Fuel Range and Reserve on a Windy Coastal Passage
A 12.5 m displacement launch displaces 14 tonnes and is powered by a single 250 hp diesel inboard turning a fixed propeller. On a calm-water trial at 1800 rpm the vessel made 8.2 knots burning 26 L/h; at 2200 rpm it made 10.1 knots burning 44 L/h. Tanks hold 480 L usable, and the skipper treats 20% as unusable reserve. The planned passage is 96 nautical miles along an exposed coast with a forecast of 20 knots on the nose and 1.5 m seas for the middle 40 miles. Past experience on this boat suggests a 15% speed loss and a 25% fuel penalty in those conditions. The crew wants to arrive before dark, about 11 hours after departure.
1. Using the trial data, calculate the fuel needed for the passage at each of the two power settings and recommend which setting to use, showing your working.
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Convert usable fuel to a working figure. Usable 480 L less 20% reserve = 384 L available for passage planning.
Calm-water consumption per nautical mile.
- 1800 rpm: 26 ÷ 8.2 = 3.17 L/nm.
- 2200 rpm: 44 ÷ 10.1 = 4.36 L/nm.
Apply the weather penalty over the affected section. The middle 40 nm is penalised by 25% fuel and 15% speed loss. The remaining 56 nm is treated as trial conditions.
1800 rpm:
- Exposed leg: 40 nm at 8.2 × 0.85 = 6.97 kn, so 40 ÷ 6.97 = 5.74 h at 26 × 1.25 = 32.5 L/h = 187 L.
- Calm leg: 56 ÷ 8.2 = 6.83 h at 26 L/h = 178 L.
- Total ≈ 365 L. That is under 384 L but leaves almost nothing beyond the 20% reserve - too tight.
2200 rpm:
- Exposed leg: 40 ÷ (10.1 × 0.85 = 8.59) = 4.66 h at 44 × 1.25 = 55 L/h = 256 L.
- Calm leg: 56 ÷ 10.1 = 5.54 h at 44 L/h = 244 L.
- Total ≈ 500 L, which exceeds the 384 L available. Not feasible without refuelling.
Recommendation. Neither setting gives a comfortable margin. The 1800 rpm plan is the only one that physically fits the fuel, and it must be run with a genuine reserve intact, so the skipper should slow to about 1700 rpm, accept a longer day, or shorten the passage. Running the fast setting on the strength of the reserve would be a planning failure, not a judgement call.
2. The crew insists on making port before dark. Evaluate the time and fuel consequences of that constraint and state what you would actually do.
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Time at 1800 rpm with weather. 5.74 + 6.83 = 12.6 h, so arrival is roughly 1.6 h after the 11-hour target - after dark.
Time at 2200 rpm with weather. 4.66 + 5.54 = 10.2 h, which meets the daylight target but needs about 500 L against 384 L available. The boat would run dry roughly 20 nm short, in exposed water, in the dark.
The trade-off. The daylight constraint and the fuel constraint cannot both be satisfied at any tested setting. Something must give: departure time, route, or destination.
Judgement. The dark arrival is the least dangerous of the failures. Options in order of preference:
- Leave earlier or plan a daylight departure so the 12.6 h run finishes in light.
- Break the passage at an intermediate harbour about 55–60 nm along, refuel, and complete in daylight the next day.
- Run at 1800 rpm, accept a night arrival, and brief the crew on lit approaches, radar reflectors and a reduced-speed entry.
What I would not do is run at 2200 rpm and treat the reserve as fuel. A reserve is a contingency for the unexpected, not a budget line for impatience. The correct answer names the conflict and resolves it by changing the plan, not by spending the safety margin.