Ship motion test

In marine engineering, hydrodynamic tests are performed with ship models, for the purpose of designing a new (full sized) ship, or refining the design of a ship to improve its performance at sea. Tests are carried out in a ship model basin or "towing tank". There are various types of test: the model may be towed along a straight line or circular path, and may be subjected to oscillations. Forces acting on the vessel are measured using a dynamometer. The tests may be evaluating the overall design, or focusing on the characteristics of a propeller.

The velocity-dependent derivatives Yv and Nv of the ship at any draft and trim can be determined from the model test carried in the towing tank. The model is towed with a constant velocity corresponding to a given ship Froude number at various angles of attack, β. A dynamometer at the origin O, measures the force Y and the moment N experienced by the model at each value of β. The dimensional ship values of the derivatives can then be obtained by multiplying the non-dimensional derivatives by the same respective combinations of ship length, ship speed and sea water density.

It can also be used to determine the cross-coupling effects of v on Yδ and Nδ and of δR on Yv and Nv.

The Rotating Arm facility measures the rotary derivatives Yr and Nr on the model, in a special type of towing tank and apparatus called a rotating-arm facility. In this facility, an angular velocity is imposed on the model by fixing it to the end of a radial arm and rotating the arm about a vertical axis fixed in the tank. The model is oriented with its x-axis and z-axis normal to the radial arm and it is attached to the arm preferably at the model’s midlength. As a result of the particular orientation, as the model revolves about the tank axis, rotates at the rate r while its transverse velocity component v is at all times zero (yaw angle of attack β=0), and its axial velocity component u1 is identical to its linear speed. The model is rotated at a constant linear speed at various radii R, and the dynamometer measures the force Y and the moment N acting on the model. The derivatives Yr and Nr are obtained by evaluating the slopes at r=0.

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