NavMan Anomaly

[Pat Anderson]

We have had a NavMan fuel computer for about a year, got it originally solely to be able to set cruise speed to best economy - and it certainly works for that. Surprisingly, as Dora-Jean observed in another thread, economy is worse between 8 and 11 knots than at 12 knots and above. It is awesome at 7 knots and below. I also started using it for the fuel used and fuel remaining, since the gas gauge supplied with the boat is awful. I don't really have it calibrated, but it seems to consistently report about 10% more use than actual (determined at each fill-up), which is fine by me - a little reserve. But here is the anomaly - there is a definite disconnect between the reported flow and the fuel used / remaining. For example, if we run for three hours at a steady speed with a continuously reported 2 gph, we would expect the fuel used / fuel remaining to change by 6 gallons - but it always uses more, in this case, the fuel used / remaining would change by 8 gallons rather than the expected 6 gallons. I do not see how this can be calibration - wouldn't calibration affect them both the same, since fuel used / remaining is simply a calculation off flow rate and time? Not a huge issue but puzzling to me...

[Dora~Jean]

Pat, just a thought, do you always fill up at the same station, same pump and orientation? On horizontal tanks of this size, I find only 1 or 2 degrees of slant difference changes the fill point by gallons!

[thataway]

Have you calibrated the Navman fuel flow meter? You need to calibate it fill up to fill up--and then change the number of gallons used. Generally the guages are withing 5% after calibration they are within 2%---except when idling. If you are doing a lot of idling the flow meter will read low.

As for the worse "mileage from 8 to 12, I would expect it to be from 7 to 14--but either way, this range is in the semidisplacement speed, when the boat uses more HP to climb over its own bow wave, and is normal.