Chart Navigation & Plotting · Distance Off — Doubling the Angle on the Bow
What is the principle behind 'doubling the angle on the bow' to find distance off?
- AIt requires the object's charted height
- BDoubling the relative bearing halves the distance off
- CIt only works when the second bearing is exactly abeam
- DThe distance run between the first bearing and the bearing at which the relative angle has doubled equals the distance off at the second bearing✓ Correct
Explanation
When the relative bearing of an object doubles (e.g., 30° then 60°), the triangle is isosceles, so the run between the two bearings equals the distance off at the second bearing. The bow-and-beam method is the special 45°/90° case of this rule.
Authority: Bowditch (Pub. No. 9), doubling the angle on the bow
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