90 
“ Screw Propulsion/’ by Robert Rawson, Assoc. I.N.A, 
Hon. Member of the Manchester Literary and Philosophical 
Society, Member of the Mathematical Society. 
Preliminary Proposition . 
1. Suppose AB in the annexed diagram to be the axis of 
the screw propeller ; draw CP perpendicular and DPE 
parallel to AB. Consider now an element of the screw pro- 
peller blade having an indefinitely small area (n) to be 
placed at P, whose distance from the axis AB is denoted by 
(r). The element (a) thus placed at P has, therefore, a normal 
line, that is, there is a line perpendicular to it which is 
called a normal to the element (a). Let GF be the projec- 
tion of this normal line upon the plane A BP ; and, further, 
suppose the position of this normal through P to be deter- 
mined by the angles a, (3, y, which it makes with AB, CP, 
and a line perpendicular to the plane ABP respectively. 
These angles are not, however, independent ; they are con- 
nected by the well known equation 
Cos 2 a -f COS 2 /3 + COS 2 y =1 1 
2. As CP revolves about the axis AB with an angular 
velocity (u), the element (a) of the screw propeller blade 
strikes the water in the direction of its normal line in 
accordance with the usual theory of fluids. And, therefore, 
