Norrbin 



comparative studies. The diagrams in Figs, 43 and 44 refer to 10° 

 course change maneuvers predicted for the tanker at two speeds in 

 deep water and at the lower speed in shallow water, all executed 

 using the same normal setting of auto pilot controls. There are 

 several overswings in shallow water, and checking helm is large. 



The final diagrams in Fig, 45 furnish a condensed illustra- 

 tion to the changing problems of course control in shallow waters 

 and in canals. These problems are also dealt with by Eda and 

 Savitsky [ 100] , and in considerable detail by Fujino , [ 101], 



It is assumed that the ship is moving at low speed on a 

 straight course parallel to the required track (in a buoyed channel, 

 say) but off- set 20 m to the starboard side. A signal proportional 

 to this lateral error, calling for i degree rudder per m off track, 

 is fed into the auto pilot. The upper curves for the free water con- 

 ditions demonstrate that the rudder ratio setting must be increased 

 (fronm normal 3 to say 10) in order to stabilize the ship on the re- 

 quired track. This control works reasonably satisfactory also in 

 shallow water, but it tends to make the ship over-shoot the centre- 

 line in the alternative case between parallel walls in deep water. 

 Obviously the presence of the near wall accelerates the first swing 

 towards and beyond the centre-line. 



The two lower curves of Fig, 45 relate to the ship in a 

 typical part of the Suez canal. In the shallow water the effect of the 

 near wall is even more pronounced, and the stern of the ship is in 

 danger of hitting the bank. However, by turning down the latercd 

 error knob to zero the auto pilot is made to behave like the ex- 

 perienced helmsman, already referred to in the introduction. Thus, 

 the ship first sheers bow -off the wall before the auto pilot applies a 

 counter- rudder in order to slowly press the ship laterzilly away 

 from the wall. The ship is seen to be almost steady on to the centre- 

 line within two ship lengths. 



REFERENCES 



1. Norrbin, N. H. , and Goransson, S. , "The SSPA Steering and 



Maneuvering Simulator , " (in Swedish), SSPA Allm. Rapport 

 No. 28, April 1969, 



2. Mandel, Ph., "Ship Maneuvering and Control," in "Principles 



of Naval Architecture" (revised). New York 1967, 



3. van Berlekom, W, , "Summary of a Simulator Study of Maneuver- 



ing Large Tankers in the Approach to a Port at Brofjorden," 

 SSPA PM No. 1643-10, Sept. 1969. (Contract Report) 



896 



