Accurate speed information is vital for safe ship navigation and efficient voyage planning. At sea, officers commonly use two speed values: STW (Speed Through Water) and SOG (Speed Over Ground). Although both indicate ship speed, they measure movement in different ways. Therefore, understanding STW and SOG at sea is essential for navigators, engineers, and maritime students.
What Is STW (Speed Through Water)?
Speed Through Water (STW) shows how fast a ship moves relative to the surrounding water. It does not consider ocean currents or tides.
How STW Is Measured
Ships measure STW using:
- Doppler speed log
- Electromagnetic speed log
- Paddle wheel (impeller) log
These instruments sense water flow along the hull. As a result, STW reflects the actual performance of the ship’s propulsion system.
Why STW Is Important
STW helps in:
- Monitoring engine and propeller efficiency
- Calculating fuel consumption
- Assessing ship handling and maneuvering
- Detecting hull fouling or machinery issues
Because STW ignores current effects, engineers often rely on it for performance analysis.
What Is SOG (Speed Over Ground)?
Speed Over Ground (SOG) indicates how fast a ship moves relative to the Earth’s surface or seabed. Unlike STW, SOG includes the effect of currents, tides, and drift.
How SOG Is Measured
Ships calculate SOG using:
- GPS (Global Positioning System)
- ECDIS
- Radar tracking systems
These systems compare the ship’s position over time. Consequently, SOG shows the vessel’s real progress along its route.
Why SOG Is Important
SOG plays a key role in:
- Voyage planning and monitoring
- ETA (Estimated Time of Arrival) calculation
- Collision avoidance
- Traffic separation scheme navigation
Therefore, deck officers depend heavily on SOG for safe navigation.
Key Difference Between STW and SOG
| Feature | STW | SOG |
|---|---|---|
| Full Form | Speed Through Water | Speed Over Ground |
| Reference | Water | Earth / Seabed |
| Affected by Current | No | Yes |
| Measured Using | Speed log | GPS / ECDIS |
| Main Use | Ship performance | Navigation & ETA |
Effect of Current on STW and SOG
Ocean currents directly affect SOG but not STW.
- Following current: SOG becomes higher than STW
- Opposing current: SOG becomes lower than STW
- Cross current: Ship experiences drift, altering course over ground
For this reason, officers must compare both values continuously.
Practical Example
Assume:
- STW = 12 knots
- Following current = 2 knots
Then:
- SOG = 14 knots
However, if the current opposes the ship:
- SOG = 10 knots
This example clearly shows why relying on only one speed value can cause navigational errors.
Why Understanding STW and SOG Matters at Sea
Knowing the difference between STW and SOG helps in:
- Safe navigation
- Accurate passage planning
- Fuel efficiency management
- Avoiding delays
- Improving overall ship performance
Modern bridge systems display both speeds so officers can make better decisions.
STW and SOG in Modern Navigation Systems
Today, STW and SOG feed directly into:
- Autopilot systems
- ECDIS
- Radar and ARPA
- Dynamic positioning systems
Because of this integration, accurate speed data becomes even more critical.
Conclusion
STW and SOG at sea represent two different but equally important speed measurements. While STW shows how well the ship moves through water, SOG shows how fast it moves over the Earth. A skilled navigator always considers both values together. By doing so, ships can operate safely, efficiently, and on schedule.
📌 Disclaimer
This article is intended for educational and informational purposes only. While care has been taken to ensure accuracy, shipboard equipment, operating procedures, and navigational practices may vary by vessel and company. Readers should always follow their company’s Safety Management System (SMS), bridge procedures, and official maritime training materials. The author and website are not responsible for any actions taken based on the information provided.
