Navigation Log and Fuel Reserve Judgement
A trainee navigator keeps a manual log during a short coastal passage. The log records: departure 09:15, speed made good 9 knots, distance to waypoint 27 nautical miles. At 10:45 the trainee notes the vessel has covered 13.5 nautical miles. The engine burns 12 litres per hour at this speed, and the tank held 80 litres at departure. The trainee claims the vessel will arrive at the waypoint by 12:15 with 'plenty of fuel to spare, since we only burn 12 litres an hour.' The coxswain asks for an independent check of the timing and the fuel margin, allowing a 20% reserve on fuel actually burned for the passage.
1. Verify the trainee's arrival time and the distance covered by 10:45.
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At 9 knots, distance in 1.5 h (09:15→10:45) = 9 × 1.5 = 13.5 nm ✓ - the trainee's 10:45 figure is correct, confirming the speed. Remaining distance = 27 − 13.5 = 13.5 nm. Time to cover it = 13.5 / 9 = 1.5 h. Arrival = 10:45 + 1:30 = 12:15 ✓. Arrival time 12:15 is correct.
2. Compute the fuel actually burned for the whole passage and check it against the 20% reserve rule.
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Total passage time = 09:15 to 12:15 = 3.0 h. Fuel burned = 3.0 × 12 = 36 L. Reserve = 20% of 36 = 7.2 L. Total needed = 43.2 L. Tank holds 80 L, so remaining = 80 − 43.2 = 36.8 L. The passage is comfortably within fuel limits with reserve intact.
3. The trainee says 'plenty of fuel to spare, since we only burn 12 litres an hour.' Identify the weakness in that reasoning even though the conclusion happens to be right.
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The statement gives a rate but never converts it into a total requirement or compares it to the reserve rule. '12 litres an hour' alone says nothing about whether 80 L is enough - you must multiply by passage time (3 h → 36 L), add the 20% reserve (7.2 L), and compare to 80 L. The conclusion is correct only because the numbers happen to work out; the reasoning would give the same confident answer on a much longer passage where it would be dangerously wrong. Correct answer, faulty method.