Vessel Tracking and AIS Intelligence-
What Vessel Speed, Course, Destination, and Draft Can Reveal About a Voyage.
A vessel’s position is only one part of its story. To understand what a commercial ship may be doing, maritime analysts also examine its speed, course, declared destination, and draft—more commonly spelled draught in international shipping.
Individually, each data field provides limited information. When combined with vessel type, historical movements, port records, weather, and route data, they can reveal important details about a voyage.
They may indicate whether a ship is underway, delayed, changing routes, approaching port, waiting at anchor, or potentially carrying a heavier load. They can also help platforms such as VesselPing detect inconsistencies requiring closer examination.
However, AIS information does not always tell the complete truth. Some fields are produced automatically by shipboard sensors, while others depend on manual crew entry. The distinction is critical.
Four important voyage indicators
| AIS field | What it primarily indicates |
|---|
| Speed over ground | How fast the vessel is moving relative to the Earth |
| Course over ground | The direction in which the vessel is actually travelling |
| Destination | The port or location reportedly entered by the crew |
| Draught | The vessel’s reported vertical depth below the waterline |
Together, these fields can help reconstruct a ship’s operational situation and likely intentions.
What vessel speed can reveal
AIS normally reports speed over ground, often abbreviated as SOG. This measures how quickly the vessel is moving relative to the Earth’s surface.
It is different from speed through the water because ocean currents can assist or resist a ship’s movement.
Normal passage speed
When a commercial ship maintains a relatively consistent speed along a recognized route, it is probably making an ordinary sea passage.
Typical operating speeds vary according to:
VesselPing should compare a ship’s current speed with its own history and similar vessels rather than applying one universal definition of “normal.”
Reduced speed
A gradual reduction in speed may indicate:
Commercial ships may also deliberately practise slow steaming to reduce fuel consumption and emissions.
Very low speed or no movement
A vessel reporting little or no speed may be:
At anchor
Berthed
Drifting
Waiting offshore
Conducting repairs
Participating in a ship-to-ship operation
Performing specialized work
Experiencing an emergency
Position history provides the necessary context. A stationary ship located at a recognized anchorage is less unusual than one remaining motionless in an isolated offshore location.
Sudden speed changes
Rapid acceleration or deceleration may deserve attention, particularly when accompanied by a route change, AIS gap, or close encounter with another ship.
It can indicate an operational event, but it can also result from a faulty sensor or incorrect AIS report. VesselPing would need to validate the change across several consecutive positions.
What course can reveal
AIS normally reports course over ground, abbreviated as COG. This is the direction in which the vessel is actually moving across the Earth.
Course over ground should not be confused with heading.
Wind, waves, currents, and manoeuvring can cause these values to differ.
Following an established route
A stable course aligned with a recognized shipping corridor generally indicates ordinary passage.
VesselPing could compare the vessel’s current track with:
Expected route to its destination
Previous voyages
Official traffic lanes
Canal and strait approaches
Navigational hazards
Weather-routing recommendations
A course change
A change in course may indicate:
Route correction
Collision avoidance
Weather avoidance
Port approach
Traffic-separation compliance
Diversion to a different port
Search-and-rescue activity
Military or security restrictions
Mechanical or navigational problems
A single turn is rarely suspicious. The location, size, timing, and duration of the deviation matter.
Course inconsistent with destination
If a ship declares Rotterdam as its destination but consistently travels in the opposite direction, several explanations are possible:
The destination field was not updated.
The voyage changed after departure.
The ship is calling at an intermediate port.
The destination was entered incorrectly.
The transmitted information may be misleading.
VesselPing could flag the inconsistency without assuming deliberate deception.
What the declared destination can reveal
The AIS destination field provides an indication of where the ship says it is going. It can help cargo owners, ports, and logistics companies organize expected arrivals.
The field can support:
However, the declared destination is normally entered manually. It may contain abbreviations, port codes, spelling errors, old information, or general descriptions such as “FOR ORDERS.”
A destination might be recorded in different forms:
SINGAPORE
SG SIN
SGSIN
SIN
SINGAPORE OPL
A maritime-intelligence platform must normalize these variations before analyzing them.
Destination changes
A destination change may reflect:
New commercial instructions
Charter-party decisions
Cargo sale while at sea
Port congestion
Weather disruption
Political instability
Sanctions or regulatory concerns
Mechanical problems
Medical or safety emergencies
Frequent or unexplained changes may be worth monitoring, especially if the vessel’s route and destination repeatedly conflict.
What draught can reveal
A vessel’s draught is the vertical distance between the waterline and the lowest part of its hull. In general, a heavily loaded ship sits deeper in the water and has a greater draught than the same ship when lightly loaded.
Reported draught can therefore provide clues about loading condition.
A possible loaded voyage
A significant increase in draught after a port visit may suggest that the vessel took on cargo.
For example:
A tanker may have loaded oil or petroleum products.
A bulk carrier may have loaded coal, grain, or ore.
A cargo vessel may be carrying a heavier shipment.
Draught alone usually cannot confirm exactly what cargo was loaded. Vessel type, terminal specialization, port activity, customs information, and commercial data are needed for a stronger conclusion.
A possible discharge event
A reduction in reported draught after visiting a terminal may indicate that cargo was discharged.
Analysts can compare:
Draught before arrival
Time spent at the terminal
Draught after departure
Vessel type and port facilities
Subsequent route
This can help VesselPing identify likely loading and unloading events.
Partial loading and ballast conditions
A vessel is not simply “full” or “empty.” It may be partially loaded, carrying ballast water, redistributing cargo, or adjusting its condition for safety and stability.
Environmental factors can also influence observed draught, including:
Water density
Fuel consumption
Freshwater and supplies
Ballast operations
Waves and vessel motion
Moreover, the AIS draught field is usually manually entered. It may be outdated, rounded, incorrect, or deliberately manipulated. It should be treated as an indicator rather than an independently verified cargo measurement.
How the four indicators work together
The greatest intelligence comes from combining the fields.
flowchart TD
A["AIS voyage reports"] --> B["Speed analysis"]
A --> C["Course analysis"]
A --> D["Destination check"]
A --> E["Draught comparison"]
B --> F["Voyage interpretation"]
C --> F
D --> F
E --> F
Scenario 1: A normal loaded voyage
A bulk carrier departs an iron-ore terminal with:
Together, these indicators support the inference that the vessel loaded cargo and is proceeding normally.
Scenario 2: Port congestion
A container ship approaches its declared destination but then:
This pattern likely indicates waiting or congestion rather than a route failure.
Scenario 3: Voyage diversion
A tanker changes course away from its declared destination, increases speed, and begins moving toward a different region.
Possible explanations include changed commercial orders, weather avoidance, regulatory concerns, or a new destination not yet entered into AIS.
Scenario 4: Possible offshore transfer
Two compatible vessels meet in open water and:
Reduce speed simultaneously
Remain close for several hours
Show draught changes before and after the encounter
Resume travel in different directions
This pattern may indicate a ship-to-ship transfer. It could be legitimate, but the location, authorizations, ownership, and reporting behaviour should be reviewed.
Scenario 5: Possible data manipulation
A vessel reports:
A destination inconsistent with its course
A draught exceeding plausible physical limits
Sudden impossible speed changes
Conflicting identity information
The combined inconsistencies may indicate incorrect configuration, sensor problems, human error, or deliberate AIS manipulation.
Turning voyage data into VesselPing intelligence
VesselPing could analyze these fields through a voyage-intelligence engine that:
Learns normal speed ranges for each vessel
Compares current and historical routes
Standardizes destination names and port codes
Calculates whether the destination matches the course
Detects major draught changes around port calls
Identifies prolonged stops and abnormal speed profiles
Predicts arrival times
Assigns confidence levels to voyage interpretations
Alerts users to important inconsistencies
An alert should explain its reasoning. For example:
Possible voyage diversion: The vessel is 120 nautical miles outside its expected corridor, its course no longer aligns with the declared destination, and its destination field has not been updated for 36 hours.
This is more useful than a generic “suspicious vessel” warning.
Improving arrival predictions
Speed, course, and destination are central to estimated time of arrival calculations.
A VesselPing prediction model could consider:
Current speed and course
Remaining route distance
Recent speed changes
Historical performance
Weather and currents
Port congestion
Canal waiting times
Vessel category
Previous voyage duration
If a vessel reduces speed substantially, the arrival estimate should change. If it is sailing away from the destination, the platform should reduce its confidence in the declared ETA.
Historical data can help determine whether a speed reduction is temporary or typical for that part of the route.
Important data limitations
AIS information must be interpreted carefully.
Speed, course, and position are usually produced automatically, but they can still be affected by sensor faults, equipment problems, or manipulation. Destination and draught generally require manual entry and may be outdated or inaccurate.
VesselPing should therefore show:
Time of the latest report
Source of the information
Whether the value is automatic or manually entered
Historical changes
Data-quality warnings
Confidence level
Supporting evidence for any conclusion
Where important legal, financial, or security decisions are involved, AIS should be checked against port records, vessel registries, radar, satellite imagery, weather information, and cargo documentation.
Reading the story behind the voyage
Speed reveals how a vessel is moving. Course shows where that movement is taking it. Destination communicates its declared intention. Draught provides clues about its loading condition.
None of these fields provides a complete answer alone. Together, however, they can reveal whether a voyage appears normal, delayed, diverted, lightly loaded, potentially carrying cargo, or inconsistent with its declared plan.
That is how VesselPing can progress beyond plotting ships on a map. It can connect separate data points into a coherent operational story—while clearly distinguishing facts from estimates and informed inferences.
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