Radar Contact on Reciprocal Bearing
Own ship is on course 090, speed 15 knots. In fog with visibility 2 nautical miles, radar shows a single contact bearing 030, range 12 nautical miles, course 210, speed 20 knots. The contact is steady on that course and speed. There are no other contacts or navigational hazards. The officer of the watch has 20 minutes before handing over and asks for a relative-motion assessment and an avoiding-action recommendation. The ship is not in a restricted channel and has sea room to starboard. The captain is asleep but has ordered to be called for any close-quarters situation. The radar is set to true-motion, north-up, and the contact's echo is clear. The OOD has not yet altered course or speed. The ship's turning circle at 15 knots has a tactical diameter of about 450 meters, and a 30-degree starboard turn can be steadied in about 2 minutes.
1. Determine the contact's aspect and the COLREGS situation. What action is required?
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Own course is 090; the contact bears 030, which is 60 degrees to port of the bow. From the contact, own ship bears 210. The contact's course is 210, so own ship is dead ahead of the contact.
This is a head-on situation, not a crossing: each ship sees the other ahead or nearly ahead. Under COLREGS both are give-way and both should alter course to starboard, early and substantially. A 30-degree starboard turn is a reasonable start, but it must be obvious to the other ship and monitored on radar. Do not turn to port. Reduce to safe speed if necessary and sound the appropriate fog signal. The captain should be called because a close-quarters situation is developing.
2. Compute the closing speed and time to CPA if neither ship alters.
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The bearing line is 030/210. Own ship's component toward the contact is 15 cos 60 degrees = 7.5 knots, because own course is 60 degrees off the bearing line. The contact's component toward own ship is 20 cos 0 degrees = 20 knots.
Closing speed = 27.5 knots. Time to collision if neither alters = 12 / 27.5 = 0.436 h = 26.2 minutes.
A proper relative-motion plot gives a CPA of about 5.1 nm in about 21 minutes, not zero, because the contact is not exactly on a collision bearing. But 5.1 nm is still a close-quarters situation in 2 nm visibility. The exact CPA should be plotted, not judged by eye.
3. If own ship immediately turns 30 degrees starboard and steadies in two minutes, what is the new CPA, and is that enough?
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Using the same initial positions, own velocity on 120 degrees is 15 knots: east component 12.99, north component -7.5. Contact velocity on 210 degrees is east -10, north -17.32.
Relative velocity = (-22.99, -9.82); relative speed = 25.0 knots. Relative position from own ship to contact = (6, 10.392). Time to CPA solves r dot v = 0: (-239.9) / (624.9) = 0.384 h = 23 minutes. At that time the range is about 7.2 nm.
So the turn improves CPA from about 5.1 nm to 7.2 nm, but that is not a large margin in 2 nm fog. I would recommend a larger starboard alteration, 40 degrees or more, or a bold course change combined with speed reduction, then verify that the bearing is drawing aft. Call the captain now, because the CPA is inside the close-quarters threshold.