Demonstrating the Effect
of Vessel Weight Change with PropExpert
A HydroComp Technical Report
Report 126
One
interesting application for PropExpert is to demonstrate the effect
on performance of an increase in vessel weight. This might be
used to evaluate potential increases in engine and propeller load,
cavitation or fuel consumption. There are two simple techniques
that can be used to show this effect.
Correlation
to a Sea Trial
The
most accurate way to demonstrate this is to build a new speed
prediction for an increased weight from a sea trial of the boat
at its expected operating condition. For example, let's say that
a builder wants to modify his boat with heavier engines, additional
equipment and more fuel, and that this amounts to a weight increase
of 20%.
We
would first define the vessel for its initial condition, say 14000
kg, with a speed prediction based on a prior trial at
this condition. Once the sea trial condition has been analyzed
in PropExpert  finding its drag, thrust, efficiencies, cavitation
 this information can be saved to PropExpert's Vessel
database.
Then,
the weight of the vessel can be increased by 20% and the speed
prediction is now based on a similar vessel  the original
lighter weight trial. This technique "nondimensionalizes" the
basic speedto drag parameters so that a proper higher drag is
predicted for the increased weight.
Continue
with the sizing and system analysis, and you will see the new
results, including the reduction in achievable top speed and the
increase in power and fuel consumption.
The
accompanying graphs show a beforeandafter example of a 20% weight
increase on a planing hull. The top speed of 23 knots was reduced
to about 20 knots and the prop curve shows a noticeable shift
to the left  indicating potential engine overload.
An
approximation
If
you are lacking sea trial data or want a very quick look at the
effects, you can simply modify the multiplier for the Average
hull formula. This will shift the drag curve and you can produce
the same kind of results, albeit without the real correlation
to the actual vessel.


Before 
After 
