Prediction of- Steering and Manoeuvring of Ships 



-15000 



-10000 

 - 5000 



1500 



1000 



500 





 



20 25 30 



Ship Speed , knots 



Fig. 28 - Variation of dimen- 

 sionless drift-angle coeffi- 

 cients with speed 



measurement of the coefficients Y^, 



N...... 



Y' 



Yl. 



N' N' N' 



and 



Forces and moments due to rudder angle depend on the flow velocity 

 around the rudder. This is influenced by ship speed, and, for a rudder in a 

 propeller slip stream, by propeller revolutions. The speed of the origin is then 

 not a representative velocity, since the flow velocity can vary markedly with 

 propeller revolutions for constant ship speed. 



Figures 29 to 31 inclusive show measurements of side force, turning mo- 

 ment, and longitudinal force due to rudder angle, made in "static-rudder-angle" 

 tests at 7 to 15 knots in steps of one knot, with the Mariner model described in 

 Ref. 5. The tests were made with constant propeller revolutions corresponding 

 to the ship self -propulsion point at 15 knots. The numerical values and coeffi- 

 cients of the polynomials are given in Table 9. 



In contrast to the results of the "static -drift -angle" tests described above, 

 the nondimensional forces and moments vary markedly with speed, becoming 

 infinite at zero ship speed. The coefficients of the fairing polynomials are 

 plotted against nondimensional speed Au' = (u-Ui)/u, in Fig. 32. 



The crosscoupling terms Y^^, \,^,^, N^^, N^.g^, and \,,^ obtained from 

 these data are of importance in the prediction of radical manoeuvres when speed 

 loss is appreciable. Since the propeller slip stream has a strong influence on 



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