ship Maneuvering in Deep and Confined Waters 



8 ■ 



0-5 • 



?s^7. 



Vl^ 



Fig. 24. Asymmetric forces and-raoments on a twin-screw tanker 

 model moving parallel to the vertical walls of canals of 

 differing widths and depths (From DTMB test data) 



The theoretical results for bow-away-from-wall moments 

 are somewhat modified in practice, where bow wave and screw 

 action contribute to make the tendency felt in ships of all types. 

 Thus, in general a ship that moves off the centre-line of a canal 

 must use helm towards the near wall and it takes up a small bow- 

 from-wall equilibrium, angle. Typical values derived from [94] 

 for the 721' tanker off-set 50' from centre-line of the 500' X 45' 

 section are 15° helm and 1° drift. 



The motion in shallow port approaches may involve much 

 larger drift angles , and the behaviour of the ship is nnarkedly 

 affected by the increase of lateral cross flow resistance due to 

 under-keel blockage. The diagram in Fig. 25 is compiled from 

 shallow water test data in Ref . [ 49] , and from Japanese data in 

 Ref. [ 96] , which also include measurements in presence of a wall. 

 Again the parameters C, and r| are used for the presentation. For 

 moderate C, cross-flow drag increases in proportion to t, , just as 

 the linear force derivatives, but the dependence on r\ is of higher 

 order. The cross -coupling between t, and t] may probably be 

 ignored in practical applications. 



875 



