52 
MR. RENNIE’S SPIRAL PROPELLER. 
Fig. 41. Fig. 42. 
is to reduce the loss of effect which is occasioned near the centre of a screw by 
the large proportion which the diagonal bears to the direct effort. By referring 
to the Tables, page 32, it will be seen that very little of the power expended 
near the centre is employed to propel the vessel, and hence a reduction in the 
length must obviously be attended with advantage.