Efficiency Test

August 21, 2017  Hours spent building to date:  3,908

John ad I took the Tardis out last week at slack tide in light winds to get some measurements of fuel burn versus speed and get a better idea of ideal running parameters prior to our first short cruise.  We ran reciprocal courses with and against what wind there was at various RPM levels and took the measurements when all the outputs from the GPS seemed pretty steady.  The boat was at about half load with tanks half full, but no anchor on as yet and not fully provisioned.

Here are the results (click on the image for full size):

FF is fuel flow in gallons per hour, and NMG is nautical miles traveled per gallon burned (just like MPG in a car) and knots are speed in nautical miles per hour.  A knot is 1.15 statute miles.  The numbers on the bottom are engine RPMs.

The results were a nice surprise.  Most semi-displacement boats like the Tardis are very fuel-stingy loafing along with the hull in the water, start burning more fuel in the mid-range, but then just drink it up at high speeds.  Indeed, if you want to poke around in Tardis at 6-7 knots (which observation shows no powerboat on Long Island Sound actually does), fuel burn is only just over a gallon an hour.  When the hull starts to lift out of the water at about 3,000 RPM and above-sailboat speeds, fuel burn gets above two gallons per hour.  But speeds above that just don’t seem to cost you much — fuel burn and speed simply rise in about the same proportion.  We didn’t take a measurement, but fuel burn only goes off the chart (8+ gallons per hour) at wide-open throttle (5,600 RPM).

Optimal speed is more a function of the noise level you want (and Tardis is never really noise) and sea state.  In most conditions 3,800 RPM and 9 knots is really “economy cruise”  a 50% improvement over Memsahib’s 5.9 knots top.  But against wind and tide, maintaining that speed at 4,100 is no problem.  And pushing up to 12-13 knots to beat out weather or keep from getting fatigued when single-handling will not drain the tanks.  And on a nice day in flat water 14 would feel pretty good if you stay out of wakes.

Mark will know what is happening here hydrodynamically, but I think this simply means that the hull, weight of gear and engine are pretty well matched.  You could pound along at 18 knots in a plywood hull with a 150hp engine, but it would really stress out both the boat and passengers.  Or you could build an Olga 28 a lot lighter without the fridge, extra tankage, big batteries and bow thruster and run 9 knots at 4,300 and 14 at 5,000 with a 70-75 hp engine and save some money and fuel.  But so far, I am very happy with the choices we made.




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