Computer Cutting vs. Manual Construction Methods
What is NC...? It simply means Numerically Controlled...
Even though computer cutting has been around for large ships for several decades by now, it is at times quite an undertaking to overcome the
nearly unbelievable reticence people have in embracing computer cutting technology for smaller metal vessels, in particular for yachts.
For metal yachts of the size used for family voyaging (say under 80 feet), it is
(fortunately, at last) becoming more and more common in the United States to find boat builders
who are willing to take advantage of computer cutting. While it is almost completely inconceivable to build a commercial vessel of any size by manual lofting and cutting
(it just is not done - it's too time consuming and therefore too expensive),
at last it seems that builders of smaller boats are willing to embrace this
concept - and to reap the benefits...!
These days, a number of excellent software tools are readily available to smaller yards and design offices,
thus the 'big ship' technology has finally "trickled down." When
the remaining small yacht builders at last wake up to this fact, we will all
benefit.
The Process...
To create the computer-driven NC cutting files needed for boat
building the process will ordinarily be as follows:
First one must create the hull and superstructure geometry by computer modeling, then
those surfaces must be faired perfectly. This is most efficiently done within a dedicated yacht design program
which will most often make use of NURBS (Non Uniform Rational B-Spline) surface geometry.
If the design is one that the designer has created using computer modeling,
then the hull geometry will already be in place. In other words, in
the hands of the designer the hull geometry will have been
generated as a result of the design process itself.
Once the computerized model has been finalized and faired, the design will then be sent to a separate program that will be used to generate the vessel's structure. In the past one would have used an
ordinary CAD program to perform the structural detailing, most often as a
simple "wireframe" model. The disadvantage of this work stream is that, having translated
the design geometry from one software program (the surface modeler) to
another (the CAD program) the hull design is "frozen" in terms of its shape.
In other words, the shape cannot therefore be readily changed, at least not without a fairly large re-investment of time (and therefore money).
This has changed... Presently there are a
number of good software programs aimed specifically at generating boat structure.
Modern marine design software is available
either as an inter-related suite of programs that are optimized in a fundamental
way, or as a series of unrelated programs, each of which is able to
accomplish a specific task. In very few cases, the design file is able
to be directly opened within the structure-generating module. In most
cases, the structure detailing follows the CAD example we have already
mentioned... i.e. the design is 'frozen' and then may be further detailed.
In the 'best of all possible worlds' the
software that created the geometry in the first place, will remain able to
edit that geometry down-stream... This is in fact what has been made
possible by the authors of the Maxsurf suite of software. As a
result, I prefer to use Maxsurf and its companion software modules,
and I will explain why...
In Maxsurf one can quickly create,
analyze, and perfectly fair any new boat design (a hull and superstructure
model). Then, downstream in a separate 'structure generating' program
one can begin to define the basic structural components: Frames, stringers,
and plates. In the Maxsurf suite of software, this program is called 'Workshop.'
Its task is quite specialized, and therefore very efficient.
In other words, Maxsurf is the surface modeling environment
that originates and modifies the design; Hydromax is the analysis environment for stability and trim; and
Workshop is the program for creating structure. Each of these software components can open the original surface model and work with it as needed.
The advantage of this approach (i.e. using the
original 'native' NURBS model) is that a "parametric" relationship is preserved between the structure of the boat and the
NURBS surface geometry within the original design file.
Because of this relationship, one can for example edit the shape of the vessel using Maxsurf
at any time, and any changes to the hull shape will flow through and be re-calculated within Workshop. In other words, it will not have been necessary to "freeze" the design geometry prior to beginning the structural detailing.
In the midst of the various software packages now available, this is a huge
step forward...!
In addition, Workshop has the benefit of automating a large
part of the creation of structure in the first place. Within Workshop, frames and bulkheads are placed and defined, and are automatically fit to the surfaces. Longitudinals are generated onto the surfaces, dividing the girths as specified.
Various assigned cutouts are automatically placed into the frames and
bulkheads along the path of each longitudinal and at the edges of the
plates.
For a simple design such as the one shown here, the generation of the
structure within Workshop may take as little as a day or so to accomplish...! The screen shot below is from Workshop. The surfaces were created in Maxsurf, and the frames and long'ls were generated within Workshop within a few hours' time. Of course there is considerably more work to be done, however
we can readily see that the genesis of the basic structure
is very quickly accomplished.
Maxsurf provides free form modeling tools for developable and non-developable surfaces. Workshop provides a choice of algorithms for determining the shapes of the exterior hull surfaces. Each of the plate expansion methods
within Workshop is optimized for a specific type of hull material, whether metal plate, wood sheathing, or
for GRP / CF fabric. This latter capability is especially useful when
using pre-impregnated cloth and other high cost fiber reinforcements.
Once the structural components and plating have been defined in Workshop, they are exported to a CAD environment for further detailing. The image below shows the structural parts of a power vessel design
at the point of having been exported from Workshop and then imported into Microstation.
In the example above, once the structure has been detailed and finalized,
it must then be efficiently 'nested' onto the available plate sizes.
In other words, after editing and detailing the frames and plates, the
structural parts will be broken into smaller segments for efficient nesting
- whether this be onto metal plates or onto plywood sheets. Fortunately this
is quite an easy task within Microstation.
Once the nesting has been accomplished, the plate definitions are sent to an NC cutter.
The AutoCAD DXF format has become the file format preferred by the industry.
Since Microstation can easily and transparently open, work with, and save a
DXF file, the standards of the working world are preserved.
When the NC files have been finalized, then they are sent to the NC
cutters. Email now makes this a
rather quick and easy transfer, right from the design office to the cutter's shop.
GRP mould making is equally suited to this design stream. Planked wooden construction resists being automated to a somewhat greater extent than other hull materials, although it is certainly an easy matter to create the mold frames and templates to build any wooden vessel.
With a GRP or a wooden vessel, the resulting geometry will be sent to a computer driven router
(rather than to a plasma cutter as would be the case for a metal boat).
Anyway, in everyday use we have found that the Maxsurf / Workshop combination has proven to be highly capable, fast, and accurate. For a concise outline of the actual CAD process, please check out the article describing our CAD Design Stream.
What's Included?
Our NC cut parts include a vessel's frames and all the plating for hull, decks, superstructure, cabins,
as well as for all internal structures such as tank faces, baffles, lids,
engine girders, keels, stem, keelsons, bulkheads and rudders. For
larger craft other internals are included where appropriate such as water
tight soles, etc. Where appropriate, "insert plates" are
also included. Insert plates are especially important on aluminum
vessels, where they reinforce locations of higher stress such as around chain plates, above propellers, around keels, below windlasses and mooring cleats, and so forth.
In all, it is our goal to supply an extremely complete set of NC cutting files that
will provide all the parts that relate to the surface shape of the hull. These
hull-shape-related parts are those that save the builder the most time, as they would otherwise require a thorough lofting prior to construction, then very accurate pattern making for each part.
Our NC cutting files are not a "boat kit" per se, which if offered would include the actual metal or plywood boat
parts. Instead, we offer the NC files themselves.
As a result, our NC files can be conveniently adapted to the specific requirements of the cutter being used, and can then be forwarded directly to them via email. For example, we will ordinarily adapt the NC files to metric materials, or
possibly to the restrictions imposed by a smaller cutting table, or to special NC file requirements as to file format or layout conventions, etc.
In other words, it is our policy to provide a high level of follow-through with the cutters of choice for the sake of
accuracy, and to assure that the files can be used with a minimum of bother. For an in-depth peek at the technicalities involved please see the article describing our CAD Design Stream.
Lofting...
As an added benefit of having modeled the lines via computer, one can output the lines
(or perhaps just the body view) onto Mylar - or better yet, directly onto the building platen.
By this latter method, a builder may choose to completely eliminate lofting and proceed directly to
assembling the already cut parts...!
It may seem odd to more traditional boat builders, but most builders familiar with the accuracy offered by computerized cutting
now prefer to dispense with the lofting entirely. The technique used in this case is to erect an accurate grid on
q building platen, then to simply align the grid that has been marked on the
parts with the "master" grid on the building platen.
This degree of faith in the capability of NC cutting may seem shocking to some (it was to me at first)
yet innumerable builders have proven that these methods are both highly accurate and very efficient.
The Savings
The number of hours involved in creating a custom boat design is not so large when compared to the hours involved in
actually building the boat. Good design will therefore always have several objectives,
among which a primary goal is to save overall costs. It is for this reason
that we strongly advocate NC cutting.
Once a new design has been created, the next logical step
inevitably seems to be the NC cutting of the structure. By this means the time invested in generating and fairing the hull and superstructure design via computer modeling
is able to be leveraged - and applied directly to the building process itself.
The number of hours added to a custom boat design project in order to
include NC cutting will be compensated for several times over by the savings during building the boat. NC cutting will even permit an overall savings with an
existing vessel design, say if done as a one-off project - even if one must create the computer model for that
occasion.
Anyway, via these methods a new custom design can be created for the same or less overall vessel cost than that of a "hand built" boat done to a stock design. Here is some feedback from boat builders:
- Estimates of the building time saved will vary from one builder to another, depending the degree of experience they've had with NC cut boats. Most builders credit NC cutting with being able to effect a 40 to 60 percent savings on the hull building labor. Since a yard is always being paid by the hour to build a hull, this kind of savings adds up quickly.
- In general one can expect to save some two to four hours of construction time for each hour spent at the computer.
- One can readily begin to see the cost advantages with an NC cut project. Even with a one-off boat the fabrication time saved will usually pay for the cost of generating the NC files twice over.
- The cost of developing NC cut parts will depend on the amount of detailing required. In other words the cost of creating NC cutting files will depend on the relative simplicity of the design and of its structure.
- A vessel that is more complex will have the chance to effect an even greater savings via computer cutting, more or less in direct proportion to the amount of labor that would be required if the vessel were to be lofted and cut by hand.
- In general however, for any vessel the simplest possible structure should be used consistent with the requirements of strength.
In order to limit the cost of generating the computer model of a vessel and of developing the parts for computer cutting, it is necessary to know what is sufficient detail... This question must always be answered on a case by case basis. However we can make a few
generalizations that will nearly always apply.
- Beyond the obvious benefit of generating the vessel's framing, it is very worthwhile to also create surfaces in the model for tank faces and tops, engine girders, stem, keels, superstructure, and all of the hull plating.
- Usually though, it is not a benefit to develop the smaller fittings on deck or the rigging components. Those small fittings are still more efficiently cut and fabricated by hand.
A few other generalities should be mentioned...
-
The accuracy of the computer model is a factor when it comes time to fit the parts together. The accuracy question is the catch, really.
-
Toward that end, it is imperative that good software tools be employed and that the software operator has the skill to produce fair and accurate results.
- If the vessel is a one-off project, it will nearly always be very worthwhile to employ NC cutting, since the savings during construction will more than make up for the cost of creating the NC cutting files.
- If it will be a repeated project where the computer time can be amortized over a larger number of projects, then of course there will be much greater savings.
Acceptance
As the computers and software involved in this kind of work stream have become increasingly affordable by smaller design offices, the general acceptance of computer modeling and computer cutting has been on the rise.
We can therefore expect to see the computer cutting process more and more often in the coming years, and for smaller and smaller projects whether they are intended for production or are simply a one-off boat project.
Oddly, among yacht builders in the United States there has been a slow acceptance of NC cutting. This is mainly a matter of unfamiliarity (dare we say ignorance) among boat builders. Many builders may not have faith that a computer can be a real boatbuilding tool...!
In New Zealand and Australia this situation is quite the opposite. Builders there have come to expect that a project will be computer cut, even if it is a one-off boat.
In the final analysis, given the excellent software tools available, there is just not a more practical means to build a vessel, whether small or large, regardless of the materials of construction.
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