TRACT Vertical acceleration measurements are often used to evaluate the “rigid body” response of a planing hull to hydrodynamic forces in waves. Unfortunately accelerometers respond to both the rigid body hull motions of interest and to unwanted vibrations, which if not addressed, produce artificially higher peak acceleration values (Riley, et.al, 2010). In full scale hulls, vibrations from the propulsors are telegraphed through the hull structure to the accelerometer. In towing tank models vibrations from the carriage are transmitted by the tow post through the hull and to the accelerometer. Historically, different methods have been used to eliminate the unwanted acceleration components including engineering judgement, electronic low-pass filtering of analog signals and postprocessing of digital measurement records using computational filtering techniques.
This paper documents a study of the effects of different towing methods on planing boat model accelerations. A four foot long planing hull was tested in calm water and in waves using three different towing methods:
- Traditional heave post with model towed at constant velocity
- Self-propelled model mounted on a lightweight free-in-surge sub-carriage
- String tow bridal with spring as proposed by Savitsky 2016
Tests were conducted in regular waves which made it possible to overlay accelerations peaks from a large number of nearly identical hull slams and make direct comparisons of the magnitude and shape of acceleration peaks measured with each towing method. Details of the three towing methods and the pros/cons of each are presented. The string tow method produced significantly cleaner acceleration THE 30th AMERICAN TOWING TANK CONFERENCE WEST BETHESDA, MARYLAND, OCTOBER 2017 2 records. The plots presented make a strong case for this simple and unconventional towing method and may encourage other towing tank facilities to experiment with it in the future.
