Uneven Fuel Usage

[Apogee]

Greetings fellow Whalers. Today at fuel fill-up, the port fuel tank of my 1999 Boston Whaler 28-foot boat took 82-gallons; the starboard tank only took 25-gallons. How could this be?

Both engines were running at the same time. Both engines covered the same ground at the same throttle settings. The starboard engine fuel tank selector switch is set to starboard and the port engine fuel tank selector switch is set to port. [Later this is discovered to be incorrect. See below--jimh] Give me any thoughts.

[jimh]

What is the capacity of each tank?

[Jefecinco]

If one of a pair of engines is not developing it's full power due to any number of causes the remaining engine will pick up the load within its capacity. If one engine is producing more power it will consume more fuel. The "lagging" engine may make the same RPM as the performing engine because it's being helped along. This is a common problem with twin Verado engines when one has a faulty boost valve.

Would a slipping propeller hub cause uneven fuel consumption? I suspect so and that is probably the first thing I would check.

What engines are on your boat?

[jimh]

There are several possible explanations for the uneven consumption of fuel from the two tanks.

The first aspect to question is the accuracy of the measured fuel consumption. The method of measuring fuel consumed by re-filling a tank to a particular tank level is prone to error. When filling tanks, the fuel supply may shut off automatically at various tank levels due to problems with tank venting, problems with unusual filler hose path length or turns, and may be affected by the static trim on the boat. An error in measurement of fuel levels in the tanks by inference from the amount needed to refill them could result in an error in the amount of fuel consumed. It is also possible that at the beginning of the test period the tank levels were already uneven, so this difference continued during the test, even though fuel consumed during the test was uniform for each engine.

Another possible cause of error is the setting of the fuel distribution manifold. Perhaps there was some means for one engine to draw from both tanks, while the other engine drew only from one tank. Fuel distribution manifolds can be complicated.

If we assume that there are no errors, and that during the test period one engine consumed 25-gallons and the other 82-gallons, while both engines ran at exactly the same throttle setting and produced exactly the same power, then we are left with the only possible explanation that the two engines vary greatly in their efficiency at converting fuel to power, their brake specific fuel consumption. The ratio of brake specific fuel consumption would be directly proportional to the fuel volumes consumed during the test period, or 82/25 or 3.28-times. This a completely unreasonable outcome. There is no reasonable circumstance in which two identical engines would have such radically different fuel consumption. Even from the best to the worst engine designs, an outboard engine brake specific fuel consumption has a ratio between the rates more like 1.5 at most; it would never be 3.28 unless a very large volume of raw fuel was leaking out or passing through unburned in an engine. Typically the operator would notice such an abnormal condition.

Perhaps the most reasonable explanation for the disparate volume of fuel used is the engines were not operated at identical throttle and load settings. This, combined with the potential for fuel consumed measurements to be in error, would explain the outcome that has been reported here.

[Don McIntyre - MI]

Are your tanks metal or plastic, and (this may sound odd) are you in a geographical area where it freezes, and if so, is she stored in freezing temps?

Regards - Don

[porthole]

What power do you have?

I used about 25% more fuel then normal my last trip out.
And then had stalling problems coming back through my river.

Turns out my low pressure fuel pumps, Yamaha salt water series 2 strokes, are bad and are dumping fuel into the crankcase.

[Jefecinco]

There is a clue that Apogee, the original poster, has twin F300 Yamaha Engines on his boat which I assume is a large Boston Whaler. On another thread he posted that he had purchased an F300.

I don't know if that has any bearing on the subject as I know absolutely nothing about Yamaha engines.

[Apogee]

I think somebody messed with [the fuel distribution manifold valves]. I checked them today and they were [set for both engines] drawing from the port tank. I guess the shop that just installed my new Evinrude E-TEC 300 G2 engines messed with the [valves] before a sea trial. When I filled up [the two fuel tanks] I got 85 gallons on port, 25 gallons starboard. [The fuel distribution manifold setting] could explain [the uneven fuel usage from the two tanks].

There are fuel totalizators on each engine so I will know what is up after this weekend. After all of this refit, I am going fishing!