Prediction of Steering and Manoeuvring of Ships 



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3 6 9 12 15 

 DRIFT ANGLE, /9, degrees. 



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DRIFT ANGLE, /3. degrees 



Fig. 27 - Results of static-drift-angle tests made at different speeds 

 (a) dimensional plot, (b) dimensionless plot 



Y(v) = Y* + Y^v + Y^^^v^ 



N(v) = N* + N^v + N^^^v^ , 



which have been fitted to the points by a least-square procedure. The numerical 

 values and coefficients of the polynomials are given in Table 8. The coefficients 

 are plotted against speed in Fig. 28. 



It is seen that the nondimensional values are almost independent of speed 

 up to 20 knots. Above about 20 knots, the nondimensional forces acting on the 

 hull increase in size. This was due to sinkage of the model both forward and 

 aft. Two effects were apparent. The model sank more as the speed increased, 

 increasing the lateral area and hence the linear coefficients, and for a given 

 speed, sinkage increased with drift angle, causing the cubic terms to become 

 more pronounced. Sinkage, both with speed and drift angle, was greatest at the 

 bow, probably because the crossflow under the forebody increased the local 

 velocity and reduced the hydrostatic pressure more than was the case in the 

 afterbody, where the flow had become straightened somewhat. 



On the basis of similar results obtained for angular velocities, it can be 

 concluded that Y^, Y^, N^, and n; are largely independent of speed over the 

 speed ranges commonly covered by merchant ships. It has consequently not 

 been found necessary to include tests in the standard program (Table 6) for 



367 



