Navigation General · Radar Plotting & CPA
Own ship is on course 090°T at 12 knots. A target is plotted at 030°R, 8 miles, and 6 minutes later is at 030°R, 6 miles. What is the approximate relative speed of the target?
- A10 knots
- B20 knots✓ Correct
- C12 knots
- D6 knots
Explanation
The target closed 2 miles in 6 minutes along the relative motion line. Speed = distance/time = 2 nm ÷ (6/60 hr) = 2 ÷ 0.1 = 20 knots relative speed. Note this is relative speed, not the target's true speed; true speed is determined by completing the vector triangle.
Authority: Radar Navigation Manual (Pub. 1310), Ch. 4
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