Prediction of Steering and Manoeuvring of Ships 



Table 2 

 Typical Range of Dimensionless Motion Parameters for a Cargo Ship 



When testing with the HyA planar-motion mechanism, the range of motion 

 parameters a ship will encounter can be adequately covered with the exception 

 of yaw velocity. In the static mode of operation, for example, drift-angle tests 

 can be executed with drift angles up to ±20 degrees corresponding to v' = ±034, 

 which is comparable to the maximum sway velocity experienced by naval ships. 

 In the dynamic mode of operation, the range of motion parameters can be ad- 

 justed by varying the amplitude at the scotch yokes and the shaft revolutions. 

 Adequate coverage of the desired range of yaw and sway accelerations can easily 

 be obtained in these tests. 



For the crucial yaw velocity. Fig. 22 gives, for maximum amplitude, the 

 relationship between nondimensional yaw velocity and shaft revolutions as a 

 function of model speed and model length. For example, in the case of a cargo 

 ship model with a maximum nondimensional yaw velocity r' = 0.4, model length 

 6 metres, and model speed 1.4 metres/sec. Fig. 22 shows that shaft revolutions 

 of 12 per minute would be adequate. Although this is well within the structural 

 capability of the mechanism, experience has shown the practical limit of opera- 

 tion to be around 8 to 10 revolutions per minute. 



This limitation is due to wave reflections from the basin walls, which for 

 higher shaft revolutions interfere with the force measurements. As the prob- 

 lem is associated with a system of standing waves built up in the basin at cer- 

 tain frequencies, it is in general independent of model speed and size. 



With a maximum shaft rpm of approximately 10, it is seen from Fig. 22 

 that the HyA planar -motion mechanism gives an acceptable coverage for 6 to 8 

 metre cargo ship models, whereas for naval ships, when smaller models would 

 often be used, the desired range of yaw velocities can be covered only at lower 

 speeds. 



It would normally be desirable to cover the complete range as given in 

 Table 2. In some circumstances it is possible, however, to economize on the 

 number of captive-model experiments necessary for accurate simulation of a 



349 



