Prediction of Steering and Manoeuvring of Ships 



20 25 30 35 



MAIN SHAFT REVSJMIN. 



20 25 30 35 



MAIN SHAFT REVS./MIN. 



Fig. 33 - Results of pure sway tests: dimensional gauge forces 

 as functions of planar-motion mechanism revolutions per minute 



In the "pure yaw" tests, results similar to those shown in Fig. 33 were ob- 

 tained, the resonant ranges occurring at about the same frequencies, but the 

 variation of the measured forces in the resonant areas being less marked. This 

 is explained by the fact that the model was a more effective wavemaker when 

 oscillating with sway motions than when oscillating with yaw motions. 



To sum up, for the model and speed tested, undesirable effects due to tank 

 resonance were found to be negligible below about 12 rpm. 



While tests have been made with only one model, at and 15 knots, it is 

 considered improbable that differences in speed or model proportions such as 

 block-coefficient, length-to-draught, and length-to-breadth ratios would precipi- 

 tate the onset of resonant waves. Based on these considerations, oscillatory 

 tests made with the HyA planar-motion mechanism used in the dynamic mode 

 are normally carried out at about 8 rpm, thus allowing a reasonable safety 

 margin. 



Frequency Effects 



A more insidious source of differences between measurements made using 

 a steady-state technique such as a rotating arm and measurements made with a 

 planar -motion mechanism stems from unsteady flow effects (independent of the 

 free surface) which invalidate the assumption of quasi-steady conditions. This 

 is a similar phenomenon to the well-known unsteady lift forces which act on an 



377 



