Question 33 ONC01 - Master/Chief Mate Unlimited Tonnage

The Correct Answer is B. ### Why Option B (67.5°) is Correct Option B, **67.5°**, is correct because it aligns with the specifications set forth in the **International Regulations for Preventing Collisions at Sea (COLREGs)**, Annex I, Section 9 (Horizontal Sectors) and Rule 21(b), which apply to both International (COLREGs) and Inland waters (often adopting the COLREG standards). The regulation states that the **sternlight** (a white light placed as nearly as practicable at the stern showing an unbroken light) must illuminate an arc of the horizon of **135 degrees**. This 135-degree arc must be centered on the fore-and-aft line (dead astern). Therefore, the light must show: $$\frac{135 \text{ degrees}}{2} = 67.5 \text{ degrees}$$ on each side of the stern (dead astern). ### Why Other Options Are Incorrect **A) 22.5°:** This angle represents the sector of illumination for the forward arc of the **sidelights** (starboard and port lights) beyond the bow, or the angular difference between a masthead light and a sidelight's minimum forward visibility. It is not the sector width for the sternlight on one side. **C) 112.5°:** This angle represents the sector of illumination for **each sidelight** (starboard and port lights), showing from dead ahead to 22.5° abaft the beam. It is not related to the sternlight's arc. **D) 135.0°:** This angle represents the **total arc** (total horizontal sector) of illumination for the sternlight (dead astern plus $67.5^{\circ}$ on port and $67.5^{\circ}$ on starboard). The question asks for the degree measurement **on each side** of the stern, which is half of the total arc.