When you’re cruising the coast, fuel isn’t usually top of mind; the next dock is never far away. But once you step into the world of offshore voyaging and remote anchorages, the equation changes dramatically.

Out there, diesel isn’t just about turning the propeller. It represents safety, independence, and the ability to live comfortably away from land. For the modern blue water sailor, fuel capacity is one of the defining traits of a true passage maker.

From tradition to today

In the 1960s and 70s, plenty of voyaging designs carried very little diesel. Some dispensed with an engine altogether. These boats were built in an era before energy-hungry systems; no refrigeration (or just an icebox), windvane steering instead of powered autopilot, no watermaker, no radar, and certainly no satellite internet. A kerosene lamp and a sextant were all that most sailors needed.

Today, things are different. The modern voyaging yacht often has both a refrigerator and freezer, autopilot, watermaker, high-output communications, and a full suite of electronics. Solar and wind cover much of the load, but not all. In weeks of cloudy skies, or in the shorter days of the tropics, battery banks can struggle. That’s when having diesel-driven charging capacity, a generator or a high-output alternator on the main engine, becomes essential.

Fuel as a safety margin

Diesel isn’t just about comfort; it’s about resilience. Offshore, hundreds of miles from shelter, a dismasting or rigging failure could mean days of motoring. Even without a catastrophe, having enough fuel gives you options. Take the ITCZ as an example. Crossing it often means alternating between dead calms and violent squalls. On Wanderlust’s last passage through, I put in and shook out a reef at least eight times in a single day, while motoring for about six hours through calms to weave around the worst squalls. Without fuel in reserve, you’re at the mercy of whatever comes next. With it, you can maneuver, stay in control, and keep the passage safe.

Some traditionalists argue electronics aren’t essential, that a “real” sailor should simply ride it out under sail alone. I see it differently. Seamanship is about making good decisions with the tools available. Reliable weather data, for example, enables smarter routing, and smarter routing is safer. Access to that information requires energy, and offshore, energy ultimately comes from fuel. Using diesel to avoid punishment, preserve crew and gear, and keep critical systems – autopilot, navigation, communications – alive isn’t weakness. It’s prudence. That combination of mobility and power is one of the truest safety margins at sea.

The freedom factor

One of the greatest joys of a capable offshore sailboat is the ability to disappear into remote anchorages for weeks at a time. Fuel capacity is what makes that possible. With ample diesel on board, you can make your own water, keep the refrigerator running, and charge batteries without being tethered to a marina.

On Wanderlust, our 2014 Passport 545AC, we carry 376 gallons in three stainless steel tanks, all positioned low in the bilge. That keeps weight centered and the foredeck free of yellow jugs lashed to lifelines. More importantly, it means we can reach places where the nearest fuel dock is hundreds of miles away, or may not exist at all. In the Tuamotus, for example, fuel is scarce and expensive. Because we have the reserves, we can drop anchor in an atoll and stay as long as we want, running systems as needed, without constantly rationing every amp-hour while worrying about the next refueling run.

This is where tankage turns from numbers into lifestyle. With it, you can live off-grid, self-reliant, and free to explore on your own terms. Without it, your horizon shrinks to the distance between fuel docks. For us, generous built-in capacity has meant crossing oceans without worry, lingering in places we love, and truly living the sea gypsy life.

Tankering: A practical advantage

Large tanks have another benefit; economics. Diesel prices fluctuate wildly by region. With big tanks, you can “tanker” fuel, buying in bulk where it’s inexpensive and carrying it forward.

Since leaving San Diego in February 2023, we’ve bought fuel only in San Diego, La Paz, Tahiti, and New Zealand. Before sailing from New Zealand back to French Polynesia in March 2025, we topped off again. After the 2,500 nautical mile passage and months of cruising Tahiti, Moorea, Raiatea, Tahaa, and the Tuamotus, we still have half our supply left. That kind of range changes the way you voyage.

How to calculate range under power

Knowing how much diesel to carry is one thing. Knowing what it gives you in real terms is another. Calculating your range is straightforward if you know two things:

• Fuel consumption at cruising RPM (gallons per hour).
• Average boat speed at that RPM (knots).

The basic formula is:

 Range = Usable Fuel x Speed/(Burn Rate)

For imperial units, range is in nautical miles, usable fuel in gallons, speed in knots, and burn rate in gallons per hour.

For metric units, range is in kilometers, usable fuel in liters, speed in kilometers per hour, and burn rate in liters per hour.

A few key points:

• Always use usable fuel, not total tankage. Most tanks can’t be completely drained without sucking up sludge or air.
• Average speed should be based on real-world conditions, not just flat-water sea trials.
• Fuel burn curves (often in the engine manual) are a good starting point but confirm them with actual log data.

The Wanderlust Example

On SV Wanderlust, we run a Yanmar 4JH4-HTE:

• Cruising RPM: ~2000
• Burn rate: ~1.3 gph
• Usable fuel: ~350 gallons (out of 376 total)
• Average speed at 2000 rpm: ~6.5 knots

Plugging into the formula:

1,750 nm = 350 Gal x (6.5 kts)/(1.3 GPH)

That’s unusually generous for a 55-footer. A more typical target for a true bluewater boat is the ability to motor 1,000–1,200 nautical miles. This gives enough reserve to handle calms, detours, or emergencies.

The critical lesson is to know your numbers. If the manufacturer doesn’t provide a curve, measure your own consumption in calm conditions and log it against RPM and boat speed. That knowledge isn’t trivia; it’s a planning tool and a genuine safety margin.

How much is enough?

Every skipper’s answer depends on their boat, cruising style, and appetite for risk. But for a 45–55 foot passage maker, 250–400 gallons is a proven sweet spot. Smaller boats can get by with less, but the same principle applies; capacity equals freedom.

Freedom and safety through fuel

Diesel tankage is more than steel or fiberglass tanks filled with liquid; it’s stored freedom. It lets you cross oceans, sit happily in remote anchorages, tanker fuel when prices are low, and keep essential systems alive even in poor weather.

In the end, the right amount of fuel isn’t measured only in gallons. It’s measured in freedom, safety, and the kind of cruising life you want. If your dream is to cross oceans, stay far from the beaten path, and choose your own horizon, then generous, well-designed fuel capacity is what transforms a capable sailboat into a true bluewater cruiser.