Fuel and Water Management

Fuel Range Calculations

Every skipper should know their boat's fuel consumption at various RPM — and most don't. This number, combined with tank capacity, determines your motoring range. On an offshore passage, motoring range determines whether you can motor through calms, charge batteries, and make water.

Measure consumption: The only reliable way is to measure it. Fill the tank, motor for a known period at a known RPM, then refill and measure what you used. Repeat at 1,500, 2,000, 2,500, and cruising RPM. Record: RPM, speed in knots, fuel consumed per hour. This gives you your boat's specific consumption curve.

Range calculation: Range = (usable fuel ÷ consumption per hour) × speed in knots. Example: 200 litres usable fuel, 3 litres/hour at 2,200 RPM producing 6 knots = 66.7 hours of motoring = 400 nautical miles. But this is dead-calm, flat-water range. In a head sea, consumption can double while speed drops by 30–40%. Real-world range in adverse conditions may be half the calm-water number.

The reserve rule: Never plan to use more than 70% of your fuel capacity. The remaining 30% is your reserve for emergencies — motoring off a lee shore, powering into harbour against a head wind, running the engine for battery charging and watermaking. On an offshore passage, fuel reserves are not optional — they are the margin between 'inconvenient calm' and 'emergency.'

Fuel Management Practices

Fuel management is more than knowing your range. It's about keeping the fuel clean, monitoring consumption, and making smart decisions about when to motor and when to sail.

Fuel quality: Diesel degrades over time — biological growth (diesel bug) thrives in the water that condenses in tanks. Treat fuel with a biocide additive when filling. Carry spare primary fuel filters (Racor-type) and know how to change them at sea — a clogged fuel filter is the most common engine failure on cruising boats. If the engine starts surging or losing power, the fuel filter is the first suspect.

Jerry cans: For extended range beyond your tanks, carry fuel in proper jerry cans lashed on deck or in cockpit lockers. Metal jerry cans are safer than plastic (no static charge, more durable). Secure them so they can't move in a seaway. Transferring fuel from jerry cans to the tank in a rolling sea is messy — use a proper funnel with a filter, and do it in calm conditions when possible.

Monitoring consumption: Keep a fuel log. Record engine hours and note every time you add fuel. Over time, you'll know exactly how much fuel your engine uses per hour in various conditions. When passage planning, use your actual consumption data — not the manufacturer's optimistic specification.

The motoring decision: On a passage, every hour of motoring consumes fuel that you may need later. In a calm, the choice between motoring at 6 knots and drifting at 1 knot is tempting — but burning 3 litres per hour for 12 hours uses 36 litres that might be critical later. Consider: is this calm going to end? Can you wait? Will you need that fuel for charging batteries or making water? The patient sailor often arrives with more reserves.

Water Management

Water is life. On a coastal passage, running low on water is inconvenient. On an offshore passage, it's an emergency. Every crew member needs a minimum of 2–3 litres per day for drinking alone. Cooking, washing, and cleaning add more.

Water budget: For a 4-person crew on a 16-day passage: 4 × 3 × 16 = 192 litres minimum for drinking. Add 1 litre per person per day for cooking = 64 litres. Conservative total: 256 litres. With 25% reserve: 320 litres. If your tanks hold 400 litres, you have adequate margin. If they hold 200 litres, you need jerry jugs, a watermaker, or strict rationing.

Watermakers: A reverse-osmosis watermaker converts seawater to fresh water. They range from small manual units (producing 5–8 litres per hour) to powered units producing 40–100+ litres per hour. A powered watermaker transforms water management — instead of rationing, you produce what you need. But watermakers fail. Membranes clog, pumps break, power systems fail. Never rely solely on a watermaker — always carry enough stored water to complete the passage without it.

Rationing: If water supply is limited, ration from day one — don't wait until the tanks are half empty. Assign each crew member a daily water bottle (filled from a central supply each morning). Cooking water can be seawater for many purposes (boiling pasta, rice, vegetables). Saltwater soap exists for bathing. The crew that starts rationing early barely notices the restriction; the crew that starts late suffers.