuel-injected we don't know what that type of pump requires as the input pressure but my gut's telling me that it's related to the pressure created by the fuel line bulb.

 

[fatchance]

Re: Adding Under Belly Fuel Tank

I just took two underbelly fuel tanks off our new pontoon as we don't use that much fuel going to and fro the sandbar. and they are avail. I can take pix if you'd like.

 

[FlyboyTR]

Re: Adding Under Belly Fuel Tank

I just took two underbelly fuel tanks off our new pontoon as we don't use that much fuel going to and fro the sandbar. and they are avail. I can take pix if you'd like.

Fatchance,
I would like to see the pictures! I just repowered my 20' toon with a 90 4-stroke and I am considering a belly tank up front to offset the extra weight from the new motor! Thanks!