If you are underway at 10 knots and note a RADAR contact dead ahead at a range of 10 miles, and at 1812 the contact is still dead ahead but at a range of 8 miles, what is the estimated speed of the contact?

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The main idea is that a contact that stays exactly on the forward line (dead ahead) as you move indicates no relative motion perpendicular to your track. Here, you are making 10 knots, and the range to the contact decreases from 10 to 8 miles while its bearing remains dead ahead. The only simple interpretation that keeps the line of sight unchanged while you close the distance is that the contact itself has no speed relative to the water. In other words, it is dead in the water.

If the contact were moving at any forward or crosswise speed, you would generally expect some change in its apparent bearing or a more complex rate of range change that wouldn’t keep it exactly dead ahead over time. The test’s point is that a constant bearing with decreasing range under your own motion points to a stationary target, hence zero speed.