Maglevs for ships

Several approaches have been devised for suppressing the rolling motion of ships at sea. Active systems that position a movable mass to provide a countering torque can rapidly damp the rolling, but the inherent friction not only is noisy but also produces constant wear. Researchers from the Korea Institute of Machinery and Materials recently demonstrated that the technology employed to reduce friction in magnetically levitated trains can do the same for ships. Indeed, their antirolling device is essentially a maglev train car on a short track that runs side to side across the ship’s midline. For the demonstration, the team built a 118-kg, small-scale model with the cross-sectional shape of a twin-hulled catamaran. A C-shaped, 4kg mass—the “train car”—wrapped around a 1-m-long rail along the top of the model; electromagnets at the bottom of the mass faced the rail from underneath and provided the levitating force. Mounted in the rail was a linear motor, like those used to propel maglev trains (and some roller coasters). Whereas the more common rotary motors produce torque, linear motors produce linear forces. In the researchers’ setup, as the motor controller received signals from a tilt sensor on the ship, it quickly moved the mass to the appropriate position to damp the rolling motion. In tests, the model ship’s free oscillations were suppressed within 4 seconds. Though it is more expensive than conventional alternatives, the researchers think maglev antirolling technology could have broad potential, from pleasure boats to oceanographic research vessels. (C. H. Park et al., Rev. Sci. Instrum. 81, 056102, 2010 http://dx.doi.org/10.1063/1.3402287 .)