If you are underway at 5 knots and see a RADAR contact 10 miles directly astern, and 12 minutes later the same contact is 8 miles directly astern, what is the estimated speed of the contact?

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Multiple Choice

If you are underway at 5 knots and see a RADAR contact 10 miles directly astern, and 12 minutes later the same contact is 8 miles directly astern, what is the estimated speed of the contact?

Focus on relative motion along the line of travel. With the contact directly astern, the change in the range is due to how fast the contact moves forward compared to you. The distance shrinks from 10 to 8 nautical miles in 12 minutes. That’s a closure of 2 NM in 0.2 hours, which is 10 NM per hour, i.e., 10 knots of closing speed. You are moving at 5 knots. Since you are closing on the contact, the contact must be moving faster than you by that closing speed: contact speed minus your speed equals 10 knots. Therefore, contact speed = 5 + 10 = 15 knots.

Focus on relative motion along the line of travel. With the contact directly astern, the change in the range is due to how fast the contact moves forward compared to you. The distance shrinks from 10 to 8 nautical miles in 12 minutes. That’s a closure of 2 NM in 0.2 hours, which is 10 NM per hour, i.e., 10 knots of closing speed.

You are moving at 5 knots. Since you are closing on the contact, the contact must be moving faster than you by that closing speed: contact speed minus your speed equals 10 knots. Therefore, contact speed = 5 + 10 = 15 knots.

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