ship Maneuvering in Beep and Confined Waters 



of the "proportional + rate control" type, although an integrator 

 control shall be added to take care of stationary deviations. Com- 

 mercial type auto-pilots include special features, which shall be 

 included in simulator applications. 



With the simple ideal auto -pilot "calling for" the rudder 

 angle 6"^ = Y^" '^ ''"'4^ the transfer function of the feed-back loop is 



Y(i + -s) 



Typical values for the gain or "rudder ratio" -y and rate constant 

 a of a tanker auto pilot in deep water setting are Y = 3 (degrees 

 helm per degree heading error) and cr = 135 seconds (or 135 degrees 

 helm per degree per second of change of heading). (See Section XIII.) 



Although the course keeping characteristics of say an inherently 

 unstable tanker may be studied in a Bode diagram by use of a total 

 system open loop transfer function YiYg, where the ship dynamics 

 open loop Y| is defined from the linearized equations , this method 

 is mostly avoided if an analogue computer is available. In case of 

 small value non-linearities — such as dead zones or lags — in gyro 

 compass and telemotors the equations -of- motion technique is un- 

 avoidable. 



Much effort has been devoted to present the function of a 

 manual helmsnaan in terms of a transfer function. The helmsman 

 is a highly adaptive control system, which makes the task more 

 difficult, but which also makes it more important. In many cases it 

 is impossible to run real-time simulations because of lack of time, 

 in other cases it is impossible to run comparative simulations just 

 because of the learning ability of the operator. 



Hooft tried to evaluate criteria for manual steering of large 

 tankers by use of a transfer function, in which the gain and time 

 constants were derived by extrapolation from high frequency pilot 

 dynamics, [69] . Undoubtedly new basic information is required. 



Propeller Thrust and Shaft Torque 



A majority of ocean-going ships are propelled by fixed-blade 

 screws driven by diesel engines or steam turbines, the normal 

 steady state outputs of which in principle are characterized by 

 constant torque Q — proportional to fuel pump stroke — and constant 

 power — proportional to steam inlet pressure — respectively. 

 In running conditions the mechanical torque losses Q depend on 

 sign of r.p.m. but are more or less independent of its magnitude. 

 Shaft r.p.m. is governed by the simultaneous equations for longi- 

 tudinal resistance and for thrust and torque, for u > here given as 



861 



