Direct Measure of Rigid Body Accelerations for Wave Impact of a Planing Hull

Abstract

Measurements of accelerations during wave impact of a planing hull are usually contaminated by nonhydrodynamic disturbances inherent in full-scale tests and by vibrations induced by the test model and towing carriage in towing tank tests. As a consequence, researchers have resorted to the use of electronic filters to extract rigid body hydrodynamic impact accelerations. This article first calculates the rigid body impact acceleration time histories of two-dimensional deadrise wedges impacting on a level water surface as a function of initial contact velocity, deadrise angle, and unit drop weight. It also calculates the spectral content of these time histories. It then demonstrates how the time histories, time to peak, and spectral content of these 2D wedges are distorted by the use of standard “one-way” electronic filters when processing the data. In a sense, this section of the article can be taken as a simple tutorial on the impact process. The Davidson Laboratory suggests and demonstrates the use of a rigid “free-running” model that is not rigidly connected to the towing carriage and is thus devoid of carriage-induced disturbances. This obviates the use of filters in processing the recorded data. Hence, the directly measured impact accelerations are thus rigid body hydrodynamic accelerations. These are compared with measurements made with the model rigidly attached to the carriage to demonstrate its effect on contaminating the recorded hydrodynamic signal. It is recommended that other towing tanks consider the use of this or other “free-model” test procedures to identify the possible contamination of the recorded acceleration time histories introduced by their carriage and model disturbances. It is also recommended that for those full-scale tests, where the “Standard G” method of data reduction has been applied, that a model be built and tested using the “free-model” test procedure suggested in this article. This will compare the derived rigid body accelerations with the true hydrodynamic impact accelerations as obtained in these free-model tests.