10 15 20 



TOTAL THRUST COEFFICIENT C, 



Fig. 11 - Ducted and unducted propellers: 

 comparison of efficiencies and diameters 



8_ 



Ct rotor 



001 ooa 



TIP CLEARANCE RATIO 



003 004 05 



TIP CLEARANCE 



INTERNAL DUCT DIA. AT ROTOR TIP 



Fig. 12 - Effect of tip clearance on ducted 

 propeller efficiency (calculated for a con- 

 stant power coefficient Cp IZ) 



3. Vertical-axis propellers: 



The much lower propulsive efficiency of vertical axis propellers makes it 

 clear that the main reason for using such propellers is the very consider- 

 able advantage they give in providing a large steering power at low ship 

 speeds. However, engineering developments have so far limited the total 



910 



