10 feet diameter having a surface of 40 square feet would produce
a very different thrust from that given by a screw having the same
surface but 13 feet diameter. Moreover, experience has shown that
with two blades the total surface may be smaller than with a pro-
peller of the same diameter and pitch, but with three or four blades

Professor Cote rill has shown in a very interesting paper read
before the members of the I.N.A. that it is only a common screw,
that is, one with very broad tips, that can make a complete column of
water, and then only at its outer part.
The author, for these and other reasons, therefore, prefers to take
diameter and square root of blade area multiplied together as one of
the governing functions instead of area simply. He also finds that
within the limits of practice thrust varies inversely with the pitch
ratio.
Then, if A is the aggregate area of active blade surface in square
feet, D the diameter in feet, V the speed of the screw in feet per
second, P r the pitch ratio, and G the ratio of the distance of the
centre of gravity of the blade face from the boss to half diameter of
screw, the rule is : —

™ . . , Dxn/AxV 2 ,
lhrust in pounds = p x G.
