In navigation and ship handling, understanding the relationship between Course to Steer (CTS), Course Made Good (CMG), Speed Through Water (STW), and Speed Over Ground (SOG) is crucial. A common question among cadets and bridge officers is: “Why do CPA (Closest Point of Approach) and TCPA (Time to Closest Point of Approach) not change when measured in STW or SOG?” Let’s break this down in simple terms.
Understanding the Basics
- CPA (Closest Point of Approach)
- CPA is the minimum distance a ship will come to another vessel if both maintain their present course and speed.
- It’s a static measure, representing only the closest separation between two moving vessels.
- TCPA (Time to Closest Point of Approach)
- TCPA is the time remaining until the ship reaches CPA.
- It depends on the relative motion of the two vessels.
- STW vs SOG
- STW (Speed Through Water): The speed of the ship relative to the water. It ignores currents.
- SOG (Speed Over Ground): The actual speed of the ship relative to the Earth’s surface, including the effect of currents.
Why CPA and TCPA Are Not Affected by STW or SOG
The key point is that CPA and TCPA are based on relative motion between vessels, not absolute speed relative to water or ground.
- Relative Motion Concept
- CPA and TCPA are calculated using relative speed and relative course between two ships.
- This relative motion automatically accounts for the effects of currents.
- Whether you use STW or SOG for an individual vessel, the relative motion between the two vessels remains the same if the current affects both equally.
- Effect of Current
- A current may change the actual ground track of a ship (SOG), but the relative positions of two vessels moving in the same water are unchanged.
- This is why navigators can safely use CPA and TCPA for collision avoidance without worrying about whether STW or SOG is used.
- Practical Example
- Imagine Ship A and Ship B are on crossing courses in a river with a uniform current.
- Ship A has STW = 12 knots, Ship B STW = 10 knots.
- The current is 2 knots for both.
- The CPA distance and TCPA time calculated using relative motion remain the same whether you measure the speeds relative to water (STW) or ground (SOG).
Key Takeaways
- CPA and TCPA are based on relative motion between two vessels.
- Currents affect both ships equally in terms of ground speed, so CPA/TCPA remain unchanged.
- These measures are independent of STW or SOG; they are essential tools for collision avoidance.
- For navigators, this ensures simpler, reliable decision-making even in strong currents.
Conclusion
Understanding why CPA and TCPA do not change with STW or SOG highlights the importance of relative motion in navigation. By focusing on relative positions and courses, navigators can effectively avoid collisions without having to adjust for currents or individual vessel speeds. This principle forms a core part of safe navigation practice and bridge resource management.
Disclaimer: This blog is intended for educational purposes for nautical students and professionals. Always follow your ship’s company procedures and COLREG rules when making navigational decisions.
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