FPB 64 Fuel Burn Tests


We’re starting to get into the meat of the sea trials, checking cruise speed against RPM, fuel burn rates, and motion. The screen above is one of 16 available from the NMEA 2000 Maretron display. This system is tied to the Deere engine monitor and amongst other things shows fuel burn, mileage, and in this case engine RPM and speed over ground from the GPS. Of particular interest is the fuel burn data.


All of these screens were photograph just off Whangarei Heads, theoretically outside the current area, in 16 to 18 knots of true wind speed. The boat is a little under half load, close to where she would average on a long ocean passage. Hydraulics and alternators are off, so the data reported is not real world, but does give us a baseline to compare to the calculations.

The first screen is running with the wind dead astern. The second is with the wind on the nose. There was a slight chop, maybe two foot (60cm) waves.


The photo above is from the engine instrument panel. Deere tells us the fuel data is computed not measured. They think it is accurate as it is a function of what the fuel injection computer is mapped to perform. We will eventually compare this data with day tank levels over long passages to quantify the accuracy.

The Maretron system has not yet had the data averaging set so the numbers are running up and down and it is hard to log the exact average reading.

We are treating this as an indicator. The real data will come from long passages and measuring day tank consumption.

With the above caveats, here is what this is telling us right now:

  • Speeds at various RPM are slightly better than projected. As the calcs for comparison are at half load, and we are a little under, this makes sense. It looks like 9.5 knots at 1600 or a hair more is feasible.
  • Fuel burn of 13.6 liters/3.51 US gallons per hour is under our projected 3.61 figure from the early phases of the design analysis. Again, in the ballpark.
  • The difference in fuel burn and speed between upwind is as expected. There is not a lot of windage or wave drag.


Now the data for 1800 RPM.

  • 17.6 liters/4.64 US gallons per hour is again a little under our projections for ten knots boat speed.
  • It looks like something in the range of 9.7 to 10 knots at 1800 RPM in smooth water depending on load is attainable.

A few general observations to put this in context:

  • It is too early to tell much other than the data appears to be in the ballpark. There are no negative surprises.
  • We may be a hair overpitched on the prop. If this is the case, the RPM boat speed relationship could change by as much as five percent, but more likely three. As right now it appears we have slightly better than expected speed this change would put the numbers more in line with what we calculated. Reducing prop pitch, if required, would not affect fuel burn.
  • It appears as if the sweet spot for maximum mileage, speed, and ambiance (the latter being noise related) is going to be around 9.5 knots.
  • For real world passaging data allowances need to be made for hydraulic and electrical loads, rougher weather, and perhaps the condition of bottom paint and propeller.
  • Faster appears to be more comfortable (as with the FPB 83) and upwind our guess now is that 10 knots may be the magic number for comfort.

More data to follow.

Posted by Steve Dashew  (March 26, 2010)

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