straight line motion of a ship . In the test the model is towed obliquely from the straight line path for different drift angles at a constant velocity . The side force and moment about the longitudinal centre of gravity are measured for every drift angle and velocity .
The factors measured in an oblique tow test are tow velocity , drift angle , water density , sway force and side force .
ROTATING ARM TEST
Figure 4 : Curves of Pull-Out Manoeuvre Test
Figure 5 : Oblique Tow Test
The pull out manoeuvre test is repeated for different speeds and rudder deflections . The recorded data of rate of change of course is plotted against time ( t ). The directional stability of the ship can be evaluated from the shape of the curves as shown in Figure 4 .
Analysis of pull-out manoeuvre determines the directional stability criteria of the vessel . All the derivatives discussed above have to be determined in order to evaluate the complete directional stability criteria of a ship when combined with the results of the oblique tow test , the rotating arm test and planar motion mechanism test which are described below .
Rotating arm tests are carried out in a special manoeuvring tank or in a large towing tank using a model to determine the derivatives of the primary derivatives of sway force and yaw moment
( Y V ′ ) with respect to a steady velocity ( N′ V
) and with respect to a steady turning rate . The set-up facilities of rotating arm test are shown in Figure 6 . The model is fitted with full appendages and installed onto a carriage and rotated about the centre of the turn-table at pre-selected rotating speeds . The model can be repositioned inward or outward for particular turning tests to change the radius of the turning curve . The rotating arm is fitted to a turn-table and can be rotated at a controlled speed . The maximum curvature for each rotation can be achieved depending on the length of the rotating arm .
Other than to obtain the above derivatives , the rotating arm test can be used to evaluate rudder effectiveness and to obtain rudder derivatives by measuring the side force and moment for different rudder angles for a range of yaw angles and curvatures . The measurements recorded in this
OBLIQUE TOW TEST
Oblique tow tests are carried out to determine the first derivatives of sway force and yaw moment
( Y V ′ ) with respect to a steady velocity ( N′ V
) in a Figure 6 : Rotating Arm Test
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