Ship- Maneuvering in Deep and Confined Waters 



VII. SPEED AND STEERING CONTROL 



In general the subject of steering and maneuvering may not 

 be separated from that of propulsive control, and this is specially- 

 true in case of ship behaviour at slow speeds. Moreover, in model 

 testing the interactions between hull, propeller, and rudder are 

 likely to cause the main problems of model-to-ship-conversion, 

 including scale effects of a hydrodynamic nature as well as other 

 model effects due to the dynamics of the testing equipment. 



Large seagoing ships are usually propelled by a single centre 

 line screw, or by wingward twin screws. In case of a tandem contra- 

 rotating propeller arrangement most of the characteristics discussed 

 below may be calculated for an equivalent single propeller. In case 

 of close-shafted twin screws of overlapping or interlocking types 

 the interaction with the rudder should be specially considered. 



It has been repeatedly proven by handling experience that 

 twin screw ships should be fitted with twin rudders. Recent model 

 tests indicate that with a suitable design of the rudders, including 

 a certain neutral position toe-out, this arrangement may favourably 

 compete with the centre line rudder alternative also from a propulsive 

 performance point of view. 



In the application of the first-order steering theory, first 

 introduced by Nomoto in 1956 and strictly valid only for inherently 

 stable ships, there appear only two constants: a (desired high) 

 "gain" K, which represents the ratio of rudder turning moment to 

 yaw damping, and a (desired low) "time constant" T, which 

 measures the sluggishness of the ship response, and which repre- 

 sents the ratio of ship inertia to yaw damping. As was subsequently 

 also shown by Nomoto [55] the non-dimensional quotient K'/T' 

 turns out to be proportional to the parameter LA^/V for ships with 

 similar stern arrangements. This quotient may therefore be looked 

 upon as a rudder- on- ship effectiveness factor, proportional to the 

 initial yaw acceleration imparted to the ship by a given helm. 



Some ten years ago maneuvering trials were run with three 

 tankers of the Gotaverken 40 000 tdw series, all similar except for 

 the stern arrangements » [56] . The SSPA analysis of zig-zag tests 

 with respect to the rudder-on-ship effectiveness factor just mentioned 

 offers a unique illustration of the merits of these arrangements. 

 Fig. 18. In particular, note that the two alternatives with rudder 

 behind screw (screws) prove to be equivalent in case of same total 

 area of rudder, and that the use of the larger area of a twin alter- 

 native therefore is especially favourable. 



A propeller or a rudder, or the combination of a propeller 

 and a rudder, acts as a stabilizing fin as well as a manoeuvring 

 device; the contributions to the fin effect from the propeller and from 

 the rudder-behind-propeller are of equal order. It should be 



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