BOTH INTERNATIONAL & INLAND The arc of...

🔍 Key Concepts

• Navigation Rules (COLREGS) – Rule 21: Definitions of lights • Meaning of sidelights and how their visibility is described in degrees relative to the ship’s head (dead ahead) and beam • Difference between total arc of visibility and how that arc is expressed relative to the beam (forward or abaft)

💭 Think About

• First, recall the total number of degrees a single sidelight must cover according to Rule 21. From there, think: does it extend only forward of the beam, only abaft the beam, or overlap both sides of the beam? • Visualize looking down on the vessel from above. Starting at dead ahead (0°), where does the sidelight’s arc stop—before the beam, at the beam, or some number of degrees past the beam toward the stern? • Ask yourself: the combined coverage of the port and starboard sidelights plus the stern light should give you almost a full 360°. How does the sidelight’s arc contribute to that overall coverage?

✅ Before You Answer

• Verify in Rule 21(b) – Sidelights exactly how many degrees from right ahead each sidelight must show its light. • Confirm what “abaft the beam” and “forward of the beam” mean in terms of angles measured from dead ahead (0°) around the horizon. • Check that the sidelight’s arc, when combined with the stern light (135°) and the other sidelight, provides continuous coverage around the vessel without gaps.

BOTH INTERNATIONAL & INLAND The arc of visibility for sidelights is from right ahead to what degree?

135° abaft the beam

abeam

22.5° forward of the beam

22.5° abaft the beam

🔍 Key Concepts

💭 Think About

✅ Before You Answer