Str^m-Tejsen andChislett 



Table 7 

 Nondimensionalizing Factors 



origin u, Nondimensional coefficients expressing such data are then also inde- 

 pendent of speed, since u^ occurs in the nondimensionalizing factors for both 

 force (or moment) and acceleration. This is supported by numerous experi- 

 mental results. For example, Fig. 22 of Ref. 11 shows complete agreement 

 between side forces and turning moments measured as functions of yaw accel- 

 eration at ship speeds of 4, 6, 8, and 10 knots, for a 53-m-Lpp trawler form. 



Forces and moments resulting from drift angle, rudder angle, and yaw 

 velocity are largely due to circulation and viscous crossflow drag. Drift-angle 

 and rudder-angle forces may be likened to lift forces acting on an airfoil having 

 an angle of attack, and yaw velocity is analogous to camber. It is thus reason- 

 able to expect the resulting hydrodynamic forces to vary proportionally with the 

 square of the low velocity as long as the flows have the same geometry, i.e., as 

 long as sinkage, trim, wave making, flow separation, etc., do not vary appreci- 

 ably with speed. 



Forces and moments due to drift angle and to yaw velocity are principally 

 dependent on the flow patterns around the hull, and the speed of the origin is 

 therefore a good representative velocity. Nondimensionalization with this ve- 

 locity results in dimensionless values which change only with flow-geometry 

 variations. 



Figure 27 shows measurements of side force and turning moment due to 

 drift angle, made in "static drift angle" tests at speeds of 5, 10, 15, 20, and 25 

 knots ship speed, with the 20-ft Mariner model described in Ref. 5. The same 

 measured values are plotted in dimensional and nondimensional form, and the 

 faired lines are polynomials of the form 



366 



