Avoid Impeding Vessel Constrained by Draught (Rule 18(d))
Chart overlay shows a narrow DREDGED APPROACH CHANNEL (blue, ~0.8 NM wide, dredged to 15 m) with wider water around it (stippled, 57 m). Your draught is 3.
Scenario briefing
Chart overlay shows a narrow DREDGED APPROACH CHANNEL (blue, ~0.8 NM wide, dredged to 15 m) with wider water around it (stippled, 57 m). Your draught is 3.0 m you have plenty of margin in the 5+ m area outside the channel. Lateral buoys mark the edges (IALA Region A, inbound from the north): RED cans on the east edge (port-hand for inbound), GREEN cones on the west edge (starboard-hand for inbound).
The CBD target ahead is a deep-draught vessel signalling "Constrained by Draught" (badge "C", Rule 28). Her keel sits only a metre or two off the dredged bottom she cannot leave the dredged channel without grounding and cannot alter sharply without losing steerage.
Rule 18(d): every vessel, other than a vessel not under command or restricted in ability to manoeuvre, shall AVOID IMPEDING the safe passage of a vessel constrained by her draught. "Impeding" means forcing her to take avoiding action. You CAN safely leave the channel 5 m under a 3 m keel is ample. Do so EARLY so she never has to alter her line. Keep CPA 0.5 NM.
Applicable COLREG rule(s)
📸 Bridge simulator scene
Captured directly from the SkipperCheck COLREG bridge simulator at scenario T = 0 — the moment the encounter begins.
Key teaching points
- Rule 18(d)(i): any vessel other than a NUC or RAM shall, if circumstances of the case admit, avoid impeding the safe passage of a vessel constrained by her draught.
- A CBD vessel shows the normal lights for her size PLUS, where they can best be seen, three all-round RED lights vertically (Rule 28).
- CBD is NOT the same as RAM — different lights, different priority level.
- Vessel constrained by her draught must navigate with particular caution (Rule 18(d)(ii)) — she has priority, but she also bears extra responsibility.
Common mistakes to avoid
- Treating three vertical RED lights as a fishing or NUC signal. Only Rule 28 vessels (CBD) show that pattern.
- Insisting on Rule 18(a)(iv) priority as a sailing vessel against a CBD bulker — courts have ruled against sailors who did this.
Why it matters
Exam relevance
Rule 28 CBD lights and the Rule 18(d) qualified priority are examined in STCW OOW orals and in Yachtmaster Offshore — examiners distinguish CBD from RAM and from NUC.
Related scenarios
About SkipperCheck simulators
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- ARPA · AIS · COLREG Bridge Simulator — 54 scenarios covering Rules 2, 5–10, 12–19, 23–30, 34 and 35.
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