The user interface has be proven to be one of the most important aspects of hull design tools ultimately affecting the efficiency in which shapes can be controlled. As a result of the introduction of X-Topology, PolyCAD’s user interface has been greatly enhanced from versions prior to 7. The graphics window now uses a camera based view which has allowed geometric data to accurately created and edited in 2D while viewing the model in 3D including perspective. In addition, a grid and object snapping have been introduced to allow geometric entities to have much more interaction with other in the design space.
PolyCAD can import data from a variety of sources to assist in the development of a hull surface. A wide range of CAD import formats are supported and it is possible to bring in data from Excel CSV files as well. Often, imported information can be translated into data from which a hull surface can be directly generated. For example, X-Topology curves can be created from polyline data.
PolyCAD allows image data to be imported and traced over using curves. This allows definition geometry to be made to match scanned images of hull form data and resulting surface contours verified. Images are presented in the graphics window on orthogonal planes calibrated to the scale of the design. All three views of a scanned image lines plan to be presented simultaneously.
Curves are the simplest way of representing a hull surface and many specialist ship design analysis, particular hydrodynamics, work with raw section data. PolyCAD is one of the few tools (as most now prefer to concentrate on surfaces) to provide extensive features to manipulate section curve data allowing the repositioning of individual curves and processing of point data. There are also extensive CAD tools for slicing, joining and deleting features in section curve data.
Hull form design using BSpline Surface is very popular particular for small craft and yachts. The technique is very easy to implement and as a result there are a large number of different hull design orientated software packages available. To create a hull form using a BSpline surface, the user manipulates a mesh of control points, reviewing the shape of the surface using contours or curvature analysis until the desired shape has been reached. A good degree of skill is required to achieve the best results and it can take a long time to develop complex hull surface such as ship forms using BSpline surface control points alone.
Parametric Hull Design using YachtLINES and ShipLINES
In parametric hull surface design, mathematical functions are used to generate the hull surface without the need for extensive manipulation of the surface definition. Numerical parameters describing form characteristics such as length, breadth and displacement etc are used create a hull surface in one step. This process is very useful in concept design where the look of the hull is not greatly important but making sure that all the ships' systems fit is. The drawback of parametric hull design is that the mathematical formulations are fixed making it very difficult to change the style of the hull form produced. After all, it is very difficult to describe style numerically! PolyCAD supports two parametric hull generators which may be used to create a hull defined by a BSpline surface which is them modified manually.
- YachtLINES is a single cubic B-Spline surface yacht hull generator based on 19 geometric parameters. It follows the basic approach taken by many previous techniques using longitudinal form curves from which section shape is generated. Form curves are defined using B-Spline curves and an iterative approach is employed to modify control vertices until the desired hull properties are reached. A final NURBS hull surface representation is generated by performing a longitudinal fit to the control polygons of each section.
- ShipLINES produces a B-Spline surface of a single-screw cargo ship hull form with and without bulb based on 25 geometric parameters. Most of these are used to define local appendages such as the bulb and shaped skeg. The control polygon of the hull surface is specified directly around the areas of the stem, midship section, transom and skeg. The rows of the surface are blended in between taking account of the parallel middle body. Unfortunately, this construction technique and the requirement to produce both bulb and skeg in the surface place a significant constraint on the surface definition such that it is not possible to control the hydrostatic properties independently of the other input parameters. Despite this, the surfaces produced by this technique are reasonably good as the effect of the high level of constraint results only in a reduction in the range of hull shapes that can be produced.
Rapid Hull Design using IntelliHull
Hull surface design based on directly manipulating the surface definition and parametric hull design are both very useful techniques. It is highly desirable to be able to use both techniques to work on the same surface. However, in general this is not possible because surface definition techniques are usually arbitrarily structured and the mathematical rules used in parametric hull generation are not adaptive. IntelliHull has overcome this problem by detaching the surface definition geometry and the parametric rules from the surface representation. In IntelliHull, the definition geometry and parametric rules control a design framework which subsequently creates the hull surface.
The design framework embodies the topological "knowledge" of what geometry is required to create a hull form surface. This means that it can augment user supplied definition geometry, if it does not exist, to produced all the information required to generate a hull surface. Therefore, the user only has to provide enough definition information to describe the primary features of the hull surface and the design framework "fills" in the gaps. As there is much less definition information, numerical parameters can be used to control the design framework and modify user supplied definition using transformations rather than inflexible generation process.
IntelliHull was developed as a PhD project and as it only generates a single BSpline surface it does not have all the ability to produce the detailed features associated with a production hull surface. IntelliHull provides proof of concept for the next stage of work, X-Topology, i.e. applying the ideas to a multiple surface patch topologies.
Hull Surface Design using the X-Topology Arbitrary Curve/Surface Framework
While IntelliHull successfully demonstrates the conceptual approach, the implementation exhibits limitations when it is used to develop the detailed features of a hull surface. The subsequent development of X-Topology hull surface build on the lessons learned while developing IntelliHull and applied similar concepts to hull surface design using an arbitrary network of curves. Arbitrary curve network accurately capture the topology of the surface shape, information which can be used to introduce parametric transformation which respect the shape of the hull surface rather than distort it. The primary aim of the X-Topology development is to produce a practical toolset which make hull design as easy as sketching but provides the user with accurate surface control when they want to refine the design.
In X-Topology, the hull surface is produced by constructing a framework of curves that are skinned using an approach similar to solid modelling. The curves are based on NURBS technology, but like IntelliHull, the user does not manipulate the control points directly. This allows the user to apply constraints onto the control points which specify shape such as introducing interpolation and knuckle points. The curves are then constructed by analysing the users control points, constraint and whether the curves reference others which have additional shape controlling information. The curves are defined in a user interface environment which allows interactive manipulation, snapping to other entities and dynamic referencing. Overall this make the X-Topology experience really productive, even more so because many of the "IntelliHull" features have yet to be implemented.
PolyCAD provides a number of tools to analyse hull surface as they are being designed. All surfaces and section curve have the ability to generate surface contours which can be used to indicate hull shape. Surfaces can be rendered and it is possible to use the specular light patch to see how hull shape changes as the reflection moves around. Providing a more mathematical approach to surface analysis, the Gaussian and Mean curvature can be displayed as well as zebra reflection (Isophote) contours. The hydrostatics and section area curves can be calculated over a range of waterlines. Finally, once the hull form has been completed a Lines Plan can be generated or the surface exported to another design system using one of the many export file formats. PolyCAD also supports scale printing to allow accurate templates to be produced on large scale printers.