A 30-metre sailing yacht motoring from Barcelona to Palma de Mallorca at 8 knots burns roughly 40 litres of diesel per hour. The same yacht at 12 knots burns over 120 litres per hour — not 50% more, but 3x as much. Understanding this exponential relationship between speed and fuel consumption is the single most important skill in voyage planning.
This guide covers how to calculate fuel consumption for any yacht voyage: the formulas, the real-world data, the common mistakes, and how to build a fuel estimate you can actually trust.
Why Fuel Calculation Matters
Fuel is the largest variable cost on any yacht voyage, typically representing 40-55% of the total budget. Get the fuel estimate wrong and the entire voyage budget fails. Underestimate and you are calling the owner for more funds mid-voyage. Overestimate and you are tying up working capital that could be used elsewhere.
The difference between a good fuel estimate and a bad one is typically 15-30%. On a €30,000 voyage, that is €4,500-9,000 in variance. Over a season of 20 voyages, poor fuel estimation can cost a fleet €100,000 or more in unplanned costs.
The Basic Formula
Fuel consumption for a voyage is calculated as:
Total Fuel = Burn Rate (L/hr) × Voyage Time (hrs)
Where voyage time is:
Voyage Time = Distance (nm) ÷ Speed (knots)
So the full formula becomes:
Total Fuel = Burn Rate × (Distance ÷ Speed)
The problem is that burn rate is not constant. It changes with speed, sea state, hull condition, and load. This is where most estimates go wrong.
Fuel Burn Rates by Vessel Size
Every vessel is different, but these ranges are reliable starting points for estimation:
| Vessel Length | Typical Cruise Speed | Burn Rate at Cruise | Litres per NM | Fuel Type |
|---|---|---|---|---|
| 24-30m (80-100ft) | 8-10 kn | 80-150 L/hr | 8-15 L/nm | Diesel/MGO |
| 30-40m (100-130ft) | 10-12 kn | 150-300 L/hr | 13-25 L/nm | MGO |
| 40-55m (130-180ft) | 10-14 kn | 300-500 L/hr | 22-40 L/nm | MGO |
| 55-70m (180-230ft) | 12-15 kn | 500-900 L/hr | 35-65 L/nm | MGO |
| 70m+ (230ft+) | 12-16 kn | 900-1,500+ L/hr | 60-100+ L/nm | MGO |
Note: These are approximate ranges for displacement and semi-displacement hulls. Planing hulls have very different consumption profiles. Always use your vessel's specific data when available.
The Speed-Consumption Curve
The relationship between speed and fuel burn is exponential, not linear. This is the most commonly misunderstood concept in voyage planning.
On a typical 50-metre displacement hull:
| Speed (knots) | Burn Rate (L/hr) | Litres per NM | Time for 337nm (Palma→Monaco) | Total Fuel (litres) |
|---|---|---|---|---|
| 10 | 180 | 18.0 | 33.7 hrs | 6,066 |
| 12 | 280 | 23.3 | 28.1 hrs | 6,555 |
| 14 | 420 | 30.0 | 24.1 hrs | 7,221 |
| 16 | 600 | 37.5 | 21.1 hrs | 7,906 |
| 18 | 850 | 47.2 | 18.7 hrs | 8,833 |
Slowing from 14 knots to 12 knots saves approximately 10% of fuel on this route. Slowing from 16 to 12 knots saves 17%. Over a full season, these savings compound dramatically.
The most fuel-efficient speed for a displacement hull is typically 70-75% of maximum speed. For most superyachts, this falls in the 10-12 knot range.
Real-World Factors That Change Consumption
The tables above assume calm water, clean hull, and moderate load. Real-world conditions add 10-40% to your base estimate:
| Factor | Impact on Consumption | Notes |
|---|---|---|
| Head seas (1-2m) | +10-25% | Higher in short, steep seas |
| Beam seas | +5-15% | Rolling increases resistance |
| Following seas | -5-15% | Can actually reduce consumption |
| Hull fouling (6+ months) | +10-30% | Single biggest hidden cost |
| Full fuel/water tanks | +5-10% | Heavier displacement |
| Generator load | +5-15% | A/C, watermaker, electronics |
| Current (adverse) | +5-20% | Depends on current strength |
Practical tip: Add 15-20% to your base fuel estimate as a sea state and conditions margin. This is not contingency — it is the difference between calm-water theory and real-world consumption.
Worked Example: Palma to Monaco
Let's calculate the fuel for a 50m superyacht sailing from Palma de Mallorca to Monaco (337nm) at 12 knots:
- Base burn rate: 280 L/hr at 12 knots (from vessel data)
- Voyage time: 337nm ÷ 12 kn = 28.1 hours
- Base fuel: 280 × 28.1 = 7,868 litres
- Sea state margin (15%): 7,868 × 1.15 = 9,048 litres
- Generator (10%): 9,048 × 1.10 = 9,953 litres
- Total estimate: ~10,000 litres
At current Mediterranean MGO prices (€1.10-1.20/L), the fuel cost for this voyage is approximately €11,000-12,000.
Common Mistakes
Using a single burn rate for all speeds. As the table above shows, burn rate changes dramatically with speed. Using your 10-knot burn rate to estimate a 14-knot voyage will underestimate fuel by 30-40%.
Ignoring sea state. Calm-water consumption is a theoretical minimum. Real voyages encounter waves, current, and wind. Add 15-20%.
Forgetting generator consumption. Main engines are not the only fuel consumers. Generators, especially with A/C load in the Mediterranean summer, add 5-15% to total consumption.
Using straight-line distance. The great-circle distance from Palma to Monaco is 337nm. The actual navigable route, accounting for traffic lanes and coastal passages, is closer to 350-360nm. Use the actual planned route distance, not the straight-line distance.
Not accounting for manoeuvring. Departing and arriving in port, holding for traffic, and course changes add 30-60 minutes of low-speed running per port call. For a voyage with 2 port calls, add 50-100 litres.
How SeaWise Calculates Fuel
SeaWise automates the fuel calculation process using vessel-specific burn curves, live bunker pricing, and actual route distances (not straight-line). When you enter your origin and destination, SeaWise:
- Calculates the great-circle distance using the Haversine formula
- Applies waypoint-based routing for shipping lanes and navigable water
- Estimates voyage time based on your planned speed
- Calculates fuel consumption using burn curves matched to your vessel size
- Applies current bunker prices from Mediterranean and global hubs
- Outputs total fuel, cost, time, and distance in one view
This replaces the 2-4 hours of manual calculation, spreadsheet building, and price checking that most captains and fleet managers go through for each voyage.
Frequently Asked Questions
How accurate do fuel estimates need to be?
For owner-funded voyages, aim for ±10% accuracy. For charter operations where fuel is charged to the charterer at cost, ±5% is the standard. The biggest source of error is not the calculation method — it is using outdated burn rate data. If your vessel has not been profiled recently, the single most valuable thing you can do is log actual consumption at different speeds over 3-5 voyages and build your own burn curve.
What is the most fuel-efficient speed for a superyacht?
For displacement hulls, the most efficient speed is typically 70-75% of maximum speed, which for most superyachts falls in the 10-12 knot range. Semi-displacement hulls have a wider efficient range. Planing hulls are a different calculation entirely — they are most efficient at or near planing speed. The key insight is that the "efficient" speed is not the slowest speed — it is the speed that minimises litres per nautical mile, not litres per hour.
How do I account for fuel price differences between ports?
Bunker fuel prices vary by 10-20% between Mediterranean ports. Gibraltar and Rotterdam are typically the most competitive. Monaco and Cannes are among the most expensive. If your route allows flexibility, planning your bunkering stop at a cheaper port can save €500-2,000 on a single fill. SeaWise displays live prices at major hubs so you can factor this into your voyage plan.
How much fuel reserve should I carry?
IMO guidelines require sufficient fuel to reach the nearest safe port plus a 10% reserve. For Mediterranean operations, most captains carry 15-20% above the calculated voyage requirement. For offshore or ocean passages, 25-30% reserve is standard. This is not optional — running out of fuel at sea is a safety emergency and a professional failure.
Can I use SeaWise for fleet-wide fuel tracking?
SeaWise currently supports individual vessel profiles and voyage calculations. Fleet-wide tracking and comparison features are on the roadmap. For now, fleet managers can create profiles for each vessel and compare voyage costs across the fleet using the export function.