A Practical Guide for Deck Officers and Recreational Sailors

Whether you’re preparing for your MCA Oral Exam, building confidence as a newly qualified OOW, or simply improving your seamanship as a recreational sailor, one skill is absolutely fundamental:

📍 Accurately assessing the risk of collision.

It sits at the heart of Rule 7 (Risk of Collision) and underpins almost every anti-collision decision you make at sea. Misjudging the situation — even slightly — can result in late action, ineffective manoeuvring, or a complete breakdown in the application of the Collision Regulations.

This guide explains exactly how to assess a risk of collision, using the principles from Rule 7 alongside practical methods used by professional mariners every day.

1. Start With the Most Important Question: Is There a Risk of Collision?

Rule 7 states very clearly:

“If there is any doubt, such risk shall be deemed to exist.”

This is the golden rule.
If you are unsure — you assume there is a risk and act accordingly.

But how do we determine it?

2. The Three Core Indicators of Collision Risk

Professional mariners rely on a combination of visual compass bearings, radar relative motion, and electronic aids to make a sound judgement.

A. Constant or Nearly Constant Compass Bearing

A vessel approaching on a steady compass bearing is the strongest indicator of collision risk.

• Take three successive compass bearings.

• If the bearing is not changing, the risk of collision is high — regardless of distance.

• Small bearing changes at long range can still result in danger at short range.

This is why Rule 7 warns:

“such risk shall exist when the compass bearing of an approaching vessel does not appreciably change.”

For sailors without an ARPA, this remains the single most reliable method. Bearings can be taken with either gyro or magnetic compass – the principle is the same.

 B. Decreasing Range

Even if the bearing changes slightly, if the vessel is closing in range, a dangerous situation may develop. A vessel who’s range is dcreasing is termed ‘an approaching vessel’ and therefore falls into the Rule 7 principle -“such risk shall exist when the compass bearing of an approaching vessel does not appreciably change.”

Signs include:

• Bow wave angle increasing
• Target appearing larger
• Radar range closing
• AIS range decreasing

Range should be monitored together with bearing.

 C. CPA / TCPA (Closest Point of Approach)

ARPA and AIS provide numerical risk indicators:

• CPA: The closest distance the ships will pass

• TCPA: The time until CPA occurs

As a very rough rule of thumb:

• CPA < 1 nm = high risk at any time

• CPA < 2 nm = potential risk depending on vessel size and sea area

A low CPA with short TCPA is indicative that a dangerous sitation maybe developing and action needs to be taken without further delay in accordance with the rules.

Note: Small errors in speed/course inputs can distort CPA, so don’t rely on electronics alone. Rule 7 warns against this explicitly. AIS cannot be used soley for collision avoidance as it used data from another vessel, the qulaity of which we cannot reley upon. AIS is great for situational awareness but is unaccounted for in the COLREGS.

3. Relative Motion: The Skill MCA Examiners Look For on Radar Plots

Understanding how your ship and the other vessel move relative to each other is a core oral exam topic.

Signs of risk on the radar relative motion vector or trail include:

📍 The contact is moving directly towards your radars origin or central common reference point
📍 The vector / trail remains parallel to a constant bearing line
📍 The relative track is heading inside your minimum CPA variable range marker.

This analysis helps you predict whether:

• You will cross ahead

• The other vessel will cross ahead

• A close-quarters or risk of collision situation is developing

4. The basic “Scenario Method” – five steps to avoiding collision:

Here’s a practical 5-step method that every officer should adopt:

1️⃣ Step 1 – Identify the Vessel and Situation

Head-on, crossing, overtaking, restricted visibility, responsibility (R18) scenario.

2️⃣ Step 2 – Take Three Bearings and Compare

3 – 6 minutes apart, unless very close, in which case constantly monitor the bearing.

3️⃣ Step 3 – Check CPA and TCPA

Use ARPA or AIS but confirm visually. Remember AIS cannot be used as a stand alone method.

4️⃣ Step 4 – Apply the Correct Rule according to the actual situation being observed,

Assess both your obligation and the other vessel’s duty.

5️⃣ Step 5 – Act Early and Substantially

Rule 8: any alteration should be early, bold, and large enough to be apparent to other vessels.

This method forms the backbone of how collision avoidance is taught professionally.

5. Common Collision Assessment Errors

These appear frequently in MCA Orals:

🟥 Assuming a vessel is safe because it ‘looks like’ it will pass ahead — the bearing may still be steady
🟥 Relying solely on AIS / ARPA — electronic data can lag or misrepresent.
🟥 Altering course too late – early action is always best
🟥 Making small, ineffective course changes – rule 8 warns again making a series of small alterations of course and/ or speed. 
🟥 Not monitoring the effectiveness of our action until past and clear.

These factors feature in real-world collisions and MAIB reports.

6. A Realistic Scenario (Used in Orals, Yachtmaster, or Sail Training)

A container ship is approaching from your starboard side at night, bearing 045° crossing starboard to port.

• First bearing: 045°

• Second bearing: 045°

• Third bearing: 044°

Range is decreasing rapidly.
CPA is 0.4 nm.
TCPA is 14 minutes.

This is a clear crossing situation (Rule 15).
You are give-way vessel (Rule 15 and 16).
Action should be early, substantial, and avoid passing ahead if the circumstances allow (Rule8)
Follow with continuous monitoring of the effectiveness of your action including ensuring the compass bearing is now appreciably changing and the CPA has opened up to a safe distance.

This type of scenario is exactly what appears in OOW, Chief Mate and Master’s exams — and in real life.

7. Summary: How to Quickly Assess Risk of Collision

A professional mariner should instinctively follow this checklist:

✔ Is the bearing steady?
✔ Is the range decreasing?
✔ What is CPA / TCPA?
✔ What does radar relative motion show?
✔ What are my obligations under the rules?
✔ Have I acted early, decisively, and according to the Rules?
✔ Have I monitored the result?

If in doubt → risk continues to exist.

Further Help 

If you want structured training with scenarios, graphics, animations and real-world examples, explore:

👉 COLREGs Super Module – Complete Rule-by-Rule Video and Scenario Training
👉 COLREGs Quiz App (500+ Exam-Style Questions, Study Aids & Analytics)

These resources are designed to support:

• MCA Oral Exam candidates

• All Tickets

• RYA Yachtmaster

• Recreational sailors improving safety

 Please contact us info@captainbarrysadler.com