FPB 64-3 Preliminary Performance Numbers

Iron LAdy First Run 303

We hesitate to give out early performance information, but since the preliminary data off yesterday’s runs with Iron Lady are in the ballpark for what was expected, we thought we’d give you an early look. Iron Lady displaces roughly 34 metric tons / 75,000 pounds, in these early runs, as as she has a bit of fuel and water onboard. There were two runs, one up and one down tide, and these were averaged. Speed is from the GPS, fuel burn from the John Deere CPU display. Water is brackish and a touch shallow, both of which retard performance. The engine is a 235HP John Deere six cylinder diesel.

At 1600 RPMĀ boat speed was 9.4 knots burning 13.2 liters / 3.49 US gallons per hour. At 1800 RPM boat speed was 9.95 knots consuming 17.6 liters / 4.65 US gallons per hour. Top speed was 11.6 knots. The cruise speed is about 1/4 knot faster at 1800 RPM, compared to the boats without the extension and about eight percent more efficient.Top speed jumped to 11.6 from 11.1.

Note, in the real world of crossing oceans, with hydraulic and electric loads plus increased drag from wind and wave, these numbers will deteriorate. Iron Lady is also about six tons / 13,000 pounds light of full load. But for comparison purposes to Sarah Sarah, the numbers give a real world indication of the ocean crossing gains to be expected.

Over the next few weeks we’ll get more detailed numbers, and feedback on uphill motion and sound levels, both of which we expect to improve.

Posted by Steve Dashew  (February 2, 2011)

2 Responses to “FPB 64-3 Preliminary Performance Numbers”

  1. Nils Pettersson Says:

    Hi Steve!

    Again, impressive numbers! 9.4 knots seems to be a little over the theoretical hullspeed according to the formula. How would slowing down one knot or so affect fuelconsumption and range. Or would this speed put the engine in a less optimal efficiencybracket?



  2. Steve Dashew Says:

    Hi Nils:
    “Hull speed” is a function of hull volume distribution, waterline length, and displacement. You can push the hull speed (really a drag hump) with a design like the 64, so that ratios as high as 1.4 (times the square root of the LWL) are possible with modest power. More important is the sweet spot, for efficiency, mileage, and noise, which in the extended FPB 64 is likely to be about 9.7 knots.

    Dropping a knot will help range, and we will have real world number shortly, but this is a little slow to work weather on trans-ocean passages. It will also be hard to keep the engine loaded properly at this speed, unless it is derated.