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7. Mediterranean Sea, Supertanker Bilging
This photograph was taken approaching the Libyan port of Benghazi. The main ship wake in this frame, cutting from center right toward bottom left, is much more readily identified than the supertanker itself, although that is also visible and useful for scale. Astronauts have observed stern wakes extending for more than 150 kilometers behind a vessel. A conspicuous wake such as the one illustrated here is almost certainly characteristic of an oil tanker pumping its bilges. This wake extends for about 40 kilometers and represents about 2 hours of tank bilging for a supertanker moving at about 10 knots. Without their cargo of between five and seven million gallons of crude oil, a supertanker is fragile. Water ballast is kept in the storage tanks to keep the vessel's profile low in the water and is pumped out when making to port. Notice how the oil has spread in the stern wake with the passage of time. Also, with the passage of time, a spiral eddy has begun to deform the wake toward the right edge of the frame..
Above the main ship wake and virtually parallel to it is another, older wake. We can tell that it is older because the eddy field has closed much more closely over it, making the wake appear fainter and narrower. It is also clear from the distention of this wake that this second ship (out of view) was traveling in the opposite direction from the ship that produced the brighter, newer wake. A third wake can be seen parallel to and above the second wake.
STS-41G, October 1984. Picture #17-38-063