Analysis of Dissimilar EngineGearPropeller
Systems with PropExpert
A HydroComp Technical Report
Report 117
Product link: PropExpert
HydroComp
PropExpert is a program for the selection and analysis of propellers
for motor yachts and workboats. The architecture of PropExpert
assumes all enginegearpropeller systems are identical (i.e.,
single screw, symmetrical twinscrew). However, it is possible
to analyze dissimilar systems by treating each system separately.
Overview
Some vessels have propulsion
systems designed to allow for independent operation in cruise
and sprint modes. For example, such a vessel might have a principal
centerline inboard propeller with twin retractable sterndrives
for use in sprint mode.
The procedure to size and
evaluate such a system is as follows:
 Choose a speed and determine
the total thrust requirement at this speed.
 Calculate the thrustmaking
capability of the principal propeller at this speed.
 Determine the required
thrust for the sterndrives by deduct the principal propeller
thrust from the total required thrust.
 Size the sterndrive
propellers and check the thrust.
Determine total thrust
requirements
You will set up PropExpert
with the principal propeller to determine the total required thrust
for the vessel. Remember that in equilibrium there must be a match
between resistance and delivered thrust. So, at the defined speed
there is only one value for required thrust  regardless of the
propeller or engine. This allows us to use any propeller and find
the total thrust required by the vessel.
 Enter the vessel information
into PropExpert and set up for the principal propeller.
 Enter the Engine data
associated with this propeller.
 Enter the propeller
Sizing data, and size the needed parameters (e.g., pitch) as
necessary.
 The analysis will find
the total required thrust, and you should copy this figure down.
You will note that the power and RPM are likely to be out of
range. This is not a problem because, as stated before, we are
only interested in the equilibrium required thrust.
Calculate the real thrustmaking
capability of the primary propeller
The engine that is driving
the primary propeller can only operate within its own rated power
and RPM, of course. Therefore, we must use PropExpert to determine
the actual thrust generated by this propeller at the design speed.
 Back in the Vessel page,
turn the Auto update checkbox OFF to retain the proper propulsive
details.
 Then change the Service
to Bollard (rated RPM). This forces an analysis of the propeller
using the rated RPM, rather than finding the equilibrium condition.
 Rebuild the analysis
in the Service page. This will display a Thrust which is the
maximum available thrust from the primary propeller.
Find the required trust
for the additional propellers
Simply deduct the available
thrust determined above from the total required to find the thrust
needed from the additional propellers. Write this figure down.
Size the parameters
for the additional propellers and evaluate the results
From this point on, you
will be running PropExpert as you would normally  with one small
exception. You will be defining the drag of the vessel based on
the needed thrust figure from above.
 Modify the Vessel information
as needed (i.e., change 1 propeller to 2, change Service back
from Bollard). Let PropExpert update the Details section as
appropriate.
 Set Prediction to Based
on similar vessel, and click Edit speed prediction data. Enter
the vessel parameters, and enter the appropriate Vessel drag.
Drag is found by multiplying the needed thrust by (1  thrust
deduction). A System efficiency is not needed.
 Enter the appropriate
Engine data.
 Enter the propeller
Sizing data, and size the needed parameters (e.g., pitch) as
necessary.
 Check to see if the
system has the ability to meet or exceed the design speed, and
you are finished.
