Fuel Reserve on a Two-Leg Patrol
Your vessel must transit 90 nautical miles from base to a surveillance box, patrol on station, and return to base. Fuel capacity is 1,600 L. Standing orders require that 20% of capacity remains in the tanks at docking, and that reserve may not be drawn on. Consumption figures are measured, not estimated: 120 L/h at 18 knots, 85 L/h at 12 knots, and 40 L/h while loitering on station at 6 knots. Loiter burn is independent of the transit speed used. The operations room wants the longest possible on-station time to support a boarding operation beginning at first light. Weather is forecast calm on both legs, so no speed penalty applies. Refuelling at sea is unavailable and there is no alternate port in range. The engineer's figures are authoritative and must not be second-guessed.
1. What is the longest on-station time the vessel can achieve, and which transit speed profile delivers it? Show the comparison that rules out the alternatives.
Read the worked answer
Step 1 - usable fuel. Reserve = 20% × 1,600 = 320 L, so usable = 1,280 L.
Step 2 - cost per nautical mile, not per hour.
- 18 kn: 120 ÷ 18 = 6.67 L/nm
- 12 kn: 85 ÷ 12 = 7.08 L/nm
Faster is cheaper per mile here, because the hourly saving at 12 kn does not offset the extra hours. The round trip is 180 nm.
Step 3 - compare the three candidate profiles.
- All 18 kn: 180 × 6.67 = 1,200 L → 80 L left for loiter → 2.0 h
- All 12 kn: 180 × 7.08 = 1,275 L → 5 L left → 0.1 h
- 18 kn out, 12 kn back: 600 + 637.5 = 1,237.5 L → 42.5 L left → 1.06 h
Answer: transit out and back at 18 knots, giving 2 hours 0 minutes on station. It is the longest loiter available and also the most fuel-efficient transit.
2. The boarding team now needs 3.5 hours on station. Set out the options and recommend one.
Read the worked answer
Requirement. 3.5 h × 40 L/h = 140 L of loiter fuel.
Transit budget. 1,280 − 140 = 1,140 L. At 6.67 L/nm that buys 1,140 ÷ 6.67 = 171 nm round trip, i.e. 85.5 nm per leg - 4.5 nm short of the planned box.
Options.
- Move the rendezvous 4.5 nm closer to base. Costs nothing but a small loss of surveillance coverage; no fuel or weight penalty.
- Carry an additional 200 L bladder. Capacity becomes 1,800 L, so usable becomes 0.8 × 1,800 = 1,440 L. That leaves 240 L of loiter fuel = 6.0 h on station. Condition: deck space and weight must not force a speed reduction, and the reserve rule must be applied to the new capacity, not the old one.
- Do nothing and accept a shorter boarding window, or refuel from another asset at a rendezvous after the boarding.
Recommendation. Option 2 if the weight can be carried without losing 18 knots; otherwise option 1. Do not simply slow down - slower transit reduces loiter time, it does not extend it.
3. A junior officer proposes transiting at 12 knots 'to save fuel'. Quantify the error in two sentences.
Read the worked answer
The correct comparison is litres per nautical mile, not litres per hour: 120 ÷ 18 = 6.67 L/nm against 85 ÷ 12 = 7.08 L/nm, a penalty of 0.42 L/nm.
Over the 180 nm round trip that is 0.42 × 180 ≈ 75 L wasted, which at 40 L/h is 1 h 53 min of on-station time thrown away. Slowing down only pays if the vessel would otherwise be stationary and burning loiter fuel anyway.