Predicting Speed with PropExpert
Overview
A propeller selection is based
on solving for a proper combination of diameter, pitch, blade area
ratio and RPM at a particular "design point". Each design point
is described by a design power, design RPM and design speed.
Design power is typically some
percentage of the engine's rated power. This percentage of power,
usually in the range of 85% to 100%, is sometimes called the %MCR
 or the percentage of the engine's Maximum Continuous Rating. Design
RPM is almost always the engine's rated RPM, although it may be
less if the design is to be based on a cruising speed, for example.
Both of these values are based on published engine specifications
and are generally welldefined.
The design speed, on the other
hand, is often based on a calculated prediction of top speed  or
even a guess. One of the biggest challenges of propeller sizing
is to determine the correct design speed  particularly with new
installations where we know very little about the potential performance
of the boat. In many cases, all we know about the boat is an overall
length and maybe the weight. A prediction of speed based on such
little data needs to be used with an appropriate amount of care.
However, there are some strategies that you can use to reliably
predict speed. The following are a few suggestions to help you consistently
get an accurate speed prediction with PropExpert.
Predicting speed with the
"Average hull formula"
PropExpert has a method  the
"Average hull formula"  that helps it predict the speedpowerthrust
relationship. It is a simple approach used with simple data. Although
we continuously check the method against real boats to find ways
to improve the prediction, there will always be boats that are better
or worse than any "average" prediction.
If you find
that your boats (or your client's boats) are consistently better
or worse than "average", there are a few things that you can do
to improve the prediction. (In other words, if your boats run faster
in service than PropExpert predicts, you can use historical information
to help PropExpert predict speed more accurately.) Two ways to alter
the prediction are a) to change the vessel weight or b) to apply
a multiplier to the "Average hull formula" (see the figure to the
right).
The effect of altering the
multiplier is to shift the speedpowerthrust curves. (Using a multiplier
of 0.900, for example, reduces the power and thrust required at
each speed to 90% of the "average hull" value.) Look at the figures
below to graphically see the effect of using a multiplier or changing
weight. The curve on the left is the "average hull" prediction,
the revised curve is on the right. If we were installing 1000 hp,
for example, the top speed prediction would change from about 18
knots to about 21 knots. Our design speed would thereby increase,
as would the pitch selected by PropExpert.
So how do you determine what
multiplier to use? One way is to keep track of actual performance,
and go back and review your sizings. A better way, however, is to
use sea trial data to develop these figures (see below).
Predicting speed with sea
trial data
Sea trial data is generally
not available for new boats before delivery, but it should be after
delivery and for repowers. Predicting speed for repowers is easy
and quite accurate with the "Based on prior trial" option. By entering
information about the existing engine, gear and propeller  and
the actual speed and RPM at trial  PropExpert can accurately determine
the required propeller thrust and vessel drag at the trial speed.
(It is critical, of course, that all data be accurate and that the
"prior trial" analysis passes all of the data checks.)
The "Average hull" curve is
then fit through this point of trial performance. Any new prediction
of speed for the repower will then be based on this shifted performance
curve. This has the same effect as manually entering a multiplier,
but we have let PropExpert calculate a proper multiplier based on
actual performance on the water.
You can also use the "prior
trial" analysis to evaluate a boat after delivery to help you develop
multipliers for future predictions. Make a chart and keep track
of how your boats compare against the "average hull". You should
find consistent trends before long.
A more thorough approach is
to add sea trials that you have evaluated into PropExpert's "vessel
database". You can then predict speed "Based on a similar vessel".
To use this approach, choose a sea trial of a boat that is as close
as possible the new boat. PropExpert will make all of the necessary
corrections for length and weight, and will predict speed by looking
at both the "average hull formula" and the actual performance of
the boat in the database.
