Fuel Range Planning for Passages

Motor Sailing · Passage Planning

Fuel range planning is the least romantic part of passage preparation and, for boats doing any significant motoring, the most consequential. The planning failure that leads to running out of fuel offshore is not ignorance — it is optimism. Sailors assume the tanks are full when they are not, assume they will have less wind than they get, assume they will make better speed than the sea state allows, and assume the fuel dock at the destination is open when they arrive. Any one of these assumptions failing is inconvenient. Several of them failing together is a safety event.

The 75% rule and a simple fuel consumption benchmark for your engine are the two tools that prevent this. Applied consistently at the planning stage of every passage with a significant motoring component, they remove optimism from the calculation and replace it with a number you can trust.

Why the 75% Rule

The rule is simple: never plan a leg that requires more than 75 percent of your tank capacity in fuel. The 25 percent reserve is not arbitrary — it represents the combined probability mass of four things that will, over enough passages, all happen at least once.

You will not depart with full tanks. Fuel gauges on sailboats are notoriously imprecise, and the physical process of filling a sailboat's tank through a deck fill while the boat is rolling at a dock is imperfect. Assume you are leaving with 90 to 95 percent of stated capacity at best — and if your gauge reads "full," that is a gauge reading, not a measurement.

You will burn more than your calm-water benchmark. Head sea conditions reduce boat speed and increase fuel burn simultaneously. A 20 percent degradation from plan is common in an active sea state. A boat that should make 5.5 knots on 0.65 L/hr in calm water makes 4.5 knots on 0.75 L/hr into 2-foot chop. The same tank that should last 55 hours now lasts 43, and the passage that should take 48 hours takes 55.

You cannot run a fuel tank dry. Diesel engines do not sputter and die gracefully when they run out of fuel — they air-lock. Air in the fuel system requires bleeding the injectors, which on many auxiliary diesels is a 30-minute procedure requiring access to the engine in conditions that may be anything but ideal. On a boat that just ran out of fuel 40 miles offshore in a building sea, this is at minimum miserable and potentially dangerous. Always maintain a reserve sufficient to prevent running the tank below the point where the pickup draws air.

The fuel dock may not be available. Fuel docks at small marinas and anchorages close at predictable hours — typically 5 pm in summer, earlier off-season. They close for weather. They run out of diesel. They are occupied by a vessel that took three hours to fuel because of a credit card problem. Arriving with 20 percent in reserve means you can wait, anchor overnight, or divert to the next fuel stop without a crisis. Arriving with 5 percent means any of these possibilities becomes an emergency.

Fuel Consumption Benchmarks

The relationship between engine horsepower and fuel consumption in marine diesels is approximately linear at cruise throttle. The working rule is 0.05 liters per horsepower per hour at 70 percent throttle in calm water — a close enough approximation for planning purposes that will be conservative in light conditions and accurate in typical ones.

A 20-horsepower diesel (Yanmar 2GM series, older Atomic 4 replacements, small Volvo units) burns approximately 1.0 L/hr at cruise. This is the smallest common auxiliary range; these engines power boats in the 25 to 32-foot range.

A 30-horsepower diesel burns approximately 1.5 L/hr. The Yanmar 3GM30, one of the most common auxiliaries in the world, falls in this range. It powers a huge proportion of the world's 35 to 40-foot cruising fleet.

A 40-horsepower diesel — Volvo D2-40, Yanmar 4JH series at moderate throttle, Westerbeke 40 — burns approximately 2.0 L/hr. These engines are common in 40 to 45-foot boats.

A 50-horsepower diesel burns approximately 2.5 L/hr. Above 50 hp, the linear rule still holds but the boats involved are typically large enough to have correspondingly large tank capacity, so range is less commonly an issue.

Deviations from these benchmarks are real and worth understanding. Bottom fouling — antifoul that is past its service life, or a boat that has not been cleaned in two seasons — increases resistance and therefore fuel burn. A propeller in poor condition (bent blade, worn leading edge) reduces efficiency. Heavy displacement boats with a hull speed penalty in chop burn more fuel per mile than the benchmark predicts. Know your specific boat's consumption from your engine log rather than relying purely on the rule.

The Range Calculation: Mental Math for the Passage Plan

The calculation has four steps. Run it before every leg with significant motoring.

Step one: establish usable fuel. Multiply tank capacity by 0.75. A 40-liter tank has 30 liters of usable fuel. A 60-liter tank has 45 liters.

Step two: calculate motoring hours. Divide usable fuel by your engine's burn rate at cruise. A 30-liter budget at 1.5 L/hr gives 20 hours of motoring. A 45-liter budget at 2.0 L/hr gives 22.5 hours.

Step three: convert to distance. Multiply motoring hours by expected boat speed. Twenty hours at 5.5 knots gives 110 nautical miles. Twenty-two hours at 5.5 knots gives 124 miles.

Step four: apply a sea state reduction. If the forecast is for anything above flat calm, reduce your calculated range by 15 to 20 percent. 110 miles becomes 88 to 93 miles of realistic motoring range in typical coastal conditions.

If the planned leg is shorter than the calculated realistic range, you have adequate fuel. If the leg is longer, you need an intermediate fuel stop, supplemental jerrycan capacity, or a revised plan that relies more heavily on sailing.

Extended Motoring: Jerrycan Management

For passages where tank capacity is marginal, jerrycans extend range at the cost of some complexity. The standard offshore jerrycan is a 20-liter polypropylene container — not metal, which corrodes and introduces rust into fuel systems, and not the style of container sold for gasoline, which is not rated for diesel.

Stowage and securing is a safety-critical detail that offshore sailors get wrong more often than any other aspect of fuel management. Jerrycans stowed unsecured in the cockpit are a fire risk (if they leak near an engine exhaust or hot surface), a crew injury risk (if they shift in a seaway), and a loss risk (if they go overboard). The correct location is in a deck locker, bilge compartment, or purpose-built deck mount with positive retention. If the cockpit must be used, the can must be strapped with dedicated jerrycan straps — not bungee cords — to a structural attachment point, and the cap must be sealed and checked before departure.

Transferring fuel underway from a jerrycan to the main tank requires a transfer pump for anything other than flat calm conditions. Tipping a 20-liter can over a deck fill in a seaway results in fuel spilled on the deck, which is both wasteful and a slip hazard. A small hand-operated transfer pump with a hose long enough to reach from the can to the deck fill takes the variables out of the process. Some sailors use a 12-volt pump; either works.

Finding Fuel Offshore

Fuel dock hours are one of the most consistent planning failures for sailors moving along coastlines or island-hopping. The standard operating hours for small marina fuel docks in temperate latitudes is 7 am to 5 pm, seven days a week in peak season. Off-season, winter hours may be reduced to five or four days per week with no advance notice. The dock closes when it closes.

Planning arrivals for early morning provides the most flexibility: the dock is open, the staff is fresh, and if there is any waiting, you have the full day ahead. Planning arrivals for late afternoon in an unfamiliar area is how you end up anchored outside a marina that closed an hour before you arrived, waiting for tomorrow with less fuel than you intended.

Fuel price varies dramatically by location. Caribbean fuel is expensive by developed-world marina standards — in some anchorages, diesel costs two to three times North American marina prices. Southeast Asian fuel, where available at all, is among the cheapest in the world. The practical implication for passage planning is that cheap-fuel ports are worth topping off at even when you do not strictly need fuel, and expensive-fuel ports are worth bypassing if you have adequate range to the next stop.

Diesel quality is a legitimate concern in developing-country ports and in isolated anchorages where fuel has been sitting in local storage for an extended period. Water contamination, microbial growth, and fuel that has oxidized over time can cause injector fouling and filter blockage. A Racor or equivalent primary fuel filter installed between the tank and the engine is mandatory equipment for any boat doing extended cruising — it is the last line of defense before bad fuel reaches the injectors. Carry spare filter elements; they may need changing mid-passage if you took on questionable fuel.

The No-Engine Contingency

Every passage that includes motoring legs should have a plan for completing the arrival under sail alone. This is not pessimism — it is the same thinking that underlies any other contingency plan. Engines fail. Fuel runs out. The impeller blows at the worst possible moment. The plan for arriving without the engine should be thought through before the passage, not improvised at 2 am in the entrance to an unfamiliar harbor.

The relevant questions are: Can the destination harbor entrance be negotiated under sail in the prevailing conditions? What is the tidal timing, and can arrival be arranged to coincide with a favorable current? Is the approach clear of hazards that require precise maneuvering that only the engine provides? Is there an alternative anchorage accessible under sail if the primary destination is not?

Many yacht harbors can be entered under sail by a competent crew in reasonable conditions. Some cannot — a narrow entrance between breakwaters with a cross-current, commercial traffic, and no room to maneuver is genuinely an engine-required situation. Knowing this about your planned destination before departure allows you to carry adequate fuel or to have a genuine alternative identified. The harbors that require an engine to enter safely are the ones that most need a fuel-planning discipline that ensures you arrive with power available.

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Frequently Asked Questions

What is the 75% rule for fuel planning and why does the reserve percentage matter? The 75% rule states: never plan a leg that requires more than 75% of tank capacity. The 25% reserve covers four compounding probabilities — you will not depart with full tanks (fuel gauges are imprecise, a 'full' reading is not a measurement), you will burn more than the calm-water benchmark (head sea conditions reduce speed and increase burn simultaneously), you cannot run a tank dry (diesel engines air-lock when fuel runs out, requiring a 30-minute bleeding procedure offshore), and the fuel dock may not be available at arrival (small marina fuel docks close at predictable hours, run out of diesel, or are occupied). Any one of these failing is inconvenient; several together is a safety event.

What are the fuel consumption benchmarks for common marine diesel engines? The working rule is 0.05 liters per horsepower per hour at 70% throttle in calm water. A 20-horsepower diesel (Yanmar 2GM) burns approximately 1.0 L/hr; a 30-horsepower diesel (Yanmar 3GM30, one of the most common auxiliaries in the world) burns approximately 1.5 L/hr; a 40-horsepower diesel (Volvo D2-40, Yanmar 4JH) burns approximately 2.0 L/hr. Deviations from these benchmarks are real — bottom fouling, propeller condition, and heavy displacement in chop all increase actual burn above the rule. Know your specific boat's consumption from your engine log.

How do you calculate realistic motoring range for a passage? The four-step calculation: (1) Multiply tank capacity by 0.75 to establish usable fuel — a 60-liter tank has 45 liters available. (2) Divide usable fuel by the engine's burn rate at cruise to get motoring hours — 45 liters at 2.0 L/hr gives 22.5 hours. (3) Multiply motoring hours by expected boat speed — 22.5 hours at 5.5 knots gives 124 nautical miles. (4) Apply a 15–20% sea state reduction for any forecast above flat calm — 124 miles becomes 99–105 miles of realistic motoring range. If the planned leg is longer than the calculated realistic range, you need an intermediate fuel stop, supplemental jerrycan capacity, or a revised plan.

What are the correct standards for offshore jerrycan selection and stowage? The standard offshore jerrycan is a 20-liter polypropylene container — not metal (which corrodes and introduces rust into fuel systems) and not gasoline-style containers (not rated for diesel). Stowage is the safety-critical detail most often gotten wrong: jerrycans stowed unsecured in the cockpit are a fire risk near exhaust or hot surfaces, a crew injury risk in a seaway, and a loss risk if they go overboard. Correct location is in a deck locker, bilge compartment, or purpose-built deck mount with positive retention. If the cockpit must be used, strap with dedicated jerrycan straps — not bungee cords — to a structural point. Transferring fuel underway requires a transfer pump, not tipping a full 20-liter can over a deck fill in a seaway.

Why is diesel quality a concern for offshore sailors and how should it be managed? Diesel quality is a legitimate concern in developing-country ports and isolated anchorages where fuel has been sitting in local storage for extended periods. Water contamination, microbial growth, and oxidized fuel cause injector fouling and filter blockage. A Racor or equivalent primary fuel filter between the tank and engine is mandatory equipment for any boat doing extended cruising — it is the last line of defense before bad fuel reaches the injectors. Carry spare filter elements, as they may need changing mid-passage after taking on questionable fuel. Cheap-fuel ports are worth topping off at even when not strictly needed; expensive-fuel ports (Caribbean diesel runs two to three times North American marina prices) are worth bypassing if range allows.