Fuel Economy

[ArrowNautical]

I am going to describe the situation with as many details as possible
I have just purchased a 1997 2800 SCR with a 7.4 carbureted engine.
I plan to do the loop in this boat so I am trying to figure out the fuel economy before I go so I can budget correctly
So my friend and I took the boat out onto the lake that it is on and we traveled 35 miles recording it with four different devices to come up with that average, before we set off we topped off the fuel tank.
We then proceeded to boat for 5 1/2 hours at 7 kn.
We went directly back to the fuel dock and filled the tank, it only took 1/2 of a gallon.
The entire trip was at 1300 RPMs.
There is no possible way that we boated for 5 1/2 hours even at that low speed and only burned a half a gallon of fuel.
We checked the vent afterwards to make sure that the fuel tank was vented properly, it was.
I disconnected the vent tube from the fuel tank and blew into the fill hose from the rear of the boat and fuel came out of where the vent tube goes in so I know that the gas tank is full, or is it.
Am I missing something?

Eric

[mmwjr]

To be clear the 35 miles and the 5.5 miles were all at 1300 rpm. Missing information; what level was the tank at when you started and how much did it take to fill after the 35 mile run?

At 1300 rpm I would expect a burn rate of at most 5 gallons per hour. At 3500 rpm around 15 gallons per hour.

Note doing the loop is completely different than operating in a lake. You’ll be subject to tides, current as well as winds all of which will impact economy. Whatever you estimate I would double it for margin.

[ArrowNautical]

35 miles taking 5.5 hours at 7kn at 1300rpms.
Tank was full when we started.
It took 1/2 gallon to fill after 35 mile trip.
Tank vent is venting properly.

I completely agree about the loop. Just baffled by our experiment.

[mmwjr]

Originally Posted by ArrowNautical

35 miles taking 5.5 hours at 7kn at 1300rpms.
Tank was full when we started.
It took 1/2 gallon to fill after 35 mile trip.
Tank vent is venting properly.

I completely agree about the loop. Just baffled by our experiment.

Ok but you still haven’t said how much gas it took to go the 35 miles. I.e. how much to fill up after the 35 mile leg?

[ArrowNautical]

1/2 gallon

[mmwjr]

Originally Posted by ArrowNautical

1/2 gallon

Duh! Now I got it. One trip of 35 miles and used 1/2 a gallon. Have you gone back to verify the tank is full and that the pump didn’t kick off prematurely?

[shrew]

How hot was it? Were the fuel tanks from the gas station stored underground? At 1300 rpms I'd expect more than a 1/2 gallon in 5 hours. The engine burns more than that an hour at idle. However I wouldn't expect it to be much.

I wonder if the fuel was stored underground and cool, then you began experiencing a bit of thermal expansion as the fuel warmed up in the boat. Then 10 gallons or so that you would have burned could have been taken up by the fuel expansion.

Was the second fueling at the same location as the first fueling? A faster pump might have overwhelmed the tank vent and caused some burping.