Zhang, Haiyan; (1998) Determination of control characteristics of ship models by tow line tests. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis investigates both the theoretical and experimental work carried out to determine the control characteristics of ship model by tow line tests. A knowledge of the hydrodynamic derivatives are necessary for the mathematical model used in the simulation and prediction of ship manoeuvres. For more than two decades this information has been obtained largely by testing captive models on a rotating arm facility and a Horizontal Planar Motion Mechanism (PMM). These tests involve many runs in the tank with the risk of cumulative errors and are impaired by the interference of the towing struts. The work in this thesis originated from a search for an alternative simple but reliable test method to determine the control derivatives of a ship model. In the tow line tests the configuration is that of a free model on which the tow line exerts a positional force and moment when the hull departs from the in-line tow position. Some combinations of towing point and tow line length result in persistent lateral oscillations of the model, which provide a way of evaluating the control characteristics of a ship model. A linear analysis of the equations of motion for a towed vessel generated a fourth order linear differential equation in which the coefficients consisted of the same terms that occur in a free model plus terms arising from the tow line geometry. It has thus been argued that measurements of the oscillatory motion of the model would allow analysis of the hydrodynamic control derivatives of the hull form. A time stepping computer simulation of a towed vessel with known derivatives was developed and this provide evidence that for certain locations of tow point position the vessel performed regular oscillations. Small scale model tests in the Defence Research Agency (Haslar) Circulating Water Channel confirmed the simulated results in qualitative terms. Further tow test experiments were conducted in the Glasgow University Towing Tank on two models, one of the MARINER class hull form and the other of BRITISH BOMBARDIER. Some study of experiment procedure was carried out to provide a valuable guide for practical conduct of tow line tests. The analytical methods to determine the linear control derivatives from the measurements of the transient response of the tow line tests have been explored. Based on an observation of the phase relationship in the measured motions, an analytic approach similar to that used in PMM analysis was evolved. For general use a robust System Identification analysis was developed and various types of methods for modelling the sway and yaw motion equations were investigated, each being suited to particular dynamic systems. The relative merits of several alternative SI techniques were investigated with reference to the limitations imposed by the operation of model tests. The computation results for MARINER and BRITISH BOMBARDIER tests generated satisfactory and accurate sway and yaw derivatives. Simulation using the estimated derivatives was compared with measurements and the study of the resulting residuals provided further confidence of the tow line test approach to obtaining ship manoeuvring derivatives.

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