representing a true sector of a circle.

All screw propellers having a pitch equal to their diameters have the
same angle for their blades at their largest diameter.

When Pitch Is Not Uniform.

A screw propeller not having a uniform pitch, but having the same angle
for all portions of its blades, or some arbitrary angle not a true
pitch, is distinguished from one having a true pitch in the variation of
the pitch speeds that the various portions of its blades are forced to
travel through while traveling at its maximum pitch speed.

On this subject Mr. R. W. Jamieson says in Aeronautics:

"Take for example an 8-foot screw propeller having an 8-foot pitch at
its largest diameter. If the angle is the same throughout its entire
blade length, then all the porions of its blades approaching the hub
from its outer portion would have a gradually decreasing pitch. The
2-foot portion would have a 2-foot pitch; the 3-foot portion a 3-foot
pitch, and so on to the 8-foot portion which would have an 8-foot pitch.
When this form of propeller is caused to revolve, say 500 r.p.m., the
8-foot portion would have a calculated pitch speed of 8 feet by 500
revolutions, or 4,000 feet per min.; while the 2-foot portion would have
a calculated pitch speed of 500 revolutions by 2 feet, or 1,000 feet per
minute.

Effect of Non-Uniformity.

"Now, as all of the portions of this type of screw propeller must travel
at some pitch speed, which must have for its maximum a pitch speed
in feet below the calculated pitch speed of the largest diameter, it
follows that some portions of its blades would perform useful work
while the action of the other portions would be negative--resisting
the forward motion of the portions having a greater pitch speed. The
portions having a pitch speed below that at which the screw is traveling
cease to perform useful work after their pitch speed has been exceeded
by the portions having a larger diameter and a greater pitch speed.

"We might compare the larger and smaller diameter portions of this form
of screw propeller, to two power-driven vessels connected with a line,
one capable of traveling 20 miles per hour, the other 10 miles per hour.
It can be readily understood that the boat capable of traveling 10 miles
per hour would have no useful effect to help the one traveling 20 miles
per hour, as its action would be such as to impose a dead load upon the
latter's progress."

The term "slip," as applied to a scre