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Plotting collision by radar

Jul 26, 2007
Jan/Feb 1999

The Gulf of Mexico area just outside the mouth of the Mississippi River must rank among the busiest in the world. Ships from all over the globe converge there, along with innumerable tugs, barges, supply boats and even the occasional yacht in a round-the-clock parade of maritime traffic.

The murky brown water is churned by both propellers and currents as unimaginable amounts of inland river water pours out of the entrance of Southwest Pass leading up to New Orleans and from there into America's heartland. Considering, as well, that the night-time horizon is in places ablaze with the lights of oil and gas platforms, it is no place for a lack of focus on the part of a ship's navigator.

Imagine yourself, then, as watch officer aboard the 780-foot bulk freighter Obo Venture, loaded with grain and outbound from the Mississippi after a long night coming down the river. You've dropped the pilot and cleared the sea buoy and are presently headed southeast out the safety fairway leading into the open waters of the Gulf. Your course is 150�T and your speed is 12 knots. Just ahead are a series of platforms to the right of the fairway, so you are pretty much confined to your present heading.

The only problem is this other ship that you can see broad off your port bow, which appears to be westbound in the converging fairway. There he is on the radar screen, as well, so you target him with the automatic plotting aid and within a minute or so have a pretty good idea of where he's headed.

Here's some radar data beginning at 04:48 on August 19th:

Min. 48: Rel. Bearing 322 degrees; Range 7.0 miles
Min. 54: Rel. Bearing 322 degrees; Range 5.4 miles

Questions that immediately arise include the following: A. What is the closest point of approach of the other ship? B. What time will this CPA event occur? C. What is the speed of the other vessel? D. What lights would you be observing on the other vessel? E. Speaking of visibility, what time is sunrise?

As you have probably guessed by now the Obo Venture did, in fact, collide with the approaching ship, the 740-foot Angelic Spirit, in August 1998. Obo Venture suffered considerable damage to its bow, while Angelic Spirit suffered damage to its starboard bow. The point of collision was 38�43.5'N, 89�22.0'W. Coast Guard officials reported that the Angelic Spirit's watch officer might have been distracted as he plotted the ship's position in the fairway just before the collision. Weather was benign at the time, and VHF radio calls to Angelic Spirit reportedly went unanswered. 'It was a classic case of one vessel not keeping a proper lookout,' said the senior investigating officer.

Note: Data used in the radar plot are not represented as being factually accurate.)  

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