Question 33 ONC05 - Master or Mate of LT 200 GRT

The Correct Answer is B **Explanation for Option B (67.5°):** The regulations governing navigation lights are defined in the International Regulations for Preventing Collisions at Sea (COLREGs), specifically Rule 21(b) for the stern light. The stern light must be placed at the stern and must show an unbroken light over an arc of the horizon of **135 degrees** (or 112.5 degrees according to US Inland Rules, though generally the 135-degree standard applies, which is centered on the stern). The question asks how many degrees the light shows on *each side* of the stern (dead astern). Since the total arc is 135 degrees, and this arc is centered on the direction of dead astern (0 degrees): Total arc (135°) / 2 = **67.5°** Therefore, the stern light shows from dead astern (0 degrees) to 67.5 degrees to port and 67.5 degrees to starboard. **Why the Other Options are Incorrect:** * **A) 22.5°:** This angle is one-half of the arc of visibility for the masthead light (225° total arc, or 112.5° on each side of the bow) *minus* the 90° arc covered by the sidelights (112.5° - 90° = 22.5° overlap). This is not the arc for the stern light. * **C) 112.5°:** This is the arc of visibility for *each* sidelight (port and starboard) starting from dead ahead. The stern light's total arc is 135°, meaning 112.5° is incorrect for the half-arc measurement. * **D) 135.0°:** This is the **total** arc of visibility for the stern light. The question asks for the degrees shown on *each side* of the stern, which requires dividing the total arc by two.