Ship Maneuvering in Deep and Confined Waters 



"N 





Fig. 39. 98 000 tdw tanker zig-zag test In deep water. Comparison 

 of full scale trials and computer prediction. 



v|r| and |v|r are not derived from measurements with this model 

 but taken from an analysis of rotating arm tests with another tanker 

 form, and the almost exact prediction of overswing angles might be 

 sonnewhat accidental. 



The good correlation of speed loss in the zig-zag maneuver 

 is satisfying. The phase difference is likely to be due to the stern 

 position of the ship's pressure-type speed log. 



The ship (and simulator model) is slightly unstable on 

 straight course in deep water; the total loop width is about 3.5° 

 at slow speed as well as at high. In Fig. 41 is also shown the 

 spiral prediction for shallow water (t, = 3.37 or h/T = 1.3). Here 

 the initial stability is further impaired, whereas the stability in a 

 turn is increased. A major factor governing the dependence of 

 initial stability on water depth is the change of Yyr • From Fig. 33 

 was seen that YjiVr is negative for this model, so that the value of 

 U' = (x^ - Nil, - NjrU/(i - Y^'r - Yjirtt,) may diminish much faster 

 with increasing C T:han does l^' = (nJ^ + N^'^^ t,)/(Y^'^ + Yj;^ (,), 



891 



