Tips for Predicting Speed with
PropExpert
A HydroComp Technical Report
Report 130
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 (see the report
on this topic) 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.
