20 December 2006
This page contains a brief summary of how we
use CAD during the development of a new yacht design,
and then leverage that work for actually building the vessel by
direct use of the computer-generated model.
We view the Design of a new yacht as being divided into three
basic stages:
I. The Preliminary Design Sketches
II. The Study /
Estimating Plans
III. The Final Building Plans.
Those are the basic 'design stages' required before a new yacht design
is ready for the builder. For a thorough review of the Design
Process itself please review our separate
article on Custom Design.
Here, we'll focus instead on the CAD tools we find to be of use...
Beyond those essential 'design' tasks, there are a few actual
'boat building' tasks in which the designer can become involved.
For example, a natural extension of the designer's work is to detail the
vessel's structure for NC cutting.
Below, we'll have a look at how the 'design'
and the 'building' processes are augmented by the various computer
software tools
that we employ. Of course we make extensive use of basic office
software such as Excel for numeric analyses and Word for
written specifications, however the following will be focused on how we
use other highly specialized software tools generally referred to as "CAD"
(Computer Aided Design).
Our "CAD Design Stream" is best
understood if it is separated into five discreet 'design related'
boat building tasks undertaken by the designer, plus a sixth
task reserved for the builder... per the
following outline:
1. Creating the Design
2. Generating the Structure
3. Detailing the Parts
4. Nesting the Parts
5. Cutting the Parts
6. Building the Boat
While the timing of the work ordinarily
flows smoothly from one to the next, the following 'design stages' are
separated on the basis of the various tasks involved, and how
each of them makes use of differing CAD tools.
I. CREATING THE DESIGN
Larger
Aft Perspective |
Larger Forward Perspective
MODELING
We begin the design process by modeling the
general shape of the hull, decks and superstructure, then we further
detail that shape using various CAD tools as needed. For the
initial surface modeling, we find the Maxsurf suite of boat and
ship design software to be intuitive, highly capable, and accurate.
Right at the beginning of any boat design
project, a model of the new design will be created within Maxsurf as a
collection of faired NURBS surfaces. Most often this is done by
modifying an existing Maxsurf design to suit a new requirement.
When it is warranted, we will create an entirely new model.
The Maxsurf model is refined as needed for the
emerging concept design. When the model has been sufficiently
developed it will be used as the "container" for sketching in the
preliminary accommodations layout. With feedback from the
preliminary layout, the Maxsurf surface model will be revised to suit.
Thus, the Maxsurf model evolves along with the
design so that at the end of the design process the model can be plotted
as a complete, faired and finalized lines drawing. The final model
will include the hull, bulkheads, soles, tank faces, decks, bulwarks,
houses, superstructure, coamings, rudders and other appendages. In
other words, when completed, the model is as complete as it can be in
terms of the structures that define the external shape and the internal
compartments.
ANALYSIS
In addition to having excellent
modeling and fairing tools, Maxsurf also contains a variety of tools for
assessing preliminary hydrostatics, including a programmable calc sheet
which can be set up to provide nearly any hull-form related calculation
(e.g. proposed sail area, speed, power, fuel requirements, etc.).
For further analyses we make use of various
other components of the Maxsurf software family such as Hydromax for
stability and trim, Hullspeed for resistance, SPAN for sailing
performance or Seakeeper for sea keeping / motion analysis.
Our primary analysis tool is
Hydromax, wherein we can model the vessel's tanks and analyze
stability and trim. Hydromax additionally has very extensive
built-in tools for assessing compliance with various international
stability criteria such as those published by the IMO or the US Coast
Guard.
DRAWING
Once we have created and faired the model within
Maxsurf and analyzed the resulting design within Hydromax, we will then
begin to detail the design for construction. Since we use other CAD
software to create our detail drawings, once
the hull geometry has been finalized within Maxsurf we will export the
lines as 2D or 3D DXF wireframe views for further detailing within a
CAD drafting program.
We find that CAD offers a
number of efficiencies that may not be readily apparent. For
example, with CAD
we are able to migrate certain details from one design to the next - an
efficiency not readily accomplished with hand drafted details.
Also, we
are able to make changes more quickly as the design evolves.
Further, the CAD drawings can be
easily shared across our local network or by email, allowing us to
co-develop the drawings regardless of our design team members'
locations. Of equal value is that "screen shot"
images or PDF plots of the emerging design can be sent by email for use as a
communication tool with our clients - also independently of our
respective locations.
With CAD, when the design is
completed, we can offer the same kind of support to the boat builders in
the form of PDF plots and / or directly via the CAD files themselves.
II. GENERATING THE STRUCTURE
Larger
Top View |
Larger Side View |
Larger Aft View |
Larger Plates View
Once the hull and superstructure lines have
been finalized and faired, if the design is to be NC cut the Maxsurf-generated
3D NURBS surface model will be opened directly within Workshop, the
construction module of the Maxsurf suite of programs.
As can be seen in the image above and the links given below
it, Workshop automates the generation of the
vessel's plating and internal structure, including placement of
stringers and frames. Workshop will calculate the shape of the frames so that
they precisely match the hull shape, and will calculate and place the various cutouts for the stringers to pass through the frames. Plates are
also defined within Workshop, and their shapes are then 'expanded' onto flat
sheets.
When finished, the entire collection of Workshop
parts are exported as 2D DXF entities or as a 3D collection of
parts for further detailing and nesting
within a separate CAD system.
III. DETAILING THE PARTS
Larger Image of Above |
Frames Fully Detailed Fwd
| Frames Fully Detailed
Aft
DETAILING
We use Microstation for CAD drafting,
for NC parts detailing, and for NC plate nesting. "NC" just
means "Numerically Controlled" and is a general term we use to refer to
the process of cutting out boat parts by a computer driven plasma or
abrasive water jet cutter.
If the design will be NC cut, the parts will
first be generated by Workshop, then exported in DXF format for
further editing and detailing within Microstation.
Microstation can work directly with DXF and
DWG files from any version of AutoCAD. This is a trick
that even AutoCAD itself cannot (or rather will not) do.
Microstation is also able to import and work with IGES files if
desired for high quality rendering or further NURBS surface modeling or
detailing.
As can be seen in the image above and in the images linked
below it, within Microstation the Workshop-generated
parts are being edited and further detailed as needed. If necessary,
additional parts are generated. By convention, within Microstation
we superimpose the frames imported from Workshop upon the body view
imported from Maxsurf in order to verify that the various frame shapes
have been correctly interpreted. We will also superimpose any
prior interior or other relevant views by attaching CAD reference files as
needed.
Once detailed, the parts are labeled and grid lines
are edited or added as needed. Text, marking lines, cut lines and plate
edge entities are placed on separate layers and color coded according to
the requirements of the NC cutting service being used. Prior to
being finalized, slices will be introduced where the parts will
be cut into smaller pieces for the sake of efficient nesting.
PLOTTING
At this stage we create "Plot Files"
of the assembled parts. In other words, we will arrange the
various components into plots that illustrate their layout and assembly.
For example, frames are shown as whole entities (prior to nesting) and
the plates are arranged to show their relationship to each other and to
the whole.
These "Plot Files" are turned into a series of
PDF files which are then sent via email to a plotting service.
Once plotted, they're shipped to the owner and the builder, or wherever
else they may be needed. Alternately, as PDF documents the "plots"
can be put onto a CD and given to the builder or they can be sent by
email wherever needed. The owner, builder, and the builder's
subcontractors or vendors can plot the PDF drawings and documents to
scale locally using any convenient copy shop or plotting service.
Additionally, these parts assembly files are
occasionally useful to the builder, in which case they will be provided
in AutoCAD format directly to the builder - usually by email.
IV. NESTING
Larger Nestings
Image
At this point, the "Plot Files" of the parts
assemblies are preserved as-is for future reference. Those files
are then copied into a new directory called "Cut Files" where
they are further detailed for NC Cutting. Creating the actual Cut
Files consists of performing the slicing and nesting onto the locally
available plate sizes, and organizing the CAD files per the requirements
of the cutters that are being used.
First, parts are sent to separate files, each of
which will contain parts of the same plate thickness. There
will ordinarily be separate files for Frames, Plating, Tank Faces and
Lids, Engine Stringers, Stem and Keelson, and for Stringers (if there is
sufficient longitudinal curvature to warrant developing longitudinal
stringers). For example, all parts of 3/8" thickness will be
within one file.
Then the "stock" plate sizes available
from suppliers are drawn into each file to scale, and the frames or
other parts are sliced as needed for efficient nesting. In order to get the most efficient use of plate,
we do the nesting manually by trial and error. Software is
available for this task, but it is expensive and is not 'smart' enough
to make the various decisions required to achieve an efficient 'nest' of
parts.
When the nestings are complete, there will be an
array of stock sized plates of equal thickness within each file, each
having a completed 'nest' of parts. Additional labeling is
introduced to identify plate sizes, plate thickness, alloy
specification, etc. Prior to finalizing the Cut Files, all cut
lines are error checked within Microstation in order to correct any
minute line crossings, unintended small line gaps and to eliminate
duplicate entities.
Finally, the nested parts are output as DXF or DWG
files for use by the cutters (who almost universally use AutoCAD).
These are the actual completed "Cut Files."
V. CUTTING
Cut Files: Since it is far cheaper to send the parts definitions by email
than it is to send the cut metal by truck to some distant place, we will
always try to make use of metal cutting shops that are closest to where
the boat will be built. We will assemble the completed "Cut
Files" into a series of ZIP archives, and ready them to be sent
by email directly to the cutter who has been chosen for the job.
The builder will ordinarily purchase the plate and have it
delivered to the cutters. The cutters will then arrange to ship the cut parts to the building site.
Cutting and
shipping charges are billed directly to the builder.
Tabbed Parts: In the image above, the NC cut plate has been delivered to the
builder in "one piece" for easier shipping. By this means,
although all of the parts have been pre-cut, small 'tabs' have been left
intact at intervals along the part edges. This is done so that the
parts stay in an easily handled and easily referenced assembly. It
is then a simple matter to saw out the small tabs to release the
parts. This is common in New Zealand, but oddly it is not done
very often in North
America.
Markings: In the above image, take note
that the parts have been
marked with an ink-jet print head, rather than having to use the plasma
or water-jet heads for marking. This vastly speeds up the process,
and the markings are accurate and easy to read. Further, they do not impose any
heat or stress on the plates. This is relatively common
'down under' but oddly, not in North America.
Quality Assurance: During the cutting process, our follow-through is an important part of our service. We have found this to be essential in order
to accommodate the varying requirements of the many different NC cutters
we encounter. For example, if a vessel will be built in New
Zealand, we will customize the job to their specifications and adapt the
nestings to metric plate sizes, etc.
Since we work with a variety of metal cutters,
it is often our practice to first send one component through the
cutting process as a trial. Most often this is the rudder, since
it is relatively small but consists of precise foil shapes for the lifts
and a precise outline for the plating. If the trial component is
satisfactory to the builder, has a clean cut and accurately fits
together, then the rest of the NC cutting files will be sent through.
VI. BUILDING
Larger Image
- Courtesy of Loren Murphy
Why is building the vessel is listed amongst
our 'design stream' stages...? Because by
the time the metal arrives at the builder's yard, we have already done a
fair bit of the boat building work right here on our computers...!
So although we do not 'build' boats per se,
we are very much involved in the building process and we like to
maintain a close relationship with the builder.
Once we
have created a custom design, or whenever we have provided a client with one of our
stock designs, we make recommendations and introductions to qualified builders.
We prefer to stay involved during construction, for example if there may be
any clarifications desired on the part of the builder, or if there are possible
changes introduced by the owner, or if additional shop drawings may be
requested, etc.
More Information
While we regularly develop NC cutting files as described above,
we do not sell "parts kits" per se. In other words, we do
not sell the actual pre-cut metal parts - we instead offer our
services for design, analysis, and parts development.
Whether we create the NC files from scratch, or offer them as part of a
stock design package, we still include our quality control and
follow-through during the metal cutting, as well as any customization or
optimization that may be required.
We can easily develop NC files for any boat design whether it
is our own or from another source. We have a number of stock
sailing and motor yacht designs available for which we've
already developed stock NC cutting files. For pricing and ordering
information, please see our Plans List web
page.
Though the above summary has been written primarily with regard
to metal boats, NC cutting also applies to router cutting of plywood or
any other panel type of structures, whether they are for hull and deck
structures, or for boat interiors.
Yet another application for the 3D modeling methods described
above is for direct 3D cutting. Here, the NURBS surface model is
used directly by a 5-axis router to carve out the shape that has been
modeled. Most commonly this would be used to directly create a plug or
mould for GRP construction of a hull or superstructure. This approach saves an enormous amount of
the builder's in-house development labor and shop space.
Another
application of this technology is to create a scale model, whether just
for fun, or to verify the aesthetic presentation, or possibly for tank testing. For a bit more
information on this please see our brief web article on
Prototyping & Development.
A somewhat unusual project of mine is the prototype shown
below... the Piranha, which we modeled in
Maxsurf and rendered in Rhino. The intent was to have a mould cut by NC
router so that the various body parts could be easily laminated.
For the Piranha, Kevlar and vinyl ester resin would yield a structure
that's stiff, light, strong... and bullet proof...!
This kind of NC Cutting technology is well established for all manner of
manufacturing endeavors, and there is very good reason for it: Economy.
In the case of the Piranha we could elect to have the NC router carve a
mould and we would be able to laminate body parts immediately.
However the NC cut moulds
are made of reinforced foam, so they are only good for up to perhaps half a dozen uses. Alternately we could
elect to have the NC router carve a plug for each part so that we could
then make a much more durable mould for each part, and this would allow
us to make many
dozens or more GRP body parts.
Where To From Here...?
In advance of developing any new boat design
or other modeling project and prior to lofting and developing NC Cut Files we will provide an estimate based
on the scope of the work that has been proposed. For more
information about creating NC Cut files for any of our designs, or
possibly to generate NC cutting files for any other type of design, please feel
free to
contact us as needed.
Computer Modeling &
Analysis Links
NC Prototyping
| NC Parts Cutting
| NC Boat Building Advantages
| Maxsurf Information
