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Why Use NURBS Surface Modeling...?
Our rationale for using NURBS surface modeling for boat design.
Copyright 2012 Michael Kasten
As an extension of our article describing our CAD Design Stream, this page outlines our use of NURBS surface modeling to create our boat and yacht designs. The aim here is to communicate the basic rationale for using NURBS surface modeling as opposed to other 3D modeling methods, such as solids modeling for developing any type of 3D free form design. NURBS stands for a "Non-Uniform Rational B-Spline" type of surface. The technicalities of what that means, you can research on Wikipedia. The reasons for using this approach are the purpose of this article...
The development of a new boat shape can be thought of as being a series of steps, each of which takes advantage of a different aspect of the original 3D NURBS surface model in a different way, more or less as follows:
1. Creating the Design ...................... (in Maxsurf - now called Maxsurf Modeler - in 3D)
2. Analyzing the Design .................... (in Hydromax - now called Maxsurf Stability - in 3D)
3. Detailing the Design ..................... (in Microstation in 2D)
4. Generating the Structure .............. (in Workshop - now called Maxsurf Structure - in 3D)
5. Detailing the Parts ........................ (in Microstation in 3D)
6. Nesting the Parts ........................... (in Rhino in 2D)
7. Cutting the Parts ............................ (done by the metal cutter in 2D)
8. Building the Boat .......................... (done by the boat builder in 3D)
In no particular order, these 'design steps' are outlined below along with the specific reasons for choosing to initiate the model design in a 3D NURBS surface environment.
AN APPROPRIATE LEVEL OF COMPLEXITY
To start with, it is instructive to consider the question of "What is an appropriate level of complexity for the CAD model...?" This question has been elegantly addressed by Philip Christiansen and Andrew Mason of Formation Design Systems in a paper that compares large ship building CAD systems to workboat and yacht design CAD systems. Philip and Andrew wrote as follows:
When using a CAD system for vessel design and construction, it is important to choose an appropriate level of complexity for the CAD model. The idea that it will always be beneficial to create an entire "product model" of the vessel, complete down to the smallest detail, is not especially applicable to the construction of workboats.
The cost of any vessel incorporates a percentage allocated to the design. The larger the vessel, the more money is available to be spent in the design process, even though this may be a lower percentage of its total construction cost than for the design phase of a smaller workboat. After the bare minimum of design data have been generated, further time can be justified in increasing the level of design detail only if it results in an equivalent saving during the construction phase, or lower repair and maintenance costs during the life of the vessel.
As more detail is modeled in the CAD system, a point will be reached where the downstream savings are outweighed by the additional design costs. This point varies based on the cost, size and complexity of the vessel, the cost of available labor, the level of automation in the shipyard and the number of vessels being produced. For example, if a vessel is a one off design, the amount of detail modeled may necessarily be less than for a production run of many vessels.
It is with this "appropriate level of complexity" in mind that we approach the design of yachts and workboats here at Kasten Marine Design, Inc. In other words, we do not need to model every last nut and bolt as might be desirable for NASA or to build a submarine... For everyday boat design, that would be extremely unproductive, wasteful, and costly. Instead, we must engage in a streamlined CAD process that is efficient, accurate, and cost effective.
Toward that end, we have found NURBS surface modeling to be the sweet spot for boat design. The following notes are intended to outline our rationale for having made this choice.
MODELING & ANALYSIS
Working with a 3D surface is no different than working with a 2D or 3D spline using a "control point net" as opposed to a series of "through points." A 3D surface is merely a network of control points embedded in a control net that is used to define the surface. The image below is an example of a fairly complex model generated within Maxsurf, with all surfaces trimmed to their final shape and "unlocked" so that the control net is visible.
40 Meter PHINISI Charter Yacht as Viewed in Maxsurf - Click for Larger Image
There are many excellent 3D NURBS surface modeling environments, such as Rhino or Maya. However most of them, including Rhino, are "generic" tools - jacks of all trades - which do not have the specific aim of generating boat designs. They are instead general purpose industrial design or artistic modeling environments.
I will pick on Rhino here because it has become popular as a boat design tool. That has occurred primarily because it is relatively inexpensive as compared to other solutions. Although Rhino happens to be able to design boat shapes, it is not necessarily the best tool for that purpose. The following explains why, but also explains some of its strong points...
Rhino can also work with solids, but that is not its best functionality. In other words, Rhino can cope with solid models by "faking it" as a collection of surfaces that are "tied together" which Rhino calls a "polysurface." If the polysurface fully encloses a volume, it is also considered a solid, and can be assigned mass properties.
Rhino and Maxsurf both support meshed surfaces. While not sufficiently accurate to create a faired smooth surface for actual production, this can be useful for data transfer into Maxsurf and for further analysis within Hydromax.
Rhino has an open architecture, so there are many plug-ins available, including several plug-ins that can do basic hydrostatics. The most comprehensive marine design plug-ins are from Orca3D, however they do not bring Rhino even close to the functionality in Maxsurf and Hydromax. Its open architecture makes Rhino very flexible, but there several fundamental differences between Rhino and Maxsurf that should be realized...
The most fundamental drawback to using Rhino to originate a design is that surface trimming becomes an unforeseen impediment. The reason is that Rhino uses "static" trimming, which means if you trim two surfaces, then move one of them, you have to "un-trim" the prior trim region and recreate it anew. If you have a model with a hundred surfaces or so, you are left to figure out for yourself which ones might have been affected by the surface that was moved, and then edit them one by one. This alone is pretty much a deal killer in terms of my own use of Rhino.
The upshot is that while Rhino is quite an excellent tool for detailing a design AFTER the shape has been finalized, it is not an optimum tool for originating the model. For example, Rhino can do some pretty amazing operations on surfaces, such as to create a variable radius fillet between two surfaces. Rhino has many other excellent tricks.
A further inconvenience in Rhino is that both halves of the model must be managed separately. In other words, the design is not automatically "mirrored" across the centerline, so if you make a change in one half, you have to edit the other half in exactly the same way. Certainly one can just model half of the boat, then manually "mirror" the result, but it is not automatic, nor is it dynamically visual. I believe the Orca3D plug-in addresses this shortcoming, however it does so at a price. In other words, it is not part of Rhino itself, but must be purchased separately.
Yet another complication with Rhino is that when you create a part using a Rhino tool, say to extrude a shape along a path (possibly to extrude a pipe along the top of a sheer line) Rhino can do this extremely well, and also accurately, however the resulting part will have an inordinately complex control net, making it extremely difficult to edit if you wish to still maintain fairness. If you change the original geometry, say to revise the sheer line, you must delete the extruded part and re-create it.
This approach has obvious drawbacks when originating a model, where global shape changes are inevitable and on-going.
SolidWorks "fixes" most of the above issues by requiring the creation of a parametric "assembly" or "logic tree" that establishes and preserves intelligent relationships between all of the parts. This imposes an added dimension of complexity itself. Ordinarily this means the user must construct the various logical relationships in advance, or at least manage them while building a model.
SolidWorks is a parametric solid modeler, in which all parts are given thickness and mass properties in addition to being assigned a parametric relationship to other parts. That the parts are interrelated by their logical associations makes SolidWorks a powerful tool for managing changes, however for the purpose of creating boat shapes the resulting model can become quite complex and can be difficult to keep fair.
The primary utility of software like SolidWorks resides in this very ability to manage complex changes gracefully. SolidWorks is the perfect tool for creating machines, where if a piston diameter is logically related to the cylinder diameter and cooling galleries, if you change the piston, the rest is automatically updated per those relationships. It is also a powerful tool for detailing a boat's structure, thereby instantly knowing the global weight and center of gravity.
However when attempting to model free form shapes such as a boat, this more or less puts the cart before the horse. In other words, it is preferable to remain unfettered by one surface being related to another, i.e. not having to worry about what effect changing one surface might have on another surface if they were to be tied together and required to move together.
In SolidWorks, even if you have accurate mass information, an accurate CG, and an editable model via its parameters, it may not be fair and you will not have any hydrostatic properties for the shape itself. In general, SolidWorks can be a highly useful tool for detailing a hull model, but is not an especially productive way to originate a hull model. The above described "appropriate level of complexity" must be kept in mind.
In addition to its very robust solid modeling abilities, SolidWorks has good surface modeling tools. I do not know whether trimming in SW is "static" like Rhino or "dynamic" like Maxsurf. In all probability, it is parametrically defined, therefore dynamic. More on this below...
Our preference for using Maxsurf Modeler to generate new boat designs is not only because Maxsurf is an entire suite of programs specifically developed for that purpose, but also because the approach taken within the Maxsurf modeling environment makes ultimate sense.
Unlike with solid modeling, thickness is not a property of NURBS surfaces or splines, i.e. they have zero thickness. Maxsurf is a NURBS surface modeling environment wherein surfaces are used to define an envelope for fairing and analysis. Maxsurf can do many of the same modeling tricks that Rhino can do, but there are several key differences.
Modeling: In the above image, you can see that Maxsurf only needs to model one half of the vessel since the other half is automatically and dynamically mirrored across the centerline. If you specifically want asymmetry, that can be set as a property of any surface, so that a differently shaped surface can be modeled on the other side - say for designing a proa. In the image above you can also observe that when modeling NURBS surfaces, there is a great benefit when the control net is kept as simple as possible, whereby fairness is easily achieved.
Trimming: Maxsurf uses "dynamic" trimming. This means that if you trim a few surfaces using a spline or another surface, and you then move one or more of those surfaces or the spline, all of the affected surfaces are all dynamically re-trimmed on the fly. Naturally, this is computationally intensive, and that’s why Maxsurf allows you to turn trimming on or off globally (as in the above image). With trimming off, you can move surfaces and splines around quickly. With trimming on, and precision set to high, it can take a few moments for Maxsurf to figure out all the correct trim regions after each change. If the model is complex, this can take several moments even on a fast machine. As a middle ground, precision can be set to a lower value so that trimming can be displayed correctly with updates calculated much more quickly.
Tools: Although NURBS surfaces have zero thickness, it is possible to set surface thickness properties in Maxsurf. This does not add another surface, nor does it create a solid, but is useful to accommodate planking thickness so that the lines and offsets output will show the planking deduction / addition, taken normal to the surface that has been modeled.
The surface stiffness can be varied in either direction, and the control point weights can be varied in order to exert more or less local control over the surface locally. Maxsurf has excellent tools for creating and editing splines, and can create extrusions along an edge or spline or create lathe turnings around an axis using any spline shape.
Maxsurf can create surfaces from cloud data by first creating a series of editable splines and edges, then lofting a surface to those curves. Alternately, Maxsurf can fit a collection of fair surfaces to a set of proven offsets, say to accurately reproduce the shape of an existing vessel.
Fairing: Maxsurf has excellent fairing tools, such as curvature porcupines, Gaussian curvature analysis, longitudinal compression, etc. as well as automated manipulation of control points, e.g. align controls to plane; align to vector; smooth control points; smooth patch; rotate / size / move / duplicate / mirror surfaces or controls.
Visualization: In addition to the standard profile, plan and body views, Maxsurf shows a perspective view which can be rendered nicely. Colors, transparency and lighting can all be varied as needed. In each view the grid and the resulting lines on the vessel's surface can each be turned on or off. If the surface is moved, the sections, buttock lines and waterlines are all dynamically updated in all views.
Parametric Variation: Maxsurf is able to automatically iterate a model according to parameters that you set, such as to achieve a specific prismatic or block coefficient, or a given displacement, water plane area or wetted surface, etc. Restraints can be defined, such as to disallow changes to the sheer line, beam, draft, displacement, and any of the other parameters, as long as sufficient degrees of freedom remain to achieve the requested variations. This parametric variation capability allows one to create a family of 'candidate' hull shapes each having slightly different characteristics, which can then be analyzed as to their performance relative to each other, or to specifically stated design requirements.
Tri-mesh Surfaces: Maxsurf can create meshed surfaces, which can be useful for data transfer into Maxsurf. As an example, Maxsurf can automatically fit a meshed surface over a cloud of data. While a meshed surface is not sufficiently accurate for a lines drawing or to make parts from the meshed surfaces, it is adequate for hydrostatics analysis in Hydromax. A meshed Maxsurf model can be created quickly, and can then be opened directly in Hydromax for a complete stability and trim analysis.
Analysis: Maxsurf provides upright hydrostatics analysis, instantly available within the program. A built-in fully programmable calculation sheet is also available, allowing nearly any parameter to be automatically calculated from the basic upright hydrostatic information, e.g. target sail area / VCG / Dellenbaugh Angle, optimum velocity / fuel capacity / endurance, etc. The model can be set to a variety of metric or imperial units without imposing any changes on the underlying geometry. This allows rapid switching between measurement systems during design development.
Data Exchange: All Maxsurf modules share a common file format. As a result there are zero file translation issues when opening the Maxsurf model in any of the programs in the Maxsurf Suite.
In addition, Maxsurf / Hydrolink support 3D NURBS data import and export via IGES, IMSA NURBS, Fastship, and the Open NURBS *3dm format (Rhino). A host of CFD and hydrostatic analysis formats are also supported, including GHS, Autohydro NUSHALLO, etc. Maxsurf can use the DXF (AutoCAD) format to export 2D and 3D polyline, face or mesh geometry. Maxsurf can import background images for each view in the gif, jpeg, png, and bmp format.
80' "FANTAIL STEAMER" Style Yacht as Viewed in Maxsurf - Click for Larger Image
Within Hydromax (now called Maxsurf Stability) one can define tanks, which ordinarily use the hull envelope as the outer perimeter, though internal surfaces can also be used. Within HM, the mass of the tank contents is defined, and for each load case, what percent of liquid is in the tank.
Hydromax then heels the model, moves the tank contents to the trim of the vessel at each heel angle, recalculates the CG based on the new position of the tank contents, and erects a righting arm for that heel angle. Over the range of heel angles, the righting curve is created. Built into Hydromax are all worldwide stability criteria, from which Hydromax will create a detailed pass-fail report based on the criteria that you select.
During the genesis of the design, since Maxsurf does not have any "structure" information, the surface areas and centers are exported to Excel, where a weight per square area for each surface is assigned in order to get the CG of the structure. In combination with a thorough list of equipment weights and their centers, tank contents, etc. an accurate CG is obtained.
With the weight and CG information being generated in Excel concurrently with the actual Maxsurf model, we can iterate the model shape in order to achieve the requisite trim and stability, or we can edit the location of equipment, tanks and ballast as needed.
With an owner involved in the decision stream during the genesis of the design, numerous changes are inevitable and are to be expected. Thus the design ordinarily goes back and forth between Excel and the various Maxsurf programs a few times before the best solution is found.
The 61' Brigantine MERMAID as Viewed in Hydromax - Click for Larger Image
Workshop (now called Maxsurf Structure) is not a "solid" modeler. Instead, Workshop is a parametric surface modeler which is able to create frames and stringers based on the underlying NURBS "surface" model. In Workshop, the structure is parametrically related to the surfaces, so if the surfaces for some reason get changed, all parts can be re-calculated and will flow to the new shape. The Workshop structure model has a very complete materials library, and therefore we do end up with accurate mass properties based on the materials and sections that we assign to the parts. Once the structure is defined, Workshop will calculate an accurate weight and CG.
The 56' Ketch SHIRAZ as Viewed in Workshop - Click for Larger Image
The analysis of the adequacy of the structure is done in Excel by using the appropriate ABS Rule. For more information about how we use the ABS rule to advantage, please see our article on Designing Boat Structure.
By knowing the required plate thickness in combination with the frames and the stringers, a weight per square area is found for each region (bottom, sides, deck, house, bulkheads, tanks, etc.). With the accurate square area and centroid derived in Maxsurf for each surface, and a "generic" weight per square area for each surface calculated in Excel per the ABS Rule, that info is brought together in a Weight Analysis spreadsheet, and the resulting CG is fed back into the loop, possibly resulting in a revision of the hull model as needed.
If it is desired to analyze a vessel's dynamic motions, Seakeeper is used (now called Maxsurf Motions). For this, the weight per square area is entered, plus other loadcase weights as needed, and SK will calculate the accelerations in a variety of standard sea states. Although I have Seakeeper, it is rarely within an owner’s design budget to indulge in this level of analysis. Instead, roll period, pitch, heave, etc. are readily calculated in Excel using basic formulae published within the volumes of Principles of Naval Architecture, although the results are not nearly as thorough or accurate as would be calculated by Seakeeper.
OUR PREFERRED PROCESS...
We can see from the above that once created, the Maxsurf "surface model" is used first to check that the shape is fair and that the trim regions are behaving correctly. Then still within Maxsurf, the upright hydrostatics can be quickly checked in order to provide feedback for editing of the shape.
Once the preliminary model is nearly finalized, Maxsurf can then calculate the surface areas and centroids for export to Excel, where weights per unit of area are introduced and the CG determined.
This very same Maxsurf model will be used in all of the other Maxsurf modules, such as for analysis in Hydromax, Seakeeper, and Hullspeed, and for modeling the structure in Workshop. For analysis, the Maxsurf model is simply opened in Hydromax, allowing complete hydrostatics including tanks and their contents without having to convert the design file into another format.
Throughout the modeling and stability analysis, none of the actual structure is present in the model, however bulkheads, soles, tank faces, girders, etc. can be introduced in order to have their surface areas calculated by Maxsurf, and for use as boundary surfaces for tanks in Hydromax. All other structure such as framing, stringers, insert plates and other structural components are separately accounted for in the Excel Weight Analysis, which will provide a single point CG for use in the hydrostatic and large angle stability analyses.
Then after the design has been finalized, if the vessel will have its parts pre-cut the original Maxsurf model can be directly opened in Workshop to begin creating frames, stringers and plates based on the surfaces present in the model.
This arranges the design process in a logical order… i.e. first the desired faired surface shapes are created and basic hydrostatics are performed in Maxsurf, then the CG is calculated in Excel, then the large angle stability analysis is performed in Hydromax, then the Maxsurf model can be edited as needed according to those results.
Once the design is "fixed" or nearly so, the Maxsurf model can be brought directly into Workshop in order to create the internal structure and expand the shell plating. This is all done without file translation by direct use of the original faired Maxsurf surface model, thus no other input file formats are needed.
To review this design process in greater detail, please see our Design Stream article. To see all of this in flow-chart format, please see our Design Flow Diagram.
FILE CONVERSION HEADACHES...
Maxsurf is able to directly export the Maxsurf model to the Rhino *.3dm file format. A Maxsurf model exported to Rhino will ordinarily open flawlessly in Rhino. Maxsurf provides a Maxsurf Plug-In for Rhino, which allows the Maxsurf Assembly Tree to be preserved for use in Rhino, as well as when subsequently opening the *.3dm Rhino file in Maxsurf. If the model is not changed much or at all in Rhino, bringing it back into Maxsurf ordinarily works very well.
However if the model has been substantially enhanced or detailed in Rhino, there can be entities that Maxsurf will not recognize, which will be deleted from the model on import to Maxsurf. Examples are any text or dimensions added in Rhino; complex entities such as ports; mechanical items, etc. For the most part though, since both Rhino and Maxsurf are inherently surface modelers, the round trip between programs works well for the basic surface model.
Rhino is one of the best programs available for translating one CAD format into another. For the most part, this works fairly well when bringing various CAD file formats into Rhino. However it is often problematic when trying to use Rhino as a possible intermediary in order to translate a CAD model into a third format -- say to get a non-Rhino originated model into Maxsurf.
For example, a SolidWorks "solid" model (with thickness and all) can be opened in Rhino using a variety of CAD file formats including the SolidWorks native format which Rhino can read fairly well. Other options are to export the SolidWorks model in an intermediary file format such as STEP or STL. These all seem to open fairly faithfully in Rhino. However on attempting to export such a model into Maxsurf, they are too complex for the basic surface modeling environment in Maxsurf. Even if simplified, the original model’s trim regions will be lost, as well as any of the original parametric relationships between surfaces.
The result of trying to get a SolidWorks Model into Maxsurf via Rhino is that when it is opened in Maxsurf, there are several surfaces present for each part which "illustrate" the thickness of the part, and which enclose the edges. But... there is nothing in between the surfaces..! In other words there is no mass. The model no longer contains "solids" but instead just contains a collection of surfaces that are unrelated to each other except by their proximity.
At this point, in order to be of any use in Maxsurf, or if a hydrostatics analysis will be done in Hydromax, the model must first travel through Maxsurf where there will be considerable work to be done to delete all of the non-essential surfaces, possibly involving re-modeling the vessel from scratch in a NURBS modeling environment such as Rhino or Maxsurf. The net result is a lot of wasted time and effort.
The upshot is that SolidWorks or any other solid modeling environment is inordinately complex for the basic task of creating a fair NURBS surface envelope.
This is the essential rationale for the PREFERRED PROCESS outlined above, i.e. starting with a relatively simple trimmed NURBS SURFACE model generated within MAXSURF which can then be properly analyzed in HYDROMAX, eventually progressing to a parametric STRUCTURE model created within WORKSHOP, then to a 3D layout or parts editing environment where the model will be easily received down stream, say within RHINO, AUTOCAD, MICROSTATION or SOLIDWORKS.
Unfortunately this process does not work at all well in reverse...! For example to get a complex parametric SolidWorks "solid" model into Hydromax for analysis...
It is possible that within SolidWorks a "surface" model can be created, then transferred to Maxsurf via IGES or the Rhino 3dm file format, both of which Maxsurf can read. However SolidWorks has several shortcomings with regard to Surface Modeling. For example, it is not possible to expose nor to directly manipulate the surface control net in SolidWorks, since that would violate the history based parametric relationships among parts. Further, SolidWorks is not able to write a 3dm Rhino file, so that basically leaves IGES as a possible avenue into Maxsurf unless Rhino is present to act as intermediary. Even then, the result will require quite a lot of re-work to re-trim the surfaces within Maxsurf prior to being able to move the model into Hydromax.
In the PREFERRED PROCESS outlined above, we are working entirely within the Maxsurf Suite of programs in order to create and analyze the design, and to detail the basic structure. Thus, there is no file translation required, and we therefore experience zero issues from one Maxsurf program to the next.
But there may be occasions where a model will have originated elsewhere. A model originally created in another NURBS surface modeling environment such as Autoship or Fastship can be imported directly into Maxsurf, or alternately imported via IGES. A Rhino originated model will usually open reliably in Maxsurf, provided that the Rhino model is kept simple. This does not mean the number of surfaces need to be restricted, only that entities that do not exist in Maxsurf should be avoided, such as text, dimensions, solids, etc.
A non-NURBS modeling program of possible interest is MultiSurf by AeroHydro. Though MultiSurf is a surface modeler, it takes quite a different approach. MultiSurf creates a surface model using "relational geometry" (RG) whereby a variety of various types of points, curves and surfaces are used to define key shapes such as the sheer, centerline profile, stem, transom, midsection, etc. Among them, parametric relationships are established to assure that they will move together and remain "related." By this means, MultiSurf builds a surface model from multiple types of elements.
It is a powerful approach, though it does require that parametric "logical relationships" first be established, similar to the "assembly tree" used in SolidWorks. The similarity between MultiSurf and SolidWorks is such that a specialized version of MultiSurf has been developed for the SolidWorks environment, called SurfaceWorks... essentially a more CAD oriented clone of MultiSurf without its hydrostatics analysis capability.
Though modeling with MultiSurf is a much more complex process, relational geometry is capable of extreme accuracy. John Letcher, creator of MultiSurf, presents a favorable case for using RG modeling where a CFD analysis will be required down-stream.
It should be pointed out that the surface model created by MultiSurf and SurfaceWorks is not a NURBS surface model, rather it is a relational model composed of a variety of points, curves and surface types. Unfortunately the relational model that is created is not recognized within most common CAD systems, nearly all of which have standardized on NURBS surfaces. Thus, in order to be used in other CAD systems the relational model must first be transformed into a NURBS surface model. Fortunately MultiSurf includes very good tools which automate that process.
However... the NURBS model that results is not precisely the same as the original MultiSurf relational model -- rather it is an approximation. By virtue of having a NURBS surface that is an approximation of the original geometry, the bi-directional file transferability to and from other CAD systems is lost. Presumably if one is working entirely within a SurfaceWorks / SolidWorks environment this may not be an issue.
If the destination format will be NURBS based for the sake of compatibility with general CAD systems, the question arises: "Has this exceeded the appropriate level of complexity required to create the model, analyze it, and build from it...?" I don't know the answer, although the MultiSurf system is liked by many.
Certainly if it were necessary to analyze the hydrostatics and stability of a MultiSurf model in Hydromax or another NURBS based environment, it should be relatively easily accomplished, since the degree of precision in the NURBS approximation created by MultiSurf is very unlikely to make any difference to the analysis. One caveat though is that in creating a NURBS approximation of the underlying relational model, it is possible there would still be considerable re-work involved in order to have trim regions be properly recognized.
The upshot is that if a NURBS surface environment will be the eventual destination for the design, the most efficient path will be to originate the design using NURBS surface modeling.
DETAILING THE MODEL
I use Microstation as a tool for detailing and illustrating a vessel's layout and structure. AutoCAD is more or less equivalent, and is by far the most common CAD program in use. And at long last, the newest version of AutoCAD 2012 is able to recognize NURBS... a feat that Microstation has been able to do since day one...!
DRAWING THE LAYOUT
Presently I find it faster to work in 2D to create the Building Plan Drawings, especially since they will ultimately be output to 2D plots on paper, or as 2D PDF files. To achieve this, first a "fixed" set of 2D lines are exported from Maxsurf as 2D DXF files, then the lines are brought into Microstation for further detailing. Admittedly, in light of the above discussion of 3D modeling, working in 2D might seem primitive, but it is relatively quick to achieve, and since the output must be in 2D anyway, there is not much to recommend against it.
An exception would be if it is desired to create a 3D model for a photo-realistic presentation or as a 3D "walkthrough" or in order to prove that the spaces in the layout are as intended. This can be helpful, but the time required in order to create a detailed 3D layout model -- and the resulting cost involved -- are ordinarily difficult to justify for yachts and workboats of less than around 60 to 80 feet.
However, if a detailed 3D layout model were to be desired, in all likelihood Rhino would be the program of choice.
DETAILING THE PARTS
If a design will be detailed for NC cutting, I will use Microstation to refine, edit, detail and prepare the 3D parts that have been exported from Workshop. For this, I prefer to work in 3D, which results in a model that can be used for illustrating all the parts. The following image shows a screen shot of a vessel's structure from within Microstation.
A cool 3D Structure Drawing in PDF format shows the same design, output directly from Microstation to a 3D PDF. You can rotate, pan and zoom the 3D model in the PDF, and you can also turn on and off the various layers in the model. This is an excellent illustration / visualization tool for communicating the structural arrangement to the builder.
Although I have no doubt that Rhino could be used equally well for the 3D parts detailing downstream from Workshop, I do not use Rhino except as an occasional tool for creating an interesting illustration of the model, as CAD translating tool when needed, and as a nesting tool after the parts have been fully detailed within Microstation. This latter trick is made possible by Rhino’s open architecture, and various readily available plug-ins such as Rhino Nest.
The VALDEMAR 53 Structure as Viewed in Microstation, Ready for Nesting - Click for Larger Image
CUTTING THE PARTS
Note that NONE of the above described 3D modeling and detailing regime involves a "solid" model. Since the third dimension is not needed for actually cutting the parts, it is only necessary to define the "outline" of each part so that it can be exported into a 2D environment for cutting. This works quite well via NURBS surface modeling.
In other words, we have employed an "appropriate level of complexity" and no more.
NC Cut Parts for the 25' BOOJUM Tabbed to Sheet for Easy Shipping
BUILDING THE BOAT...!
After all the parts have been cut and shipped to the builder, naturally it is all moved back into a 100% 3D environment...!
Our 96' Schooner ZEBULUN design, in Frame
Kasten Marine Design, Inc. is a distributor of Maxsurf software in the USA. For more information about the Maxsurf Suite of products described above for boat, yacht, and ship design, please see our separate Maxsurf web site. If you'd like more information about Maxsurf by email, please inquire.
Please see our CAD Design Stream article for a complete description of how we implement the above software solutions to create our boat designs and to generate NC cutting files in order to pre-cut a boat's structure.
Although we regularly develop NC cutting files as described above and in the CAD Design Stream article, we do not sell "parts kits" per se. In other words, we do not sell any pre-cut materials. Instead, we offer Building Plans and NC Cutting Files for any of our pre-existing designs, or for new designs we offer our services for design, analysis, and parts development.
Once we have completed a new custom design, or if we have provided a client with one of our pre-existing stock designs, we will then make recommendations and introductions to qualified builders who we consider to be suited to the task at hand. For example, some builders will prefer to provide a bare hull, others a power-away package, and yet others will only take on the construction of a turn-key yacht.
Although our Building Plans packages are very complete, we very much prefer to stay involved during the boat's construction in case there might be clarifications desired on the part of the builder, or if there are possible changes introduced by the owner, or if additional shop drawings might be requested, etc.
WHERE TO FROM HERE...?
For pricing and ordering information on any of our pre-existing boat designs and NC cutting files, please see our Plans List web page. Whether we create the NC files from scratch, or offer them as part of a stock design package, we still include our follow-through during the metal cutting.
In advance of developing any new boat design or other modeling project and prior to developing NC Cut Files we will provide a written Design Proposal that includes an estimate for our work that is based on the scope of the project 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 design, please contact me as needed.
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