The Super Rudder

Efficient as they are in transporting oil across the oceans, today's huge tankers are clumsy giants, difficult to maneuver and hazardous near shore. Starting a voyage, a typical 250,000-ton tanker may require two hours or more to reach cruising speed (approximately 16 knots). Stopping is no less difficult. Even with props reversed, the steel leviathan will frequently coast up to ten miles before coming to a dead halt. A tanker can reduce that distance to less than two miles by a tactic called "slaloming"--turning in one direction and then in the other, like a racing skier.

But in crowded shipping lanes like the English Channel, this maneuver may be an invitation to disaster; the lumbering vessel needs vast stretches of open water for such turns.

To make the big tankers more maneuverable--and thereby safer--British researchers have now proposed an ingenious modification of that ancient steering device, the rudder. In essence, a rudder works by altering the flow of water around it so that one side begins to experience greater pressure than the other. If the rudder is swung to starboard (right), for instance, pressure on that side will increase and lessen on the other. As a result the stern, or tail end, of the boat will swing to port (left) and the bow, or front, to starboard. Moreover, the turn will become sharper as the rudder angle is increased. But if the angle becomes larger than 35DEG the rudder will stir up so much turbulence in the vicinity that it will rapidly lose its "grip" on the water and thus its steering ability.

Running Circles. To reduce the turbulence and at the same time increase the rudder's effective steering angle, a group led by Naval Architect Barry Steele, of Britain's National Physical Laboratory, revived an idea once proposed for aircraft wing flaps. They fitted rotating cylinders around the rudder posts of several ship models (see diagram). Equipped with its own small motor, the cylinder can spin in either direction. Thus when the rudder is pushed hard to port (left), for instance, the cylinder is rotated in a clockwise direction. This directs a flow of water against the back of the rudder, smoothing out the turbulence there and making the rudder effective at angles much greater than 35DEG.

Encouraged by the tests with their models, the British researchers recently equipped a 200-ton cargo vessel with their super rudder. Tested off the Isle of Wight, the vessel ran circles around other ships of its size: it could turn on its own axis, stop in only seconds, and effectively operate with its rudder turned up to an angle of 90DEG. The British scientists concede that the device will probably not work as spectacularly with heavier ships. Their calculations show, however, that a 250,000-ton tanker should be able to turn completely around in only about 180 yds., and even without slaloming, come to a halt in about a third of a mile--that is, about a quarter of the distance that it now takes to stop.

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