See Diesel Fuel Consumption for some initial thoughts. We have a mystery.
The diesel system is a well-understood transformation from pounds of fuel to horsepower and torque. Indeed. It's 0.4 pounds of fuel per hour per horsepower (about 18 fluid ounces.) 59 HP is 3.25 gal/hr.
(59 hp is 1800 RPM, peak efficiency for the Lehman. If I'm reading the chart on page 10 of the owner's manual correctly.)
And this is a big however.
I've recorded times and RPM's and fuel consumption carefully. For years.
And my data doesn't completely match the theory.
My benchmark is two long legs from south Florida to Beaufort NC in April of 2014. From 10:00 April 22 to 18:00 April 26.
Total time was 117 hours of engine operation. (This does not include stopping for a night in Fernandina Beach and taking on fuel.)
RPM's varied. That means the horsepower output varied. But. We can compute RPM-hours and HP-hours and get an overall average.
So. Almost 5 days. Something like 700 nm. At about 1300 RPM on average. (The tach is off, so it would have read close to 1500.)
That's 0.15 pounds of fuel per HP hour not 0.4.
The discrepancy isn't orders of magnitude. It's a big factor of 2.667 (or 0.375.)
Did I measure fuel or hours wrong? No. Those are easy. The log is an hour-by-hour entry from the cockpit.
Did I measure the RPM and HP wrong? Not by much. The tach error is a factor of 0.8574. It's linear, and easily measured. I have a device to read engine RPM's directly from the spinning wheels and compare those with the tach.
It ain't hull or prop or anything like that. Those have a minor effect, but a diesel engine cranks out the same HP and burns the same fuel if you're going 4 knots into the current with a foul prop or 7 knots with the current. We're not measuring speed. Only hours.
We almost always have sails up when we're motoring. Even in light air.
So. Here are two fuel consumption tables. The power-only is theoretical times based on industry average fuel consumption.
And. Motor Sailing. Based on Red Ranger's Ford Lehman and cutter rig.
This gives us a range of options. The best case is a fair wind, the fuel tank hours are infinite. The motor-sailing case is 50 hours. The worst case is 24 hours under power because it's wind-on-the-nose.