It is often desirable to test scale models of ship hulls, propellers, torpedeos, tow cables, etc., in water tanks to determine their operating characteristics before building the full scale devices. Sometimes the scale model being tested will be towed through a tank of stationary water and sometimes the scale model is held stationary while the water is circulated past it. While doing such testing, it is often necessary to measure movements and vibrations of the model being tested as the water flows past it. Photographic techniques have been used to measure such movements and vibrations in the past. However, the photographic techniques require that a submerged photo pit or a water proof protective housing be available in which to mount the camera equipment. Furthermore, accurate measurements of the magnitude of the vibrations require that one make complicated corrections for the optical distortion at the air-water interfaces. It is difficult to automate the process of reducing the data which is collected on photographic film. Another technique which has been used is to place accelerometers on the scale model being tested. Since the output of the accelerometer represents the acceleration of the test model, this output must be integrated once to obtain the velocity of the test model and integrated twice to obtain the relative position of the test model. This single and double integration presents the possibility of large cumulative errors in measuring velocity or position on long test runs. Other methods such as variable inductance transducers and differential transformers have the required resolution and sensitivity, but as a result of the size of the required coils, tend to interfer with the flow of water. A technique for measuring changes in position and vibrations of test models in water should be accurate, lend itself to automated data reduction, be easy to use, and not interfer with the flow of water around the model.