Rules 4-8

COLREGS Rules 4-8: The Action Rules

Explained for Gen Z Merchant Seafarers

Part 1: Rule 4 – Application (1 minute)

The Simple Truth:

Rule 4 says: “The rules in this section apply in ANY condition of visibility.”

Why This Matters:

You might think: “If it’s foggy or night or raining, maybe different rules apply?”

WRONG. Rules 4-10 apply EVERYWHERE, ALWAYS, NO MATTER WHAT.

Clear daylight? Rules 4-10 apply.

Thick fog? Rules 4-10 apply.

Night time? Rules 4-10 apply.

Sandstorm? Rules 4-10 apply.

Heavy rain? Rules 4-10 apply.

The keyword: “ANY condition of visibility” means no excuses.

What Rule 4 Applies To:

Rule 4 is the umbrella rule covering:

Rule 5: Lookout (maintaining awareness)

Rule 6: Safe Speed (moving at appropriate speed)

Rule 7: Risk of Collision (detecting danger)

Rule 8: Action to Avoid Collision (what to do about it)

Rule 9: Narrow Channels

Rule 10: Traffic Separation Schemes

Common Misunderstanding ⚠

❌ WRONG: “In restricted visibility, we don’t need to maintain safe speed because we can barely see.”

✅ RIGHT: Rule 4 says in restricted visibility, you MUST maintain safe speed appropriate to the visibility. This usually means SLOWER speed, not normal speed.

Real Example – Merchant Ships:

Scenario: Your bulk carrier is in thick fog in the North Sea, moving at 12 knots because you have radar.

The mistake: “We have radar, so we don’t need to slow down.”

The truth: Rule 4 + Rule 6 mean you must slow down because:

Radar has limitations (doesn’t detect small objects, can be affected by weather)
You might miss visual signals if you encounter a small fishing boat or sailing vessel
Your response time in fog needs to be longer

Correct action: Reduce speed to 6-8 knots, maintain proper lookout by radar and compass bearing checks, sound fog signals (Rule 35).

Part 2: Rule 5 – Lookout (2 minutes)

The Rule in Simple English:

“Every vessel must maintain a proper lookout at all times using sight, hearing, and all available means.”

Why Lookout Matters:

Lookout is NOT just looking out the window. It’s systematic surveillance to:

Detect other vessels early

Assess the situation (are they getting closer?)

Determine the risk of collision

Take action in time to avoid danger

Time allocation: Lookout should occupy 70-80% of the OOW’s (Officer of the Watch) time.

Methods of Lookout:

Visual Observation

Look around the horizon systematically

Check all quadrants: ahead, port, starboard, astern

During night: use darkness-adapted eyes (avoid red lights on bridge)

During the day: reduce glare, don’t stare into the sun

Hearing

Listen for fog signals from other vessels

Listen for distress signals

Listen for sound signals (ship’s whistles, bells)

This is CRITICAL in fog or restricted visibility

Radar

Use long-range scanning (12 NM setting) to spot distant traffic

Use short-range scanning (3-6 NM) to track nearby vessels

Use ARPA (Automatic Radar Plotting Aid) to calculate the closest point of approach

CRITICAL: Do NOT rely on radar alone—it has limitations

AIS (Automatic Identification System)

Shows other vessels’ positions, speeds, courses, vessel names etc

WARNING: Not all vessels carry AIS; small fishing boats often don’t

AIS can fail or be switched off

VHF Radio

Listen for other vessels’ transmissions

Can indicate traffic density and vessel movements

Helps in congested areas

The Golden Rule of Lookout:

“Use EVERYTHING available, but don’t rely on any SINGLE source.”

If AIS shows a vessel 10 NM away but radar doesn’t see it = assume the AIS target is real. If radar shows a target but AIS doesn’t = assume the radar target is real.

Common Student Mistakes ⚠

❌ WRONG: “The lookout can multitask—check emails, plan the voyage, and watch traffic.”

✅ RIGHT: Rule 5 demands 70-80% of the OOW’s attention for lookout. Everything else is secondary.

❌ WRONG: “Radar alone provides a proper lookout in fog.”

✅ RIGHT: Radar is part of lookout, but not the whole thing. Visual observation, hearing, AIS, and compass bearing checks are equally important.

❌ WRONG: “If AIS shows clear traffic 15 NM away, we don’t need to worry.”

✅ RIGHT: AIS can fail, be switched off, or have outdated information. Verify with visual observation and radar.

Real Merchant Ship Example:

Case: General Cargo Ship & Bulk Carrier in North Sea (2019)

A general cargo ship was navigating on a clear evening. The lookout was NOT maintaining proper lookout—the OOW was distracted with administration. The ARPA on the cargo ship was not being used to track traffic. Meanwhile, a bulk carrier was approaching.

What happened:

The cargo ship did NOT notice the bulk carrier

The range dropped to 200 meters before visual contact

The collision was unavoidable

Hull damage to both vessels

Why Rule 5 failed:

OOW was not maintaining 70-80% lookout time

ARPA was not being used properly (continuous systematic observation)

No compass bearing checks were being taken

Lesson: Lookout is YOUR JOB as OOW. It’s not optional. The safety of the ship, cargo, and crew depends on it.

Part 3: Rule 6 – Safe Speed (2 minutes)

The Core Rule:

“Every vessel must proceed at a speed at which it can take proper action to avoid collision and stop within the appropriate distance.”

Translation:

Safe speed is NOT:

The maximum speed the engine can produce

The speed you’re used to sailing at

The speed the company recommends

Safe speed IS:

The speed at which you can respond to danger

The speed at which you can stop in time to avoid a collision

The speed appropriate to the CONDITIONS

Factors to Consider for Safe Speed:

All vessels must consider:

State of visibility (can you see danger coming?)
Traffic density (how many other vessels are around?)
Manoeuvrability (how quickly can your ship turn and stop?)
Proximity of hazards (reefs, rocks, shallow water, fishing vessels)
Time of day (night vs. daytime)
Wind, sea state, currents (rough seas reduce manoeuvrability)
Water depth ship’s draft (shallow water affects rudder control)

Vessels with radar must also consider:

Radar limitations (what if a small object isn’t detected?)
Weather effects on radar (heavy rain, snow can degrade radar)
Risk of smaller undetected objects (wooden boats, fishing nets, debris)

The Speed Calculation:

Safe speed depends on stopping distance.

A bulk carrier at 12 knots in open water with good visibility might need 3-4 NM to stop. The same bulk carrier at 12 knots in a busy harbour approach might need to stop in 1 NM. Therefore, safe speed in harbour = 4-5 knots, NOT 12 knots.

Critical Concept: “Take All Way Off”

Sometimes the safest action is STOP THE SHIP COMPLETELY.

Rule 6(e) says: “If necessary to avoid collision or allow more time to assess the situation, a vessel shall slacken her speed or take all way off by stopping or reversing her means of propulsion.”

Example: Your container ship detects a small fishing boat on radar at 8 NM, but the bearing is steady (risk of collision). It’s night, and visual contact is not yet made. The safest action might be to:

Reduce speed from 18 knots to 10 knots
Manoeuvre to port or starboard by 20 degrees
If risk persists, STOP the ship and wait for the fishing boat to pass

Common Student Mistakes ⚠

❌ WRONG: “Safe speed means we maintain engine RPM regardless of conditions.”

✅ RIGHT: Safe speed changes constantly based on traffic, visibility, and hazards. Engine speed must be adjusted (engine room must stand by).

❌ WRONG: “If radar shows clear water ahead, we can proceed at full speed.”

✅ RIGHT: Radar has blind spots. Small objects, fishing nets, debris, and semi-submerged containers can be missed.

❌ WRONG: “In the open ocean with good visibility, full speed is safe.”

✅ RIGHT: Even in the open ocean, you must consider: Are there fishing vessels? Is your stopping distance sufficient? Can you respond to a cargo container floating at the surface?

Real Example – Merchant Ships:

Scenario: Container Ship in Singapore Strait

Your container ship (40,000 TEU) enters the Singapore Strait at night. Visibility is poor due to haze. Traffic density is EXTREMELY HIGH (50+ vessels in the strait at any time).

Safe speed analysis:

Normal ocean speed: 20 knots

Singapore Strait safe speed: 8-10 knots

Why? Because:

Poor visibility requires a slower response time
Extremely high traffic density (many vessels manoeuvring)
Your stopping distance at 20 knots would be 4-5 NM—unacceptable in such congestion
Fishing vessels and small craft operate in the strait

Failure case: A vessel maintains 18 knots in the Singapore Strait at night. A fishing boat appears on radar 3 NM ahead. There is NOT enough distance or time to stop safely. Collision occurs.

Part 4: Rule 7 – Risk of Collision (1.5 minutes)

The Rule:

“Every vessel must use all available means to determine if a risk of collision exists. If there is ANY doubt, such risk shall be deemed to exist.”

Translation:

This rule says: “Don’t assume safety; assume danger and prove otherwise.”

How to Determine Risk of Collision:

Step 1: Use ALL Available Means

Visual observation

Radar with ARPA

AIS

Compass bearing checks

Listening on VHF

Step 2: The Steady Bearing Rule

If the compass bearing of an approaching vessel does NOT appreciably change over several observations, RISK OF COLLISION EXISTS.

Example:

20:00 — Vessel bearing 045°, range 10 NM

20:10 — Vessel bearing 044°, range 8 NM

20:20 — Vessel bearing 044°, range 6 NM

The bearing stayed steady (044-045°). Range is decreasing. RISK OF COLLISION EXISTS. Action must be taken.

Step 3: Radar Plotting (ARPA)

Use ARPA to track the Closest Point of Approach (CPA):

If CPA is less than 1.0 NM → RISK EXISTS

If CPA is 0.5 NM or less → IMMEDIATE DANGER

Important: Do not rely on radar alone if CPA shows 2 NM. Radar can miss small objects. Assume risk exists and verify with visual/AIS.

Step 4: The Doubt Rule

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

Translation: ASSUME DANGER.

Don’t think: “Probably okay, the bearing might be changing slowly.”

Think: “I cannot guarantee safety, so I assume risk exists and take action.”

Common Student Mistakes ⚠

❌ WRONG: “If I only get one radar bearing, I can determine if risk exists.”

✅ RIGHT: Rule 7(c) says: “Assumptions shall NOT be made on the basis of scanty information.” You need MULTIPLE bearing checks over TIME.

❌ WRONG: “If AIS shows a vessel will pass 0.8 NM away, I don’t need to take action.”

✅ RIGHT: AIS data can be outdated, and manoeuvres are unpredictable. Take action to increase the passing distance.

❌ WRONG: “Risk of collision only exists if the vessel is moving toward us.”

✅ RIGHT: Rule 7(d)(ii) says: “Risk may exist even with appreciable bearing change, especially when approaching very large vessels, tows, or at close range.” A slow bearing change with a super-tanker at 5 NM = risk exists.

Real Example:

Scenario: Your bulk carrier, night time, fog

A vessel appears on radar: bearing 030°, range 12 NM.

You take three bearing observations (10 minutes apart): 030°, 031°, 030°. Range drops: 12 NM → 10 NM → 8 NM.

Analysis:

Bearing is steady (not changing appreciably) ✓

Range is decreasing ✓

Time to closest point of approach: ~4-5 minutes ✓

Conclusion: RISK OF COLLISION EXISTS. You must take action NOW.

Part 5: Rule 8 – Action to Avoid Collision (2 minutes)

The Rule:

“Action taken to avoid collision shall be POSITIVE, made in AMPLE TIME, and with good seamanship.”

What “Positive” Means:

Positive = SUBSTANTIAL ALTERATION

❌ NOT positive: Alter course from 050° to 053° (only 3 degrees)

✓ Positive: Alter course from 050° to 035° (15 degrees) or Reduce speed from 18 knots to 10 knots

What “Ample Time” Means:

Ample time = EARLY ACTION

When traffic is 10+ NM away: PERFECT time to alter course/speed

When traffic is 5 NM away: Still a good time, but less room for error

When traffic is 2 NM away: LATE, more dramatic action needed

When traffic is <1 NM: EMERGENCY, immediate action required

Golden rule: A substantial alteration made early is better than a very large alteration made late.

Types of Action:

Action Type 1: Alter Course

Most effective when sufficient sea room exists

Make a LARGE alteration (15-30 degrees minimum)

This makes your action OBVIOUS to other vessels watching radar or visually

Course alteration is better than speed change (because speed changes are harder to observe)

Action Type 2: Reduce Speed

Useful in confined waters (harbours, narrow channels)

Shows the other vessel you’re taking action

Gives MORE TIME to assess the situation

Action Type 3: Combination of Both

Sometimes you need both: alter course AND reduce speed

Example: Risk exists with a vessel on starboard; alter course to port 20° AND reduce speed from 18 knots to 12 knots

Action Type 4: Stop or Reverse

Use when manoeuvrability is constrained (harbour, narrow channel)

Rule 8(e): “Take all way off by stopping or reversing propulsion”

Last resort, but sometimes necessary

Critical Rule 8(b) – Make Your Action Obvious

“Alteration of course and/or speed shall be large enough to be readily apparent to another vessel.”

Why? Because other vessels observe your radar blip or visual lights. Small changes don’t show up clearly. The other OOW might not realise you’ve taken action.

Example:

You alter course from 050° to 052° (2 degrees)

Another vessel watching radar won’t see this change (radar blip appears the same)

They might think you’re NOT taking action

They might panic and make wrong decisions

COLLISION RISK INCREASES

Correct action: Alter to 040° or 060° (10+ degree change). Now the other vessel SEES on radar that you’ve moved.

Rule 8(d) – Check Your Action

“Action shall be carefully checked until the other vessel is finally past and clear.”

Translation: “Keep monitoring until the danger is GONE.”

Don’t take one action and then relax. What if:

The other vessel also altered course?

Your alteration wasn’t large enough?

Your action made the situation worse?

Procedure:

Take action (alter course, reduce speed)
Monitor compass bearing continuously
If bearing starts to decrease (moving away), action is working
If bearing remains steady or increases (getting closer), your action FAILED
Take MORE action if needed

Common Student Mistakes ⚠

❌ WRONG: “A small speed reduction from 15 knots to 14 knots is positive action.”

✅ RIGHT: This change is barely noticeable to another vessel. Make it 15 knots to 10 knots (substantial) or alter course by 15+ degrees.

❌ WRONG: “We should ask the other vessel (via VHF) what they’re going to do.”

✅ RIGHT: Rule 8 demands YOU act independently, in accordance with COLREGS. You cannot rely on the other vessel’s response or agreement.

❌ WRONG: “After we alter course to avoid a vessel, we can return to the original course immediately.”

✅ RIGHT: Rule 8(d) says continue monitoring until the vessel is FINALLY PAST AND CLEAR. Monitor bearing and range until they’re safe.

❌ WRONG: “If there’s enough sea room, we should alter course. But if there’s any doubt, we should reduce speed.”

✅ RIGHT: If there’s enough sea room, course alteration is PREFERRED (Rule 8(c)). It’s clearer and more effective than speed reduction.

Real Example – Real Collision:

Case: General Cargo Ship collision, Baltic Sea (2019)

Two cargo vessels were proceeding in a busy shipping area at night.

Vessel A: Maintained course and speed, relying on AIS that showed the target vessel passing at 1.2 NM (deemed safe).

Vessel B: Was altering course slightly (from 090° to 100°) but making SMALL alterations (only 10 degrees total).

What happened:

Vessel A’s AIS was outdated; the actual distance was 0.8 NM, NOT 1.2 NM

Vessel B’s small alteration was not obvious enough

Vessel A did not realise Vessel B had taken action

By the time the situation became clear, both vessels were in close quarters

Emergency manoeuvres were too late

COLLISION — both vessels sustained damage

Why was this a failure of Rule 8:

Vessel A did NOT take any action (relied on AIS)

Vessel B’s action was NOT substantial enough

Neither vessel checked the effectiveness of their action

Rule 8 in Action – Step-by-Step Process:

Step 1: Detect Risk (Rule 7)

Risk of collision detected via radar/bearing check

Step 2: Assess Situation

Your ship: power-driven, loaded, in open water

Other ship: power-driven, approaching

Sea room: SUFFICIENT (open ocean)

Step 3: Take Action (Rule 8)

Decision: Alter course (preferred because sufficient sea room exists)

Action: Alter course from 065° to 045° (20-degree alteration = SUBSTANTIAL ✓)

Timing: NOW, while vessels are 8 NM apart (AMPLE TIME ✓)

Step 4: Monitor Effectiveness

After 5 minutes: Check bearing—target now bearing 020° (bearing CHANGED ✓)

After 10 minutes: Check range—target now 6 NM (but bearing 020° steadily decreasing)

After 15 minutes: Target bearing 010°, range 4 NM

After 25 minutes: Target bearing 000° (fine on your port bow), range 2 NM

After 30 minutes: Target bearing 350° (crossing your stern), range 1.5 NM

After 35 minutes: Target bearing 330° (astern), range 2 NM

Result: Vessels passed safely at a safe distance. Action was EFFECTIVE.

Part 6: Integration – Rules 4-8 Work Together

The Flow:

Rule 4: These rules apply in ANY visibility condition (no excuses)
Rule 5: Keep proper lookout using all means (see, hear, radar, AIS)
Rule 6: Maintain safe speed appropriate to conditions
Rule 7: Continuously assess risk of collision (assume danger if in doubt)
Rule 8: Take positive, early, substantial action to avoid collision

Real Merchant Ship Scenario:

Situation: Your container ship (18 knots) enters the Singapore Strait at night during reduced visibility due to haze.

Rule 4: Rules 5-8 apply DESPITE reduced visibility.

Rule 5: Increase lookout intensity.

Radar: Switch to 6 NM range setting, then 12 NM range every 10 minutes

Helmsman: Be alert, anticipate course changes

Listen: Monitor VHF for vessel traffic information

Visual: Station additional lookout on bridge wings if possible

Rule 6: Reduce speed because visibility is restricted.

Current speed: 18 knots (too fast for this condition)

New safe speed: 10 knots (allows stopping distance of ~2 NM in an emergency)

Engine on standby: Ready to increase or decrease speed

Rule 7: Continuously assess for risk.

Radar target appears: bearing 035°, range 10 NM

Take bearing: 035°

Wait 10 minutes, check again: bearing 034°, range 8 NM

Assessment: Bearing is steady, range decreasing → RISK OF COLLISION EXISTS

Rule 8: Take positive action.

Assess sea room: Sufficient (open water available to port and starboard)

Action chosen: Alter course alone (most effective)

Current course: 065°, Change to: 045° (20-degree alteration = SUBSTANTIAL ✓)

Timing: NOW while vessels are 8 NM apart (AMPLE TIME ✓)

Reduce speed further to 8 knots (POSITIVE action + good seamanship)

Monitoring:

After alteration, bearing changes from 034° to 025° (action working!)

Continue monitoring until the target is astern and the bearing is steady (safety confirmed)

Result: Safe passage through the Singapore Strait despite challenging conditions.

Part 7: Common Exam Mistakes & Misunderstandings

Mistake 1: Rule 4 – Visibility Exception

❌ WRONG: “In fog, we don’t need to follow Rules 5-10.”

✅ RIGHT: Rule 4 says “any condition of visibility” — fog means you INCREASE compliance, not decrease.

Mistake 2: Rule 5 – Lookout Multitasking

❌ WRONG: “The OOW can maintain lookout while doing administrative work.”

✅ RIGHT: Lookout requires 70-80% of OOW’s time. Other tasks are secondary.

Mistake 3: Rule 5 – AIS as Sole Lookout

❌ WRONG: “AIS shows all traffic; we have proper lookout.”

✅ RIGHT: AIS doesn’t show small fishing boats, sailing vessels, or swimmers. AIS can fail. Use all means.

Mistake 4: Rule 6 – Full Speed in High Traffic

❌ WRONG: “We have radar, so full speed in busy waters is safe.”

✅ RIGHT: Radar doesn’t detect all objects (small fishing boats, containers, nets). Reduce speed based on traffic density and visibility.

Mistake 5: Rule 6 – Engine Speed = Safe Speed

❌ WRONG: “Safe speed is whatever RPM we’re comfortable with.”

✅ RIGHT: Safe speed must allow stopping within an appropriate distance given conditions (visibility, traffic, hazards, stopping distance).

Mistake 6: Rule 7 – Single Bearing Check

❌ WRONG: “We took one bearing and the vessel isn’t on us, so no risk.”

✅ RIGHT: Rule 7 requires continuous observation over TIME. Multiple bearing checks are essential.

Mistake 7: Rule 7 – Trusting Radar Alone

❌ WRONG: “Radar shows CPA of 2 NM; no action needed.”

✅ RIGHT: Radar has blind spots. Assume risk exists and verify with additional means.

Mistake 8: Rule 8 – Small Alterations

❌ WRONG: “We altered course 5 degrees and reduced speed slightly; we’ve taken action.”

✅ RIGHT: Rule 8(b) requires alterations to be “large enough to be readily apparent.” Small changes don’t show on the other vessel’s radar or visually.

Mistake 9: Rule 8 – Stopping Monitoring

❌ WRONG: “We took action; now we relax and let the other ship handle it.”

✅ RIGHT: Rule 8(d) requires continuous monitoring until the vessel is “finally past and clear.”

Mistake 10: Rule 8 – Asking Permission

❌ WRONG: “We radioed the other vessel to ask if they would alter course.”

✅ RIGHT: You MUST act independently per COLREGS. Verbal agreements cannot override rules.

Quick Reference: The 5-Minute Takeaway

Rule 4: Rules 4-10 apply in ALL visibility conditions. No excuses. No exceptions (except local rules).

Rule 5: Maintain proper lookout using sight, hearing, radar, AIS, VHF — use EVERYTHING, depend on NOTHING alone.Lookout = 70-80% of your time.

Rule 6: Maintain safe speed appropriate to visibility, traffic, manoeuvrability, and conditions. Safe speed allows you to stop and respond to danger. Reduce speed in restricted visibility, busy waters, and close to hazards.

Rule 7: Continuously use all available means to assess collision risk. Steady bearing + decreasing range = RISK EXISTS. If ANY doubt, assume risk and take action. Use multiple bearing checks, not single observations.

Rule 8: Action must be POSITIVE (substantial), made in AMPLE TIME (early), and APPARENT (large enough to observe). Prefer course alteration if sufficient sea room. Monitor effectiveness until the other vessel is safely past.

Bridge Team Responsibilities

These rules place responsibility on:

✓The Master: Establishing bridge protocols for lookout and safe speed procedures

✓The OOW: Maintaining 70-80% lookout time, continuously assessing risk, and taking appropriate action

✓The Helmsman: Alerting the OOW to traffic and executing course/speed changes

✓Additional Lookout: Stationed on bridge wings in restricted visibility or high-traffic areas

✓The Engine Room: Maintaining the engine on standby and ready to increase/decrease power on OOW command