van Manen, Oosterveld and Witte 



effect was also measured by the authors of Ref. 13; they went to higher v u val- 

 ues than English and established that the total side force and effective moment 

 acting on their model both had a minimum for V u at about 0.4 to 0.5. 



This peculiar behaviour can be understood from the work of Keffer and 

 Baines (14). They found that for V U values higher than 0.2, a jet which was 

 initially blown through a hole in a wall perpendicular to the mainstream turns 

 90° and tends to cling to the wall creating a region of low pressure downstream 

 of the nozzle. This low pressure region creates a suction force which tends to 

 diminish the effect of the thrust unit. For ships with jet steering this is an im- 

 portant effect since the center of action of the suction force moves aft for higher 

 V/u values, hence the turning effect on the ship is no longer proportional to the 

 side force on the ship and must be assessed in terms of both side force and 

 turning moment. 



A qualitative view of the effect of the forces and moments involved is given 

 by Fig. 13. 



///// \Si^ PRESSURE REGION 



v = o 





Ms=Tx-Sy Tc-» 

 Ts = T-S 



Fig. 13 - Forces and moments on a 

 moving ship with a bow thruster 



When V is increased, the center of the suction force S moves downstream 

 decreasing the effective turning moment M^ on the ship. This turning moment 

 passes a minimum and then increases again. When S passes the center of the 

 ship, M^ has the same value as when v - 0. In Fig. 14, taken from Ref, 13, the 

 dimensionless moment M^ pAU^x is plotted against the velocity ratio V U, and 

 the curve clearly displays the effect. The line T^ ,. AU^ against V U is also 



256 



