Question 33 ONC04 - Mate of LT 500-1600 GRT

The Correct Answer is B **Explanation for Option B (67.5°):** Option B (67.5°) is correct because the specifications for the stern light are defined under the International Regulations for Preventing Collisions at Sea (COLREGs), Rule 21(d) (and identical requirements apply to inland rules in the US, 33 CFR § 83.21). The stern light is defined as a white light showing over an arc of the horizon of $135^{\circ}$ and so positioned as to show the light from dead astern to $67.5^{\circ}$ forward on each side of the vessel. Since $67.5^{\circ} + 67.5^{\circ} = 135^{\circ}$ (the total arc), this confirms the required arc for the stern light coverage on each side of the stern. **Explanation for Incorrect Options:** * **A) 22.5°:** This angle is incorrect. While $22.5^{\circ}$ is the specific angle defining the additional forward visibility arc for the sidelights (beyond the $112.5^{\circ}$ arc), it does not apply to the stern light visibility. * **C) 112.5°:** This angle represents the arc of visibility for the sidelights (port and starboard lights), which cover from dead ahead to $112.5^{\circ}$ aft on their respective side. It is not the coverage arc for the stern light. * **D) 135.0°:** This is the **total** arc of visibility for the stern light. The question asks for the degrees on **each side** of the stern, which is half of $135^{\circ}$ ($67.5^{\circ}$).