Question 33 OSV01 - Master/Chief Mate - Offshore Supply Vessels

The Correct Answer is B. ### **Explanation of why Option B ("67.5°") is correct:** The positioning and arc of visibility for navigation lights, including the stern light, are standardized under the International Regulations for Preventing Collisions at Sea (COLREGs), which apply to both international and inland waters (though specific inland rules may adopt them). The stern light is defined as a white light placed as nearly as practicable at the stern showing an unbroken light over an arc of the horizon of $135^{\circ}$ and so fixed as to show the light $67.5^{\circ}$ from dead astern on each side of the vessel. Therefore, the stern light shows: $67.5^{\circ}$ (to the starboard side) + $67.5^{\circ}$ (to the port side) = $135.0^{\circ}$ total arc. The question asks for the degree measurement *on each side* of the stern, which is $67.5^{\circ}$. *** ### **Explanation of why the other options are incorrect:** * **A) 22.5°:** This measurement is typically associated with the arc covered by the masthead light *forward* of the beam (i.e., the arc that the sidelights *do not* cover immediately forward of the bow). It is not the arc for the stern light on one side. * **C) 112.5°:** This is the arc of visibility for the sidelights (red or green), which must show from dead ahead to $112.5^{\circ}$ aft on its respective side. This is a measurement for the forward lights, not the stern light. * **D) 135.0°:** This is the **total** arc of visibility for the stern light (from port $67.5^{\circ}$ to starboard $67.5^{\circ}$). The question asks for the measurement "on **each side** of the stern," making $135.0^{\circ}$ the total, not the unilateral arc.