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Marine Propulsion Determining Shaft Speed with Custom Magnets Cruising down the highway, it is fairly easy to determine speed just by looking out the window to gauge how fast the scenery is passing by. While oceangoing vessels do not have this option, custom magnets are providing viable, reliable and cost effective solutions for one Louisiana company. For fixed propeller marine vessels, the surest way to determine propeller speed is to measure shaft speed, since the shaft is direct coupled to the engine and has the propeller affixed to it. International Marine Systems in Schriever, La., has developed a unique method to do so which imparts no stresses onto the shaft. "We wrap magnetic tape around the circumference," said Andreas Gottschalk, Engineering Layout showing magnetic tape, magnetic sensors, tach amplifier control box, and display readout panel. (Photos courtesy of International Marine) Manager of IMS, "then install a magnetic pick-up sensor. When the shaft is spinning, the sensor picks up the alternating (north/south) domains." "It creates a sine wave," said Gottschalk "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 "tape" is actually a custom designed magnetic sheet with an adhesive back, which is then affixed onto the shaft. The magnetic sheet produced by The Electrodyne Co., has high-energy magnetic fields, or stripes, within it. The stripes have alternating north and south poles, allowing a magnetic sensor to register the poles as they pass by. When the shaft is idle, there are no poles passing the sensor. As the shaft begins to turn, the time delay between passings of the poles is measured and indicated. Relatively long time periods between poles equate to slow shaft (and propeller) speed. As the shaft speed increases the time between the passings of poles decreases, indicating faster shaft (and propeller) speeds. The stripes are spaced just 0.25-in. wide, providing a continuous stream of speed sensing data. Incorporating highenergy Plastalloy magnetic material, the stripes are easily read by the sensors even at high rates of speed, and despite the close proximity of the stripes. Skewing of the stripes allows the sensors to determine the direction of the rotation or whether the vessel is going forward or in reverse. Ken Koch, Marketing Manager of magnetics supplier The Electrodyne Co. recalls the original design. "The original drawing called for a magnetic strip with offset holes to interrupt the magnetic field so that they could determine direction of rotation. We proposed a skewed strip design which is equally effective and much simpler (and more cost effective)." As important as the determination of speed and direction are, the magnetic "tape" imparts no stress onto the vessels shaft. This was an important consideration for International Marine, since the other options each included some element of stress for the shaft. 52 Maritime Reporter & Engineering News