Fuel for the Return Leg
A training vessel must complete a 96 nautical mile passage to port and then a 64 nm return trip on the same fuel load. Fuel consumption is 18 litres per hour at a service speed of 12 knots, and the tanks hold 400 litres with a reserve rule requiring 15% of capacity to remain at all times. The engineer proposes running the outbound leg at 12 knots and the return leg at 10 knots, at which speed consumption falls to 13 litres per hour. Wind and current are assumed neutral. The cadet must confirm whether the plan is safe and, if not, propose the smallest change that makes it compliant.
1. Calculate the fuel required for the proposed plan and determine whether it complies with the reserve rule.
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Outbound: 96 nm ÷ 12 kn = 8 hours. Fuel = 8 × 18 = 144 L.
Return: 64 nm ÷ 10 kn = 6.4 hours. Fuel = 6.4 × 13 = 83.2 L.
Total used = 227.2 L.
Reserve required: 15% of 400 L = 60 L that must remain. Usable fuel = 400 − 60 = 340 L.
227.2 L < 340 L, so the plan complies, with 400 − 227.2 = 172.8 L remaining - well above the 60 L floor. Margin over the usable limit: 340 − 227.2 = 112.8 L.
2. The engineer then learns the return distance is actually 88 nm, not 64 nm. Re-test the plan and state the smallest single change that restores compliance.
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New return: 88 nm ÷ 10 kn = 8.8 hours. Fuel = 8.8 × 13 = 114.4 L.
New total = 144 + 114.4 = 258.4 L. Still below 340 L, so the plan remains compliant, leaving 141.6 L - above the 60 L reserve.
To find the true breaking point, note usable fuel is 340 L. After the 144 L outbound leg, 196 L remain usable. At 13 L/h that supports 196 ÷ 13 ≈ 15.08 hours, i.e. about 150 nm at 10 knots.
So no change is needed at 88 nm. The smallest change would only be required beyond roughly 150 nm of return distance - for example trimming return speed further, or reducing outbound speed to save fuel. The correct answer is therefore: no change required; state the margin.
3. Explain, in terms a deck cadet could defend to the master, why the 15% reserve is subtracted before comparing, rather than added afterwards.
Read the worked answer
The reserve is a floor that must never be crossed, not an allowance to be spent. It exists to cover the unexpected: a slower passage than planned, a diversion, holding offshore, or a generator load not in the plan.
The logic: if the tanks hold 400 L and 60 L must always remain, then only 340 L is ever available for passage planning. Comparing total burn (e.g. 258.4 L) against 400 L would be wrong, because it silently spends the reserve and leaves no buffer at the moment it is most needed.
The correct comparison is always fuel required ≤ capacity − reserve. Stated that way, the cadet can show the master the arithmetic in one line and the safety margin is visible rather than hidden.