CPA / TCPA Calculator Guide for Collision Avoidance

Why Bridge Teams Track CPA and TCPA

Real bridge situation: You are approaching a traffic lane at night with six ARPA targets on the radar. Two targets look close on screen, but only one is actually developing into a dangerous crossing situation.

• CPA tells you the minimum predicted passing distance.
• TCPA tells you how soon that minimum distance will happen.
• Together they help the Officer of the Watch decide which targets require immediate monitoring, trial manoeuvre checks, or an early course/speed alteration.

Good bridge teams never use CPA/TCPA as a replacement for COLREG judgement, but they do use it as a fast filter for collision risk.

Inputs Required for a Reliable CPA / TCPA Result

If the target vector is unstable, the CPA/TCPA output is unstable too. Before taking action, confirm that the target has been tracked long enough and that the reported course and speed are credible.

Worked Example: Crossing Situation

Input data:

• Own ship: COG 000°T, SOG 10 kn
• Target ship: COG 255°T, SOG 18 kn
• Target bearing: 040°T
• Target range: 7.0 NM

Calculated result:

• Relative speed: 22.7 kn
• TCPA: 18.2 minutes
• CPA: 1.20 NM

Interpretation:

• A CPA of 1.20 NM may be acceptable in open sea, but it may be too close in dense traffic, reduced visibility, or a narrow lane.
• A TCPA of 18 minutes means you have some time, but not enough to delay monitoring.
• The correct bridge response is not just “CPA looks okay”; it is to compare the encounter with COLREG responsibilities, traffic density, company limits, and available sea room.

What Counts as a Safe CPA?

There is no universal “safe CPA” value for every bridge. Masters, company procedures, traffic conditions, and visibility all change the acceptable limit.

Common CPA / TCPA Mistakes

• Watching TCPA only: A long TCPA can still end with a very small CPA.
• Trusting a single scan: Always confirm that target vectors are stable over time.
• Ignoring own manoeuvre effects: A small course change can improve CPA, but it can also create a new target conflict.
• Forgetting current and drift: In strong current, the observed target geometry changes faster than expected.
• Treating CPA/TCPA as a decision maker: They support judgement; they do not replace COLREG or bridge resource management.

Using CaptainCalc for a Fast CPA / TCPA Check

CaptainCalc makes the workflow simple when you need a quick independent check away from the radar menu tree:

• Enter own ship COG and SOG.
• Enter target COG, SOG, bearing, and range.
• Read the predicted CPA, TCPA, and relative speed instantly.
• Use the result to support your radar plotting and bridge team discussion.

Open the CPA / TCPA Calculator when you want a simple constant-course, constant-speed risk check without leaving the CaptainCalc workflow.

Related Glossary Terms

• CPA and TCPA - Predicted closest approach values between two vessels
• AIS - Automatic Identification System target data
• Bearing - Direction from own ship to a target or object

Related Articles

• Voyage Planning: Professional Maritime Route Calculator
• ETA Calculation with Set & Drift

Ender Soyuince built CaptainCalc to give bridge teams faster, clearer decision support for everyday navigation calculations, from CPA checks to voyage planning and stability work.