Of course, real propellers don’t “screw” through the
water. The difference can be thought of in two ways. The
easiest is the ratio of the actual advance to the theoretical
advance in one revolution from the pitch. This ratio of pitch
times rpm to speed is called slip. If the slip is zero, the
blade element is operating at zero lift and there is no thrust.
Engineers use a different measure that compares diameter
times rpm to speed called the advance ratio, usually
represented as J. Propellers are usually compared on the
pitch to diameter ratio. The engineering graphs will all use
the advance ratio, J. For the mathematically inclined, the
common engineering relationships are shown in the picture
on page 25.
PROPWASH
24
October 2013
Props – The Theory Part One
By Lohring Miller
NAMBA Safety Chairman
Propellers are the black art of model boat racing. In this two part
series of articles I hope to improve your understanding of how props
work and how to modify a cast blank into a racing prop. The first
part will go over some basic propeller theory. The second part will
describe the details of propeller finishing, modification, and
measurement. I’m depending heavily on
Propeller Selection for
Boats and Small Ships,
a course by Chris Barry, for the theory
section of this article.
You can think of a propeller as a magic device that takes water
and throws it backward. This creates a force proportional to the mass
of water and its speed according to Newton’s law of motion. The
ratio of this thrust to the weight of water passing through the
propeller is the thrust load coefficient. The graph below shows the
result of this on efficiency of an ideal, and some real propellers.
The most important thing to observe from the graph is that
propeller efficiency is higher as the thrust gets smaller. Because the
thrust load coefficient decreases with both the propeller area and
velocity of the water thrown backward, efficiency also increases with
a larger propeller area and a small water velocity increase. Propeller
efficiency is the ratio between the engines power and the thrust
power. Thrust power is thrust times the velocity of the propeller
through the water.
Real propellers are a series of blades that rotate around an axis.
They can be thought of as wings that produce lift and drag, just like
on an airplane. The component along the shaft is thrust and the
component across the shaft is the torque needed to turn the propeller.
The angle of the blade element to the water flow is going to vary
from the hub to the blade tip. This is because the speed of the
rotating blade element increases with the distance from the hub. We
usually want to keep this angle of attack fairly constant. An easier
way to characterize this is called pitch. Propellers were originally
called screw propellers and the pitch was the amount this propeller
would “screw” itself through the water in one revolution. Another
important basic propeller characteristic is the area of the blades. This
can be measured several different ways as the diagram in the next
column shows. The standard propeller drawing presents the same
information.