Compare passage speeds using your vessel’s fuel curve
Use measured consumption at each speed to compare voyage time and fuel cost without assuming that faster or slower is always cheaper.
Choose comparable observations
A fuel curve relates operating speed or RPM to consumption. Use observations from your own vessel, recording load, hull condition, sea state and engine configuration where available. A curve copied from another vessel may be useful as a provisional assumption, but should be labelled that way.
Calculate each speed separately
An illustrative 100-nautical-mile passage at 10 knots and 40 litres per hour takes 10 hours and uses 400 litres. At 12 knots and an assumed 65 litres per hour, the same distance takes about 8.33 hours and uses about 542 litres. The faster scenario saves roughly 1 hour 40 minutes while adding about 142 litres. These are teaching assumptions, not measured vessel data.
Keep the decision wider than fuel
The lowest modelled fuel total is not automatically the right operating choice. Arrival restrictions, weather windows, machinery limitations and safe operation still matter. Compare alternatives within the operating envelope approved for the vessel; do not select a speed solely from a cost calculation.
Make uncertainty visible
Repeat the comparison with a plausible higher burn rate and longer passage time, based on your operating experience. Show the base and higher-consumption cases side by side. If a small change in the assumptions reverses the preferred option, the decision needs better input data rather than more decimal places.
Review the curve after a real passage
Record actual distance, underway time and fuel used on the same basis as the plan. Separate generator use if the original curve covers propulsion only. SeaWise’s vessel fuel-curve features provide a place to model operating points; the quality of the comparison still depends on the evidence supplied.