Practical Propeller Modeling: From
Concept to 3D CAD Model
A HydroComp Technical Report
need a propeller design. Actually, what I need is a 3D solid
model file so that I can get a propeller made for me."
Not just any propeller,
I trust, but your own special propeller - for your own special
"Correct. I think
I know what I need in the way of a basic design, but I'm
completely baffled as to how I actually develop a CAD model."
It's a tough job,
isn't it. Propellers - more particularly the propeller blades
- are complex shapes that require just the right hydrodynamic
surface. General-case CAD tools can generally handle complex
shapes and surfaces, but you have to tell the software exactly
what shape you need. So what's the problem? Propellers do
not easily conform to linear XYZ space. They rotate and
advance along an axis. Their natural coordinate system is
helical. You'll never find a 90 degree angle on a propeller.
Let's simplify the
process of propeller design into three major steps:
- identifying the
proper hydrodynamic size and shape,
- creating the blade
surfaces and propeller design data, and
- turning blade
surfaces and propeller design data into a 3D CAD model.
The graphic to the
right provides an overview of the software needed and the
flow of information to get from concept to 3D CAD model.
1. Identifying the Proper Hydrodynamic Size and Shape
From the perspective
of thrust and torque performance, all that we really need to consider
are the surfaces of the propeller blades. Of course, we eventually
need things like the hub and root fillets, but they only minimally
affect performance. They are basically along for the ride so that
we can rotate the blades with adequate strength.
eliminate the need to deal with the definition of the blade surfaces
by using a collection of propeller parameters that ultimately
represent the surface. These parameters include both global and
detailed parameters. Global parameters define overall size and
shape from the stand-point of the 2D blade sections (i.e., how
the "wing" shape actually moves through the fluid). Detailed parameters
typically identify where the 2D blade sections are located in
space, as well as considerations for vibration, clearance, strength
shape (e.g., B-series, Gawn)
Number of blades
Blade area ratio
Root fillet radius
HydroComp's PropExpert or NavCad are used to define the global
parameters. Considerations about clearance, local flow properties,
and strength affect the detailed parameters.
is a software tool for the sizing and analysis of propellers for
work boats and pleasure craft. It was designed to provide reliable
answers with a minimum of hull data and is principally used by
vendors of engines, gears and propellers.
is a more complex - and more thorough - tool for the prediction
of resistance and power, and to size and analyze propulsion systems
including the propeller. It requires a full description about
the hull, so that complete predictions of drag and hull-propulsor
interactions can be performed.
At the conclusion
of this step, we will know the global propeller parameters based
on our analysis with either PropExpert or NavCad. We can now enter
the world of geometric design.
Creating the Blade Surfaces and Propeller Design Data
We know what
we need for propeller parameters, so we can proceed to the creation
of the 2D and 3D blade surfaces and propeller design data. Trying
to do this with general-case CAD software is like wearing the
shoe box instead of the shoe. Special software is needed to take
our parameters and design constraints, and develop the blade surfaces
and the propeller design documents. For this we call on HydroComp
a propeller design document and blade surface generator. It creates
standard propeller design drawings (using an included copy of
Visual CADD as a CAD "host") and tables of offsets.
It also develops a full 3D definition of the hydrodynamic blade
surfaces and can package this data for export to conventional
CAD, CAM and modeling software.
offers an interactive environment to rapidly build design documents
and geometric properties from your parameters. A Builder
wizard defines the complete design from a collection of standard
industry propeller styles. You can also define any arbitrary section
shape and use this as a "parent template" in conjunction
with the Builder to create derivatives of your own proprietary
main PropCad design form allows us to review the design in 3D.
The various menu commands call functions to define material, strength,
tip, hub and drawing requirements.
PropCad project form
we are satisfied with the design, we can create standard propeller
design drawings and table of offsets.
This is where
many propeller manufacturing companies end their design. Using
well-established manual techniques, these companies will build
their patterns using PropCad's various drawings and tables.
additional analyses that can be performed before taking the propeller
geometry from PropCad into 3D CAD. These are strength analysis
with FEA (finite element analysis) and detailed 3D performance
analysis with CFD (computational fluid dynamics). FEA and CFD
are broad topics beyond the scope of this report. However, we
should know that PropCad can be a very useful geometric modeler
for FEA and CFD.
graphic shows a plot of stress for a propeller designed with PropCad.
The calculation was performed with a popular PC-based FEA software
package. Data was transferred to the FEA software via the 3D DXF
document developed in PropCad (which is not a true surface definition,
but a collection of faceted panels).
Turning Blade Surfaces and Propeller Design Data Into a 3D CAD
objective is to have a propeller built - either by milling, molding
or rapid-prototyping - then we need to create a full 3D solid
CAD model. Somehow we need to for PropCad to export the surface
information to a CAD modeler. Why is a CAD modeler necessary?
Why not add these features into PropCad?
quite simply is a) it has already been done (and done quite well),
b) it is not universally required by PropCad users, and c) we
would prefer to add value into PropCad by addressing features
that are unique to propeller design. (That is also why we use
Visual CADD to handle conventional CAD tasks, such as printing
a design drawing.)
complications arise with each CAD modeling software representing
and manipulating 3D surfaces in different ways, and that modeling
requirements differ for the various types of manufacture. For
example, a milled product needs to have cutter path and milling
instructions added to the design.
this broad collection of CAD modelers by creating and exporting
ASCII files that instruct the CAD/CAM modeling software ho