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The velocity-dependent derivatives Yv and Nv of the ship at any draft and trim can be determined from the model test carried in 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.
It measures the rotary derivatives Yr and Nr on the model, 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.
The rotating arm facility can also be used to determine Yv and Nv as well as Yr and Nr. By cross plotting the values of Yv and Nv obtained at each r-value against r, the values of Yv and Nv at r=0 can be obtained.
It can be used instead of Rotating Arm Test where towing tank facilities are longer and narrower. It can measure velocity dependent derivatives Yv and Nv, the rotary derivatives Yr and Nr as well as the acceleration derivatives Yύ and N ύ.
The PMM consists of two oscillators, one of which produces a transverse oscillation at the bow and the other a transverse oscillation at the stern while the model moves down the towing tank at the constant velocity as measured along the centerline of the towing tank.
Some of the formulations given by the IITC 1978 for the performance prediction method are as follows:
VRM2 = VAM2 + (0.75π nM DM) 2
RncM = VRM cM / υM
KTS = KTM - ΔKT
KQS = KQM – ΔKQ
Where,