Patrol Fuel Endurance Decision
A 30-meter patrol boat has 3,600 liters of usable diesel. At 15 knots it burns 180 liters per hour; at 10 knots it burns 120 liters per hour. Standing orders require keeping 15 percent of usable fuel as a reserve. The mission is to transit 45 nautical miles to station at 15 knots, patrol on station for 8 hours at 10 knots, and return 45 nautical miles at 15 knots. A late tasking would add a 20-nautical-mile diversion at 15 knots before the return leg. No refueling is available at sea. The fuel gauge reads 3,650 liters, but the sounding table says 3,600 liters usable; 50 liters are not usable because of trim. The executive officer wants a yes-or-no answer with numbers, not an estimate. The weather forecast is moderate, and current is negligible.
1. Compute the fuel required for the original mission and the required reserve. State the remaining margin.
Read the worked answer
Transit out: 45 nm ÷ 15 kn = 3 h; 3 h × 180 L/h = 540 L. Transit back: same = 540 L. Patrol on station: 8 h × 120 L/h = 960 L. Mission total: 540 + 540 + 960 = 2,040 L. Reserve: 15% × 3,600 L = 540 L. Fuel available for mission after reserve: 3,600 − 540 = 3,060 L. Margin: 3,060 − 2,040 = 1,020 L.
The original mission is feasible with 1,020 L of margin above the required reserve. That margin is about 28 percent of usable fuel, which is a reasonable buffer for moderate weather and minor speed changes, though it is not a reason to ignore fuel monitoring.
2. Compute the fuel required for the 20-nautical-mile diversion and decide whether the tasking can be accepted.
Read the worked answer
Diversion time: 20 nm ÷ 15 kn = 1.333 h. Diversion fuel: 1.333 h × 180 L/h = 240 L. New mission total: 2,040 + 240 = 2,280 L. Total with reserve: 2,280 + 540 = 2,820 L. Remaining margin: 3,600 − 2,820 = 780 L.
The tasking can be accepted, but with conditions. The boat still retains the 540 L reserve and has 780 L above it. That is enough for the planned diversion, but not enough to absorb a large unforecast increase in consumption. The answer should be “yes, within the stated assumptions,” not an unconditional “yes.”
3. The gauge reads 3,650 liters while the sounding table says 3,600 liters usable. Explain which figure you would use and why, and state one monitoring action if the diversion is accepted.
Read the worked answer
Use 3,600 L, not the gauge reading. The gauge may be indicating total liquid in the tank, including fuel that cannot be drawn because of trim, while the sounding table accounts for usable volume at the current attitude. The 50 L difference is exactly the amount identified as unusable, so treating it as available would understate the reserve risk.
If the diversion is accepted, I would set an hourly fuel check against the plan: record fuel consumed, compare it with the expected 180 L/h transit burn and 120 L/h patrol burn, and define an abort or return criterion. For example, if consumption exceeds the plan by enough to erode the 540 L reserve, the boat should abort the diversion and report to higher authority rather than press on. The monitoring action matters because the calculation is only valid if actual burn matches the assumed rates.