How Fast Is That Ship?

While GPS can tell ocean-going vessels where they are, the ships rely on propeller speed to figure out how fast they're going. For vessels with fixed propellers, the best way to determine this is to measure the speed of the propeller shaft, which is coupled to the engine.

THE CHALLENGE: Measure speed for ocean-going vessels
To avoid stressing the shaft, International Marine Systems, Schriever, LA, wraps magnetic tape around the shaft's circumference and then installs a magnetic pick-up sensor. The sensor picks up the alternating north/south magnetic domains on the tape as the shaft spins. According to Andreas Gottschalk, engineering manager of IMS, "It creates a sine wave. When it spins faster, the frequency of the wave changes. Our electronics can take the frequency of the wave generated and determine how fast the shaft is moving. In essence, we use the tape to interact with the sensor."

The Electrodyne Company created the magnetic tape for IMS, combining a magnetic sheet containing high-energy magnetic fields or stripes with an adhesive backing. The stripes have alternating north and south poles, so a magnetic sensor can register the poles as they pass. The shorter the time between poles passing, the faster the propeller is spinning; the longer the interval between poles, the slower the shaft and propeller are moving. The magnetic stripes are skewed, which allows the sensors to detect the direction of rotation and indicate whether the vessel is moving forward or in reverse.

While using magnets to determine shaft speed isn't new, using the tape offers a significant benefit—the tape imparts no stress to the propeller shaft, in contrast to using welded targets or bolted collars to interact with the sensor. It also requires no routine maintenance.

Contact Ken Koch, The Electrodyne Co., Batavia, OH; 513-732-2822, [email protected].