speed. A calculator is available at www.marksmarine.com.
You can get an idea of the needed pitch if you know your
boat’s speed and engine rpm. As an example assume the
boat goes 60 mph and the propeller turns 16,000 rpm. The
boat is traveling 60x5280/3600 or 88 feet per second.
That’s 88x12 or 1056 inches per second. The propeller is
turning 16000/60 or 266.7 revolutions per second so it
needs to advance 1056/266.7 or 3.96 inches per revolution.
If there’s 10% slip that means the prop will only go 90% as
far each revolution as the measured pitch. That means the
prop needs a pitch of 3.96/.9 or 4.4 inches. With that
information you can look at the various tables that show
prop pitches and select a prop.
Most companies list props with a diameter and pitch to
diameter ratio. Thus an Octura X470 has a 70 mm diameter
and a pitch 1.4 times that or 98 mm. Dividing the metric
dimensions by the 25.4 millimeters in an inch gives 2.76
inches diameter and 3.86 inches pitch. The new ABC props
use inch measurements so an ABC 2715 has a diameter of
2.7 inches and a pitch of 1.5 x 2.7 or 4.05 inches. Our 60
mph boat might need a little more pitch.
What about the diameter? Since there’s no engineering
information on model props, you need to rely on
experience. The pitch of a prop won’t change if you change
the diameter, so if the prop bogs the engine down or draws
too much current, a small reduction in diameter will help.
Look at the equations in the beginning and realize that the
torque varies as the fifth power of the diameter.
Of course there are a lot of other changes you can make
to match a prop to your boat and get the best speed. We will
cover all that in the next part in the April 2014
Propwash
.
PROPWASH
October 2013
27
All these considerations have made theoretical analysis of surface
piercing props difficult. This hasn’t stopped either full size or model
boat racers. When an early boater noticed his boat went faster with
the prop partially out of the water, the race was on. One of the first
well known examples was Rainbow IV in 1924. Since that time
nearly all the racing development has been by trial and error. This is
especially true for model props. It’s easier and probably more
accurate to try a number of props then to try to calculate what should
work.
However, a little basic prop math will help. The most important
mathematical relationship to understand is the relation of pitch to