Rules 16-18

COLREGS Rules 16-18: Give-Way, Stand-On, and Vessel Priorities

Explained for Gen Z Merchant Seafarers

Part 1: Rule 16 – Action by Give-Way Vessel (1.5 minutes)

The Golden Rule:

“Every vessel which is directed to keep out of the way of another vessel shall, so far as possible, take early and substantial action to keep well clear.”

Three Requirements for Give-Way Action:

1. EARLY Action

Not early = LATE action

WRONG: Waiting until 300 meters separation to alter course RIGHT: Altering course when the target is 5-10 NM away

Why early matters?

Gives the stand-on vessel confidence that you’re taking action

If you act too late, a stand-on vessel may think you’re NOT responding and may panic

Their panic could trigger unnecessary actions that CREATE collision risk

Real scenario: Container ship sees bulk carrier approaching (crossing situation). Container ship waits until 1 NM separation to alter course. Bulk carrier captain, seeing no action, alters course too. Now BOTH are manoeuvring in unexpected directions =collision risk increases.

2. SUBSTANTIAL Action

Not substantial = MEANINGLESS action

WRONG: Altering from 050° to 053° (only 3 degrees) RIGHT: Altering from 050° to 035° (15 degrees minimum)

Why substantial matters?

Other vessels watching radar must see your course change clearly

Small changes are barely detectable on radar

Stand-on vessel won’t realize you’re taking action

They may take their own action, creating confusion

Real metric: ARPA radar shows your course change as approximately 1-2 degrees per minute. Small 3-degree alterations take1.5-3 minutes just to become visible. By then, separation may be dangerously close.

3. KEEP WELL CLEAR

Not just “avoid collision” = KEEP WELL CLEAR

This means: Pass with SAFE SEPARATION (typically 2+ NM for merchant ships)

Translation: Don’t aim for “just barely passing without collision.” Aim for obvious, safe clearance.

The Speed vs. Course Dilemma:

Which is better: Alter course or reduce speed?

Answer: BOTH. Alter course is PRIMARY.

Why?

Course change is VISIBLE on radar (appears as a course line change)

Speed reduction is SLOWER to observe on radar (takes longer for the range to show a gap increasing)

Course alteration shows IMMEDIATE clear intent

Procedure:

1. Alter course (15-30 degrees) = PRIMARY action

2. Reduce speed (optional but recommended) = SECONDARY action

3. Both together = maximum clarity

Real Merchant Ship Example:

Scenario: Container ship crossing a bulk carrier

Initial positions:

Range: 10 NM

Container ship bearing: steady (crossing situation)

The container ship is a GIVE-WAY vessel

Rule 16 WRONG application:

Reduce speed from 18 knots to 17 knots

Alter course from 090° to 093° (3 degrees)

Result: Bulk carrier captain doesn’t notice; thinks you’re not taking action

Rule 16 CORRECT application:

Alter course from 090° to 060° (30 degrees) = SUBSTANTIAL ✓

Reduce speed from 18 knots to 12 knots

Result: Bulk carrier radar shows a clear course change immediately

Part 2: Rule 17 – Action by Stand-On Vessel (1.5 minutes)

The Core Principle:

Stand-on vessel = MAINTAIN COURSE AND SPEED (normally)

BUT with important exceptions…

Rule 17(a)(i) – Normal Stand-On Responsibility:

“Where one of two vessels is to keep out of the way of the other, the latter vessel shall keep her course and speed.”

Translation: Unless the give-way vessel fails, you HOLD your position.

Why?

Predictability = safety. If a stand-on vessel starts manoeuvring unpredictably, the give-way vessel can’t anticipate their moves.

If both vessels are manoeuvring, collision risk INCREASES (not decreases)

One vessel doing one thing; other vessel doing another = chaos

Rule 17(a)(ii) – The “Emergency Exception” – CRITICAL!

“The latter vessel may, however take action to avoid collision by her manoeuvre alone, as soon as it becomes apparent to her that the vessel required to keep out of the way is not taking appropriate action in compliance with these Rules.”

This Has Two Critical Parts:

Part A: “Becomes apparent that the vessel is NOT taking appropriate action”

NOT = The give-way vessel either:

1. Is making NO alterations (straight course, straight speed)

2. Is making SMALL alterations (3-5 degrees—not substantial)

3. Has altered but NOT early enough (vessels are now too close)

At what range does “apparent” usually mean?

Typically 1-2 NM. If the give-way vessel hasn’t acted significantly by 1-2 NM, it’s clear they’re NOT complying with Rule 16.

Part B: “Take action by her manoeuvre alone”

Translation: Stand-on vessel takes action INDEPENDENTLY, not waiting for the give-way vessel.

Actions available:

Alter course (most effective)

Reduce speed (second option)

Both (if needed)

Restrictions:

Rule 17(c): Don’t alter to port for a vessel on your port side (explained below)

Rule 17(b) – Last-Resort Rule:

“When, from any cause, the vessel required to keep her course and speed finds herself so close that collision cannot be avoided by the action of the give-way vessel alone, she shall take such action as will best aid to avoid collision.”

Translation: If we’re DANGEROUSLY close and the give-way vessel’s action won’t be enough = STAND-ON MUST ACT.

Example: Give-way vessel alters to starboard, but two vessels are already at 0.3 NM and still on a collision course. Stand-on vessel MUST also take action.

Rule 17(c) – The Port-Side Restriction:

“A power-driven vessel which takes action in a crossing situation…shall, if the circumstances of the case admit, not alter course to port for a vessel on her own port side.”

This Needs Careful Explanation:

Scenario: You are a stand-on vessel. Give-way vessel is on YOUR port (left) side. Give-way vessel is NOT taking adequate action.

You CAN do:

Alter to STARBOARD (right)

Reduce speed

Increase speed

Reverse engines

You should AVOID doing (if possible):

Alter to PORT (left)

Why? Because altering to port when another vessel is on your port side moves you TOWARD the other vessel, increasing collision risk.

Rule 17(d) – The Give-Way Vessel Still Responsible:

“This Rule does not relieve the give-way vessel of her obligation to keep out of the way.”

Translation: Even if a stand-on vessel takes action, the give-way vessel STILL has primary responsibility.

Scenario: Stand-on vessel alters course. Give-way vessel sees this and thinks “Now I don’t need to manoeuvre.” WRONG. Rule 17 (d) says: No, you’re STILL required to keep out of the way. Both vessels should be taking avoiding action once the stand-on vessel moves.

Real Merchant Ship Example – The Crossing Situation:

Your bulk carrier = STAND-ON vessel (container ship on your starboard side)

Timeline:

10 NM separation:

A container ship is a give-way vessel (has you on their starboard)

You maintain course 090°, speed 18 knots (Rule 17a(i))

Container ship alters to 070° (30-degree alteration) and reduces speed to 12 knots (Rule 16)

Result: Safe passage. You never have to take action.

Now, a different scenario—GIVE-WAY VESSEL FAILS TO ACT:

10 NM separation:

A container ship is a give-way vessel

You maintain course, speed (Rule 17a(i))

Container ship DOES NOT alter. Maintains 090°, 18 knots.

6 NM separation:

Container ship is still on a steady bearing

Still 18 knots

No course change visible

You realise: “They’re not taking Rule 16 action”

At 6 NM, Rule 17(a)(ii) triggers: “Becomes apparent they’re not taking action”

Your action at 6 NM:

Alter to starboard (045°)

Reduce speed to 12 knots

Maintain this action

Why 6 NM, not wait until 2 NM?

Earlier action is safer

Gives time for actions to take effect

Shows clear intent

2 NM separation (if needed—Rule 17b):

Evaluate: Is it still on a collision course?

If yes, take MAXIMUM action (45-90 degree turn + emergency speed reduction)

Part 3: Rule 18 – Responsibilities Between Vessels (1.5 minutes)

The Vessel Hierarchy:

Rule 18 creates a PRIORITY SYSTEM based on manoeuvrability. The LESS manoeuvrable vessel has priority.

Rule 18(a) – Power-Driven Vessel Give-Way Responsibility:

A power-driven vessel shall keep out of the way of:

Rank                                         Vessel Type

1                  Vessel NOT under command

2                  Vessel RESTRICTED in ability to manoeuvre

3                  Vessel ENGAGED in FISHING

4                  Sailing vessel

Priority

HIGHEST

VERY HIGH

HIGH

MEDIUM

Reason

No engines; completely immobilised

Limited manoeuvring due to work operations

Limited manoeuvrability due to restricting gear

Limited power compared to engines

Translation for Merchant Ships:

If your bulk carrier (power-driven) encounters ANY of these 1-4 types:

You MUST give way

They are stand-on (even if crossing, head-on, or overtaking rules would apply)

You take action per Rule 16 (early, substantial, keep well clear)

Rule 18(b) – Sailing Vessel Give-Way Responsibility:

A sailing vessel shall keep out of the way of:

Type

Vessel NOT under command

Vessel RESTRICTED in ability to manoeuvre

Vessel ENGAGED in fishing

Priority

HIGHEST

VERY HIGH

HIGH

Sailing vessels DO NOT automatically give way to other sailing vessels (Rules 12 applies instead—wind rules)

Rule 18(c) – Fishing Vessel Give-Way Responsibility:

A fishing vessel shall, so far as possible, keep out of the way of:

Type

Vessel NOT under command

Vessel RESTRICTED in ability to manoeuvre

Priority

HIGHEST

VERY HIGH

Rule 18(d) – Vessel Constrained by Draft (CBD):

“Any vessel other than a vessel not under command or a vessel restricted in her ability to manoeuvre shall, if the circumstances of the case admit, avoid impeding the safe passage of a vessel constrained by her draught.”

What This Means:

A DEEP-DRAFTED vessel (like a fully-loaded 100,000 DWT tanker in shallow waters) gets SPECIAL CONSIDERATION.

Translation:

Your normal power-driven vessel must try NOT to impede a CBD vessel

“Avoid impeding” means: before the risk of collision develops, take action

You don’t wait for crossing/head-on/overtaking rules to apply

You proactively give way to protect CBD vessel’s passage

Real scenario: Your loaded bulk carrier (20 m draft) approaches a fully-loaded Very Large Crude Carrier (VLCC, 18m draft)in a shallow-water approach (19m charted depth).

Analysis:

Your vessel: 20m draft = CONSTRAINED BY DRAFT (can’t deviate much)

VLCC: 18m draft = EVEN MORE CONSTRAINED

Application of Rule 18(d):

Both vessels should navigate with caution

Neither should impede the other

Both should communicate intentions (via VHF if appropriate)

Both should maintain safe separation

Critical Note: “If the circumstances of the case admit”

Translation: Circumstances may prevent compliance with Rule 18.

Examples where Rule 18 might NOT apply:

1. In a narrow channel: Rule 9 takes precedence (overtaking rules, starboard-side priorities)

2. In a TSS: Rule 10 takes precedence (traffic lane direction)

3. In an overtaking situation: Rule 13 takes precedence (the overtaking vessel is always the give-way)

Part 4: Integration – Rules 16-18 in Practice (1 minute)

The Decision Tree:

Step 1: Determine encounter type (crossing, head-on, overtaking) per Rules 11-15 → UNLESS a special vessel type exists (CBD, fishing, NUC, restricted)

Step 2: Check if Rule 18 OVERRIDES

Question: Is the other vessel one of these types?

Vessel NOT under command? → THEY are stand-on; you are give-way (Rule 18)

Vessel RESTRICTED in manoeuvre? → THEY are stand-on; you are give-way (Rule 18)

Vessel engaged in fishing? → THEY are stand-on; you are give-way (Rule 18)

Sailing vessel? → THEY are stand-on; you are give-way (Rule 18)

Vessel constrained by draft? → Take Rule 18(d) action (proactive separation)

If NO special vessel type:

→ Apply Rules 11-15 (crossing, head-on, overtaking)

Step 3: Determine give-way vs. stand-on roles

Step 4: Apply Rules 16 & 17

Give-way vessel: Rule 16 (early, substantial action)

Stand-on vessel: Rule 17 (maintain course/speed; take action if give-way fails)

Real Scenario – Complex Situation:

Situation: Your container ship navigates a busy sea lane. You detect:

Vessel A: Trawler fishing with nets deployed (crossing situation, bearing steady, 8 NM away)

Vessel B: Loaded tanker on overtaking course from your stern (approaching at 25° abaft beam)

Analysis:

Vessel A (Fishing Vessel):

Rule 18(a)(iii) says: A power-driven vessel gives way to a fishing vessel

REGARDLESS of whether it’s crossing, head-on, or overtaking

You are a GIVE-WAY vessel per Rule 18, not Rule 15 crossing rules

Action: Alter course away from trawler (Rule 16)

Vessel B (Tanker):

Vessel B is overtaking you

Rule 13 says: Overtaking vessel (Vessel B) is give-way; you are stand-on

Rule 18 doesn’t apply (both are power-driven vessels)

You maintain course/speed (Rule 17a(i))

Vessel B must alter around you

Your Actions:

1. First priority: Alter course to AVOID trawler (Rule 18)

2. Second: MONITOR tanker to ensure they’re taking action

3. If tanker appears to NOT alter: You take action against tanker too (Rule 17a(ii))

Part 5: Common Exam Mistakes & Misunderstandings  

Rule 16 Mistakes:

❌ WRONG: “We altered course 5 degrees; that’s substantial action under Rule 16.”

✅ RIGHT: Substantial means 15-30 degrees minimum. 5 degrees won’t be visible on radar or to a visual observer.

❌ WRONG: “We reduced speed from 18 knots to 17.5 knots; we’re complying with Rule 16.”

✅ RIGHT: Suchsmall reduction won’t be observable. Rule 16 requires “substantial” action.

❌ WRONG: “We can take action only when risk of collision becomes obvious (at 1 NM).”

✅ RIGHT: Rule 16says EARLY action. Act at 5-10 NM, not at 1 NM.

❌ WRONG: “After we alter course, we can relax and let the stand-on vessel handle it.”

✅ RIGHT: Continue monitoring your action’s effectiveness. If it’s not sufficient, take MORE action.

Rule 17 Mistakes:

❌ WRONG: “As stand-on vessel, we can start altering course immediately when we see another vessel.”

✅ RIGHT: Rule 17(a)(i) says maintain course/speed. Only take action per Rule 17(a)(ii) if give-way vessel fails to act.

❌ WRONG: “Give-way vessel altered; now we can relax completely.”

✅ RIGHT: Rule 17 requires you to monitor whether their action is sufficient. If not, Rule 17(a)(ii) & (b) require YOUR action.

❌ WRONG: “As a stand-on vessel in a crossing, we can alter to port to avoid the give-way vessel.”

✅ RIGHT: Rule 17(c) says avoid altering to port if the give-way vessel is on your port side (this moves you toward them).

❌ WRONG: “Stand-on vessel took action; now give-way vessel is off the hook.”

✅ RIGHT: Rule 17(d) says the stand-on vessel’s action does NOT relieve the give-way vessel’s obligation.

Rule 18 Mistakes:

❌ WRONG: “Our power-driven ship crosses in front of a fishing vessel because crossing rules say we can.”

✅ RIGHT: Rule 18(a)(iii) says power-driven gives way to fishing. Crossing rules (Rule 15) don’t override Rule 18.

❌ WRONG: “We’re in a TSS; a fishing vessel must stay clear of us.”

✅ RIGHT: Rule 10(a) says “does not relieve any vessel of her obligation under any other rule.” Rule 18 still applies; the fishing vessel has priority.

❌ WRONG: “We’re overtaking a vessel fishing; as the overtaking vessel, they must give way.”

✅ RIGHT: Rule 13 says “overtaking vessel is give-way,” BUT Rule 18 can OVERRIDE. A fishing vessel has priority over a power-driven overtaking vessel.

❌ WRONG: “A fully-loaded tanker (deep draft) can navigate anywhere it wants; everyone gives way.” ✅ RIGHT: Rule 18(d) says other vessels should “avoid impeding” CBD vessel. It’s not an absolute right-of-way; it’s a request for courtesy/caution.

Part 6: Quick Reference – The 5-Minute Takeaway  

Rule 16 (Give-Way Vessel): Take EARLY and SUBSTANTIAL action (15-30° course change minimum, or significant speed reduction) to keep WELL CLEAR (2+ NM separation). Single small alterations are NOT compliant.

Rule 17 (Stand-On Vessel): Maintain course and speed UNLESS the give-way vessel fails to act appropriately (Rule 17a(ii)). If it becomes apparent they’re not taking action by 1-2 NM range, take YOUR action. Do NOT alter to port for a give-way vessel on your port side.

Rule 18 (Vessel Hierarchy): Power-driven vessels give way to NUC, restricted manoeuvre, fishing, and sailing vessels. Sailing vessels give way to NUC, restricted, and fishing. Fishing gives way to NUC and is restricted. Deep-draft vessels deserve special courtesy (Rule 18d).

Bridge Team Responsibilities – Rules 16-18

For the Master:

✓ Ensure bridge team understands give-way vs. stand-on roles ✓ Brief on Rule 18hierarchy before arrival/departure

✓ Be available on the bridge during complex multi-vessel situations

For the OOW (Give-Way Vessel):

✓Act EARLY (at 5-10 NM, not 1 NM)

✓Act SUBSTANTIALLY (15-30° minimum, not 3-5°) ✓ Maintain actionuntil OTHER vessel is safely clear

✓ Continue monitoring and be ready for additional action if needed

For the OOW (Stand-On Vessel):

✓ Maintain course and speed initially

✓ Monitor the give-way vessel’s action closely

✓ If the give-way vessel hasn’t acted by 1-2 NM range, prepare to take action (Rule 17a(ii))

✓ DO NOT alter to port if the give-way vessel is on your port side

For the Helmsman:

✓ Execute course alterations smoothly and completely ✓ Hold course after alteration (don’t drift back)

✓ Report bearing changes on other vessels ✓Acknowledge all helm orders clearly

Bridge Scenario – Integration of Rules 16-18

Situation: Your container ship (course 090°, speed 18 knots) detects two approaching vessels at night:

Vessel A (Bulk Carrier):

Bearing: steady at 045°

Range: 10 NM

Range decreasing at 1 NM per minute

Visual: Red sidelight visible (YOU on THEIR port side = crossing situation)

Vessel B (Fishing Vessel):

Bearing: 055°

Range: 8 NM

Range decreasing

Day shapes: Two red balls visible (fishing vessel engaged in fishing)

Analysis:

Vessel A (Bulk Carrier):

Red light visible = crossing situation (Rule 15)

Bulk carrier has you on their port side = bulk carrier is GIVE-WAY

You are a STAND-ON vessel (Rule 17)

Action: MAINTAIN course 090°, speed 18 knots

Monitor the bulk carrier for action

Vessel B (Fishing Vessel):

Rule 18(a)(iii) applies: Power-driven (you) must give way to fishing (them)

REGARDLESS of the crossing situation

You are a GIVE-WAY vessel (Rule 18)

Action: Alter course to starboard (at least 15-30°) = 060° or less (Rule 16)

Reduce speed to 12 knots as secondary action

This alteration also creates a separation from the bulk carrier

Timeline:

At 0 minutes (10 NM/8 NM):

Assess situation

Identify Vessel A (bulk carrier) as a stand-on

Identify Vessel B (fishing) as a priority per Rule 18

At 2 minutes (8 NM/6 NM):

Execute action: Alter to 060° (30° change = SUBSTANTIAL)

Reduce speed to 12 knots

Both vessels should see your action on their radar

At 5 minutes (5 NM/3 NM):

Monitor fishing vessel: Are they seeing your action?

Monitor bulk carrier: Are they maintaining course?

Your altered course should now pass the fishing vessel safely on the port side

At 8 minutes (2 NM/0 NM):

The fishing vessel should now be crossing astern (behind)

The bulk carrier should be further separated by your course change

Both vessels are now safely clear