Fuel Burn vs. Hull Speed (NMPG) Calculator
Compare RPM, boat speed, and fuel burn to find your most efficient cruising point. The calculator shows NMPG, fuel per nautical mile, speed relative to hull speed, and the point where extra throttle starts costing disproportionately more fuel.
Fuel Economy Results
Efficiency by RPM
| RPM | Speed | Fuel Burn | NMPG | Fuel / NM | Hull Speed % | Δ Fuel | Δ Speed | Efficiency Flag |
|---|
What is NMPG?
NMPG means nautical miles per gallon. It describes how far the boat travels for each US gallon of fuel.
$$NMPG = \frac{Boat\ Speed\ (knots)} {Fuel\ Burn\ (US\ gal/hour)}$$
Because one knot equals one nautical mile per hour, dividing speed by gallons per hour gives nautical miles per gallon.
What is fuel burn per nautical mile?
This is the inverse of NMPG:
$$Fuel/NM = \frac{Fuel\ Burn}{Boat\ Speed}$$
Lower fuel per nautical mile means better fuel economy.
How do you calculate hull speed?
For a traditional displacement hull, a common approximation is:
$$Hull\ Speed = 1.34 \times \sqrt{LWL_{ft}}$$
The result is in knots when LWL is entered in feet. Hull speed is better treated as a reference transition point than as a strict physical limit.
What is the fuel economy sweet spot?
The sweet spot is the operating region where the boat gains useful speed without a disproportionate increase in fuel consumption. The calculator identifies two related things:
- Best NMPG point: the RPM with the highest calculated nautical miles per gallon.
- Efficiency knee: the first higher-RPM step where the fuel increase is disproportionately larger than the speed increase according to your selected thresholds.
How does the knee calculation work?
The calculator compares adjacent operating points:
$$Fuel\ Increase\ \% = \frac{Fuel_2-Fuel_1} {Fuel_1} \times100$$
$$Speed\ Increase\ \% = \frac{Speed_2-Speed_1} {Speed_1} \times100$$
$$Penalty\ Ratio = \frac{Fuel\ Increase\ \%} {Speed\ Increase\ \%}$$
For example, if fuel burn increases 30% while speed increases only 5%, the penalty ratio is 6. That is a strong indication that the extra throttle is buying relatively little additional speed.
Example of the fuel burn sweet spot
Imagine these two operating points:
$$1400\ RPM: 6.8\ kn,\ 3.1\ GPH$$
$$1600\ RPM: 7.1\ kn,\ 4.2\ GPH$$
Fuel burn rises by approximately 35%, while speed rises only about 4.4%. The penalty ratio is therefore roughly 8. This is exactly the type of step the knee test is designed to highlight.
Why can fuel burn increase so quickly near hull speed?
As a displacement boat approaches its practical wave-making limit, additional speed can require increasingly more propulsion power. Resistance is influenced by hull form, displacement, length, appendages, water depth, trim, and sea conditions.
Is the best NMPG always the best cruising speed?
No. The highest NMPG point maximizes fuel economy, but a skipper may reasonably choose a slightly faster operating point to reduce passage time, maintain control, meet weather constraints, or operate in a part of the engine's preferred range.
The useful result is therefore not simply "maximum NMPG." Looking at the whole RPM-to-speed-to-fuel curve gives a much better picture of the practical cruising range.
Why use several RPM points?
A single fuel-burn measurement cannot show where efficiency starts deteriorating. Multiple steady operating points let you see the shape of the curve and identify the point where additional speed becomes expensive in fuel.
Fuel Burn vs. Hull Speed Example
For a 35 ft LWL displacement hull:
$$Hull\ Speed = 1.34 \times \sqrt{35} \approx 7.93\ kn$$
A speed of 7.4 knots would therefore be about 93% of the traditional hull-speed reference:
$$\frac{7.4}{7.93} \times100 \approx 93.3\%$$
What makes a good fuel-burn test?
For a useful comparison, keep the conditions as consistent as possible. Record steady speed and fuel consumption at each RPM, ideally on similar water conditions and with similar loading, trim, wind, current, and sea state.
Why is actual fuel data better than a theoretical formula?
Real fuel economy depends on the complete propulsion system and operating condition. Propeller loading, hull fouling, displacement, sea state, wind, current, and engine condition can all change the result. Your measured operating points capture the behavior of your particular boat.
Fuel economy FAQ
There is no single good value. NMPG varies widely with boat type, displacement, hull form, engine, propeller, and cruising speed. The useful comparison is between your own operating points.
Measure speed and fuel burn at several steady RPM settings, calculate NMPG, and compare the changes in speed and fuel burn as RPM increases.
It depends on the individual boat and propulsion system. The most useful RPM is often near the point where additional RPM produces relatively little extra speed for a much larger fuel increase.
It often can for displacement boats because resistance can rise substantially as the boat approaches its practical wave-making limit, but the exact curve depends on hull and propulsion design.
No. The traditional hull-speed formula is a rule of thumb, not an absolute physical limit. Some displacement boats can exceed the traditional estimate under suitable conditions.
NMPG measures nautical miles per US gallon, while conventional MPG normally means statute miles per US gallon. One nautical mile is longer than one statute mile.