Demonstrating the Effect
of Vessel Weight Change with PropExpert
A HydroComp Technical Report
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.
to a Sea Trial
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
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
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 "non-dimensionalizes" the
basic speed-to- drag parameters so that a proper higher drag is
predicted for the increased weight.
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.
accompanying graphs show a before-and-after 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.
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.