What Is Vessel Speed?

Vessel Speed is the rate at which a ship changes position over time. It is normally expressed in knots, with one knot equal to one nautical mile per hour.

However, a single speed figure can have different meanings depending on its reference point and purpose. A ship's speed relative to the Earth is called Speed Over Ground, while its speed relative to the surrounding water is called Speed Through Water. Design speed, service speed, average speed, economical speed and safe speed describe other technical or operational concepts.

Understanding which type of Vessel Speed is being reported is essential for navigation, ETA forecasting, fuel-performance analysis and fleet monitoring.


How Is Vessel Speed Measured?

Modern ships determine speed using several types of navigation equipment.

Global navigation satellite systems, such as GPS, are commonly used to calculate speed relative to the Earth. Ship logs, including Doppler and electromagnetic logs, may measure speed relative to the water.

Vessel Speed is usually displayed in knots:

1 knot = 1 nautical mile per hour = 1.852 kilometres per hour

The unit does not specify whether the measurement is over the ground or through the water. That distinction must be identified separately.


Main Types of Vessel Speed

Speed Over Ground

Speed Over Ground, abbreviated as SOG, is the ship’s speed relative to the Earth’s surface.

SOG shows how quickly the vessel is actually progressing across the chart. It includes the effect of ocean currents and tidal streams and is particularly useful for:

  • Monitoring voyage progress

  • Calculating arrival estimates

  • Comparing movement between positions

  • Displaying speed on vessel-tracking platforms

  • Determining whether a ship is moving or stationary

SOG is part of the dynamic information transmitted through the Automatic Identification System. The current ITU AIS technical standard specifies that AIS speed should be determined as Speed Over Ground.

Speed Through Water

Speed Through Water, abbreviated as STW, is the vessel’s speed relative to the water surrounding its hull.

STW indicates the flow of water past the ship and is useful when assessing propulsion and hull performance. It may be measured by ship-log equipment.

SOG and STW can differ because the water itself may be moving:

  • A favorable current can make SOG higher than STW.

  • An adverse current can make SOG lower than STW.

  • A cross-current affects both the vessel’s ground track and the relationship between the two speeds.

The difference is vector-based, so current direction as well as current speed must be considered. SOG should not always be calculated by simply adding or subtracting two speed figures.

Average Vessel Speed

Average speed is generally calculated as:

Average speed = Distance travelled ÷ Elapsed time

The result depends on how distance and time are defined. An average sea-passage speed may exclude time at anchor or in port, while an end-to-end voyage average may include waiting, maneuvering and stoppages.

Average speed calculated from historical tracking positions may also differ from the average of individual AIS speed reports because reporting intervals and data gaps affect the calculation.

Service Speed

Service speed is the speed a vessel is designed or commercially expected to maintain under specified operating conditions.

It is not necessarily the vessel’s maximum speed. Published service-speed figures may depend on assumptions about draught, engine power, sea conditions, hull cleanliness and loading. Users should therefore check the conditions attached to a quoted figure.

Maximum Speed

Maximum speed is the highest speed a vessel can attain under defined conditions. It may be affected by propulsion power, displacement, hull condition, weather and machinery limitations.

A vessel is not normally expected to operate continuously at maximum speed. Maximum speed should not be treated as a prediction of its usual AIS-reported speed.

Economical Speed and Slow Steaming

Economical or eco speed is an operating speed selected to balance fuel consumption, schedule requirements and machinery performance.

Slow steaming refers to operating ships at substantially reduced speeds to lower fuel use over voyages or across a fleet. The IMO's GreenVoyage2050 speed-management guidance identifies speed reduction as an important operational efficiency measure.

The most efficient speed is not universal. It depends on vessel design, engine characteristics, weather, currents, cargo commitments, required arrival time and the condition of the hull and propeller.

Safe Speed

Safe speed is a navigational concept rather than a fixed number.

Rule 6 of the International Regulations for Preventing Collisions at Sea requires vessels to proceed at a speed that allows proper action to avoid collision and enables them to stop within an appropriate distance.

Safe-speed decisions take account of factors such as:

  • Visibility

  • Traffic density

  • Vessel maneuverability

  • Wind, sea and current

  • Navigational hazards

  • Draught in relation to available water depth

  • Radar limitations, where radar is in use

A speed that is safe in open water may be unsafe in restricted visibility, congested traffic or a narrow channel.

SOG vs STW: What Is the Difference?

Measurement  Reference  Includes current effect?  Common use  
SOG  Earth's surface  Yes  Tracking, navigation and ETA  
STW  Surrounding water  No  Propulsion and performance analysis  
Average speed  Defined distance and time  Depends on data used  Voyage and fleet analysis  
Service speed  Design or commercial condition  Not a live measurement  Vessel specifications  
Safe speed  Prevailing navigational conditions  Operational judgment  Collision avoidance  


SOG is generally the most relevant speed for determining when a vessel will arrive at a geographic destination. STW is more useful when evaluating the interaction between the hull, propeller and surrounding water.

Neither measurement should automatically be described as the ship's "true speed" without specifying the intended reference.


What Affects Vessel Speed?

Weather, Waves and Currents

Head winds and adverse seas may increase resistance or require a speed reduction. Following conditions may sometimes assist progress, while strong currents can significantly affect SOG without producing the same change in STW.

Loading, Draught and Trim

A vessel's displacement, draught and trim influence hull resistance and propulsion performance. The effect varies by ship type and hull design, so the same engine setting may not produce the same speed in every loading condition.

Hull and Propeller Condition

Marine growth, surface roughness and propeller damage can increase resistance or reduce propulsion efficiency. Regular inspection, cleaning and maintenance support more consistent vessel performance.

Shallow Water and Traffic Restrictions

Limited water depth can alter a vessel's hydrodynamic behavior and may require reduced speed because of squat, bank effects or local regulations. Traffic-separation schemes, port limits and Vessel Traffic Service instructions can also constrain speed.

Operational Decisions

Masters and operators adjust speed for safety, weather routing, fuel management, berth availability and schedule changes. A vessel capable of sailing faster may deliberately reduce speed to avoid arriving before a berth is available.


Practical Uses of Vessel Speed Data

ETA and Port Planning

SOG, position, route and remaining distance help estimate the vessel’s ETA. Ports and terminals use this information to plan berths, pilots, tug services and cargo operations.

Fleet and Voyage Monitoring

Fleet managers compare current speed with expected voyage progress to identify delays, route changes, drifting, anchoring or unusual operating patterns.

Speed should be interpreted together with Course Over Ground, heading, navigation status, destination and weather conditions.

Fuel and Emissions Management

Speed selection has a major influence on propulsion demand and fuel consumption, although the precise relationship varies by vessel and operating condition. Speed optimization can help reduce fuel use while meeting safe-navigation and arrival requirements.

Maritime Safety

Speed data supports traffic monitoring and collision-risk assessment. It does not replace radar, visual lookout, COLREG compliance or the professional judgment of the bridge team.


FAQ

What is Vessel Speed measured in?

Vessel Speed is normally measured in knots. One knot equals one nautical mile per hour, or 1.852 kilometres per hour.

What is the difference between SOG and STW?

SOG is speed relative to the Earth, while STW is speed relative to the surrounding water. Currents and tidal streams can cause them to differ.

Which speed is transmitted by AIS?

AIS position reports transmit Speed Over Ground. STW is normally obtained from ship-log equipment and is not a standard AIS position-report field.

Why does a vessel's speed change during a voyage?

Speed may change because of weather, currents, traffic, shallow water, loading condition, machinery limits, fuel strategy or revised arrival requirements.

Does a higher Vessel Speed always mean an earlier arrival?

Not necessarily. Route changes, adverse weather, port congestion, waiting time and speed restrictions can all affect the final arrival time.