What Is Economical Speed in Shipping?
Economical Speed, sometimes called economic speed or eco speed, is an operating speed selected to balance fuel consumption, voyage time, machinery performance and commercial requirements.
It is not a universal fixed speed or a permanent specification of a ship. A vessel's most economical speed may change from one voyage to another because of weather, currents, loading condition, fuel prices, charter terms and the required arrival time.
In some contexts, Economical Speed means the speed that minimizes fuel consumed per nautical mile. In broader commercial planning, it may mean the speed that produces the lowest total voyage cost while still meeting contractual, operational and safety requirements. The intended definition should therefore be confirmed whenever an economical-speed figure appears in ship particulars or voyage instructions.
How Is Economical Speed Determined?
There is no single formula that can calculate Economical Speed for every vessel.
A basic voyage-planning calculation begins with the speed required to meet the requested arrival time:
Required average speed = Remaining sailing distance ÷ Available sailing time
This result is only a starting point. The operator must then consider whether the speed is safe, achievable and efficient under the expected conditions.
A practical economical-speed assessment may include:
Vessel type and hull design
Loading condition, draught and trim
Engine and propeller characteristics
Fuel-consumption curves
Weather, waves and ocean currents
Hull and propeller condition
Remaining distance and required arrival time
Port or berth availability
Fuel prices and time-related operating costs
Charter-party or schedule requirements
The selected speed may be adjusted throughout the voyage as forecasts, currents, traffic conditions or the requested arrival time change.
Why Does Speed Affect Fuel Consumption?
For conventional displacement ships, the power required to overcome resistance generally rises steeply as speed increases. The precise relationship depends on hull form, displacement, weather, water depth, propulsion efficiency and the part of the speed range being considered.
IMO's GreenVoyage2050 speed-management guidance uses a cubic speed–power relationship as a rule of thumb for displacement vessels. It should not be treated as an exact formula for every ship or operating condition.
Reducing speed normally lowers propulsion power and hourly fuel consumption. However, lower speed also increases voyage duration. The ship then operates its main engine, generators and auxiliary systems for longer.
For this reason, the lowest possible speed is not automatically the most economical. Very low engine loads may also be outside the machinery's most efficient or recommended operating range unless the engine and associated systems are configured for prolonged low-load operation.
Economical Speed vs Other Speed Terms
| Term | Meaning | Is it a live measurement? |
|---|---|---|
| Economical Speed | Selected speed intended to balance fuel, time and operating cost | No—operational target |
| Service Speed | Speed the vessel is designed or commercially expected to maintain under stated conditions | No—specification |
| Maximum Speed | Highest attainable speed under defined conditions | No—capability limit |
| Slow Steaming | Deliberate operation at substantially reduced speed over voyages or across a fleet | No—operating strategy |
| Safe Speed | Speed appropriate to current navigational circumstances | Determined continuously |
| Seed Over Ground | Actual speed relative to the Earth | Yes—navigation measurement |
Economical Speed vs Service Speed
Service speed is usually a design or commercial reference value associated with specified operating conditions. Economical Speed is selected according to current voyage priorities.
A ship may operate below service speed when the schedule permits fuel-saving operations. It may need to operate closer to service speed when recovering from delays or meeting a firm arrival window.
Neither value should be assumed to represent the vessel’s current speed without checking live navigation data.
Economical Speed vs Slow Steaming
Slow steaming is the practice of operating ships at reduced speeds for an extended period or across multiple voyages. Economical Speed is the optimized target for a particular set of conditions.
The two can overlap, but they are not identical. A slow-steaming policy may establish a lower operating range, while the economical speed within that range may still vary by voyage, weather and required arrival time.
Operators must also consider whether the main engine, turbochargers, exhaust systems and auxiliary machinery are suitable for prolonged low-load operation.
Economical Speed vs SOG
Speed Over Ground, or SOG, is the vessel's actual speed relative to the Earth. Economical Speed is a target or planning value.
Currents can cause the two to differ from a target expressed as Speed Through Water. A favorable current may increase SOG without requiring additional propulsive effort, while an adverse current may reduce SOG even if engine output remains unchanged.
SOG is particularly useful for monitoring voyage progress and estimating arrival. Speed Through Water is often more relevant when evaluating hull and propulsion performance.
Economical Speed vs Safe Speed
Economical Speed must never override safe navigation.
Under Rule 6 of the International Regulations for Preventing Collisions at Sea, a vessel must proceed at a safe speed appropriate to the prevailing circumstances. Restricted visibility, dense traffic, shallow water, severe weather or maneuvering limitations may require a speed below the planned economic target.
What Changes a Ship's Economical Speed?
Weather and Currents
Wind, waves and currents affect the speed and power required to make progress. Weather routing may recommend a longer route or a temporary speed adjustment if it reduces heavy-weather exposure, fuel consumption or schedule risk.
Loading Condition
Changes in cargo, ballast and displacement alter the vessel's draught, trim and hydrodynamic resistance. A speed that is economical in ballast may not be optimal when the ship is fully laden.
Hull and Propeller Condition
Hull fouling and propeller roughness can increase resistance and reduce propulsion efficiency. A vessel may require more power and fuel to maintain the same speed than it did with a clean hull and propeller.
Machinery Condition and Limits
Economical operation must remain within the main engine's approved load range and account for maintenance condition, fuel quality and manufacturer guidance. Machinery protection and reliable operation take priority over a theoretical fuel-saving target.
Arrival Requirements
A ship may waste fuel by sailing quickly toward a congested port and then waiting at anchor. Better information about berth and service availability can allow the vessel to reduce speed earlier and arrive when required.
Economical Speed and Just-in-Time Arrival
Just-in-Time Arrival, or JIT Arrival, is an operational approach in which the ship receives reliable information about when the berth, fairway and nautical services will be available. It can then optimize speed across the voyage instead of arriving early and waiting.
IMO guidance emphasizes that JIT Arrival is not simply an absolute speed limit or another term for slow steaming. It depends on coordination and timely information exchange among the vessel, operator, port, terminal, pilots and other service providers.
By aligning speed with a requested arrival time, JIT planning can support lower fuel consumption, reduced emissions and less anchorage waiting. IMO's JIT guidance describes speed optimization as a central part of this approach.
Practical Uses of Economical Speed
Voyage Planning
Operators use economical-speed calculations to prepare passage plans, bunker estimates and arrival schedules. Plans may be updated when the weather forecast, route or berth window changes.
Fuel and Emissions Management
Speed management can reduce fuel use and greenhouse-gas emissions when applied within safe and technically appropriate limits. It is one of several operational measures that may form part of a Ship Energy Efficiency Management Plan.
Chartering and Commercial Operations
Owners and charterers may evaluate speed together with fuel consumption, hire, cargo commitments and laycan or arrival requirements. The applicable contract determines which party controls speed and how performance is assessed.
Fleet Performance Monitoring
Fleet managers can compare planned speed with actual performance to identify schedule risk, excessive consumption or unexpected speed loss. Fair analysis must account for weather, currents, draught, hull condition and operational instructions.
FAQ
What does Economical Speed mean?
Economical Speed is an operating speed chosen to balance fuel consumption, voyage time, machinery performance and commercial requirements.
Is Economical Speed the same as slow steaming?
No. Slow steaming is a general strategy of sustained speed reduction. Economical Speed is the optimized target for a particular vessel, voyage and arrival requirement.
How is a ship's Economical Speed calculated?
It is determined using distance, available sailing time, fuel-performance data, loading condition, weather, currents, machinery limits and commercial costs. There is no universal formula for all ships.
Is SOG the same as Economical Speed?
No. SOG is the vessel's measured speed relative to the Earth. Economical Speed is an operational target. Currents and changing voyage conditions can cause actual SOG to differ from the target.
Does sailing more slowly always save fuel?
Lower speed usually reduces propulsion power and hourly fuel use, but extremely low speed increases voyage time and may result in inefficient engine operation. The overall optimum depends on the vessel and voyage.