INGENIEUR
taken and recorded . The distance normal from the original path is also determined . When the ship ’ s change of course of 20 ° is attained , the rudder is again deflected as quickly as possible to 20 ° Starboard .
The tests are repeated in the same manner for a number of cycles for different speeds and the same measurements and recordings are made . The measurements recorded during the tests are plotted to generate the curves and establish the characteristics as shown in Figure 2 .
The smaller the values obtained in the analysis of the overshoot yaw angle , distance from the original path , period of cycle , the time taken to attain 20 ° change of course ( the time taken between the deflection of rudder angles ) mean that the ship possesses better response characteristics or behaviour with respect to rudder deflection , in comparison with bigger values .
SPIRAL MANOEUVRING TEST
The main objective of carrying out a spiral manoeuvring test is to evaluate the directional stability of ships in calm water . This test is sometimes called the Dieudonne spiral test to commemorate Admiral Dieudonne , a naval officer of the French navy who created the test . The test is conducted using a model ship fitted with full appendages and a self-propulsion system comprising a motor , propeller , shaft , rudder and control and monitoring gadgets . The test starts with the model moving in a straight direction or path until it reaches a constant speed . Then the rudder is deflected to 15 ° starboard until a steady or constant turning rate is achieved . The test is continued with the rudder deflected to 10 ° starboard until a steady turning rate is achieved and subsequently the rudder is deflected to 5 ° starboard until a steady rate of turn is attained .
The test is repeated in the same manner with the rudder deflected to the port for rudder angle deflections of 15 °, 10 ° and 5 °. The measurements recorded in the test are rudder angle deflection , speed , ship ’ s course for selected intervals of time , water temperature and density .
Figure 3 : Curves of Spiral Manoeuvre
The steady rates of change of course for each rudder angle deflection are then determined . This is obtained from the slope of the curve of change of course versus time for each rudder deflection to the port and starboard sides . The data of rates of change of course or steady turning acceleration ( ŕ ) is then plotted against rudder deflection ( δ ) of 15 °, 10 ° and 5 ° port and starboard . The directional stability and characteristics or behaviour of the ship are represented by the shape of the curves shown in Figure 3 .
PULL-OUT MANOEUVRE TEST
Other than spiral manoeuvre test , the directional stability of a ship can also be evaluated by performing a pull-out manoeuvre test . The test is carried out with a model fitted with full appendages and a self-propulsion system . The test starts with the model moving at a steady speed in a straight line course and the rudder is set at amidship that is in forward and aft direction . The rudder is then deflected to an angle to the port ( or starboard ) and the model is allowed to change course until a steady rate of change of course is achieved and the rudder is then brought back to the amidship or centreline .
The measurements recorded in the test are rate of change of course for every rudder deflection , speed , water temperature and density .
52 VOL 83 JULY-SEPTEMBER 2020