THE CAPACITANCE METHOD OF MEASURING THE WETTED SURFACE AREA OF A SHIP MODEL

The High Speed Marine Vehicle Committee of the International Towing Tank Conference has recommended that in tests on models of high speed marine vehicles the actual wetted surface area should be used to calculate the frictional resistance component in the scaling procedure rather than the static wetted surface area used for low speed ships. This recommendation has been made since it has been shown that the wetted surface area can vary significantly from the static wetted area, particularly at higher speeds and therefore affect the predicted power requirement. The Committee has also recommended that, since there is no universally agreed method of measuring this wetted surface area in model tests, the method of measuring the area should also be given. A new method of measuring the actual wetted surface area has been developed. The method is based on capacitance where the model hull has been given a metallic coating and then an insulating coating so that it effectively becomes one plate of a capacitor with the water of the towing tank or water channel becoming the other plate. The experiments have shown it to be a viable technique. It has the advantage that it can be undertaken at the same time as the standard resistance tests while not adding to the manpower requirement such as that necessary with visual observation of the wetted surface area.