Norrbin 



Om RPM n" 

 15 



20 



■5- 



150 



100 



50 



-50 



0- 



5L 



- Ship 

 — Model 



■~- -0 2 



0-25 



050 



075 



100 



1 25 



Fig. 21, Relative change of rudder control force with change of 



propeller advance conditions for 3,6 m model and 98 000 

 tdw tanker prototype. Diagram based on model tests at 

 VBD and SSPA , and on speed trials. 



where typical values are ky= - 0.5 and k^. = 0.5. (An alternative 

 but less explicit method to include the same phenomena is given by 

 Strom-Tejsen and Chislett [68] , who make use of a number of 

 coupling derivatives such as Yg^^, etc.) 



Helm Control 



The manual or automatic pilot exerts the control through the 

 steering gear, which is supposed to have a time constant T^, 

 causing a small delay in the rudder angle 6 obtained. The value of 

 Tp. may vary say between 0.4 and 4s, the first figure being a good 

 catalogue value and the second one not seldom realized in shipboard 

 testing. The steering gear or telemotor system often has a back- 

 lash of about half a degree. 



The function of an auto -pilot may be said to be essentially 



860 



