Marcus Bole - PolyCAD - Parametric Hull Generation
Parametric Hull Generation

Introduction

Parametric Hull Generation has played an important role in PolyCAD's mission to promote methods of easy hull surface creation. The software implements both methods which quickly generate hull surfaces using coded parametric algorithms and techniques which build up the surface using typical manual definition using an efficient user interface. There remains an aim to merge the both approaches to allow a representative early hull definition to be generated numerically that can be subsequently customised using manual definition to introduce the specific design styles and local features.

PolyCAD contains a variety of different Parametric Hull Surface Generation approaches. YachtLINES and ShipLINES are traditional 'Black Box' techniques which are entirely driven by numerical parameters and offer the user minimal control over style.

Hull surface generated using ShipLINES parameters.

Development to overcome this limitation resulted in IntelliHull, where users define the shape of principle features using definition curves. IntelliHull uses these curves to loft a hull surface and exposes numerical parameters that change the dimensions of the hull form through transformation. IntelliHull is not without limitation. The number of control points on each curve must be consistent and certain shapes in the hull form are challenging to control. This said, commercial projects have been completed using IntelliHull as a hull surface design tool.

X-Topology evolved from the need to overcome these limitations in IntelliHull. X-Topology is not a parametric hull generation technique but it is a more capable surface design tool which supports many of the ideas prototyped by IntelliHull. X-Topology Hull Surfaces can be transformed parametrically in exactly the same way IntelliHull surfaces can. These options are exposed when the Hull Transformation functions are applied to an X-Topology Surface. In development is a Parametric Hull Generation tool for X-Topology geometry based on Initial Sizing routines. An early version can be found available on the X-Topology menu for the purposes of interest but it has not received much testing and easily crashes.

Traditional "Black Box" Style Parametric Hull Generation Solutions

YachtLINES and ShipLINES are traditional Parametric Hull Generation tools where a surface is entirely generated from numerical parameters. Both methods represent the earliest experiments in PolyCAD into this technology. The work revealed limitations in this approach and identifies challenges in respect of how well this solution interfaces to the design process. YachtLINES was an undergraduate project and ShipLINES a post-graduate project. Since then further evolution of rapid hull surface design techniques with the development of IntelliHull and X-Topology mean that these approaches remain useful but with accepted limitations. Consequently, these methods are maintained but are not being actively developed further.

YachtLINES

YachtLINES generates a single cubic B-Spline surface yacht hull form 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 surface is generated by performing a longitudinal fit to the control polygons of each section. YachtLINES was developed as a final year project for undergraduate study [1]. (Read More...)

Default Hull Surface generated using YachtLINES.

An illustrative example of the use of the YachtLINES surface in PolyCAD is shown in this demonstration.

Alternative surface created by modifying parameters.

ShipLINES

ShipLINES generates 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. ShipLINES was developed as an M.Phil. study but the limitations of the technique suggested a greater scientific contribution could be made through the PhD study and led to IntelliHull [2]. (Read More...)

Default hull surface generated by ShipLINES without Bulb.
Default hull surface generated by ShipLINES with Bulb.

An illustrative example of the use of the ShipLINES surface in PolyCAD is shown in this demonstration.

IntelliHull

Tradition Parametric Hull Generation techniques like YachtLINES and ShipLINES are quick but there is limited opportunity for the user to modify the style of the hull produced beyond that allowed by the algorithm. The hull surface that's generated can have quite a detailed definition which is challenging to alter if an alternative style is desired. IntelliHULL addresses this challenge by allowing the user to design curves to which define style which is subsequently combined by IntelliHull into a hull surface.

The development explores a number of new opportunities:

  • Providing the user with the capability to define the style of the hull form using curves independently of the hull generation algorithm.
  • Using numerical parameters to change the dimensions of the surface by applying transformations to the user definition curves.
  • Use of Form Topology to identify the role of user supplied definition curves and generate additional definition curves if missing.
  • Merging of a separate parametrically defined bulb surface with the main hull surface.

Developed as a PhD project, IntelliHull offers the user the ability to generate a single non-uniform B-Spline hull surface quickly while maintaining control over the style of the vessel. Longitudinally lofting is used to generate the hull surface, requiring all design curves to have the same number of points. Curve "Modifiers" are used to introduce specific shape features into curves, a precursor to the "Constraints" used in X-Topology Curves. Finally, the bulb surface is introduced by refining the surface using Knot Insertion and shifting control points on the main surface to positions on the bulb surface.

The simple methodology that IntelliHull uses can limit precise control of shape but this also makes it quick to use. Commercial ship design has been done using the technique, including multihulls. IntelliHull is also a proof of concept for subsequent ideas developed in X-Topology. Here a network of curves is used to generate hull forms represented by multiple surfaces using blending techniques such as Coons Patches. This approach provides a level of control that cannot be achieved in IntelliHull.

Hull surface generated by IntelliHull

The process used to create an IntelliHull definition is illustrated in this example.

X-Topology: Beyond IntelliHull

A particular challenge experienced with IntelliHull is that because lofting is applied in the longitudinal direction only there is limited control of the surface in the transverse direction. This is a particular issue around the ends of the parallel middle body as the definition curves become rather stretched. X-Topology overcomes this poor configuration by defining the surface as a mesh using a network of curves. The approach is already a well-established technique used in other commercial ship design tools. Can the principles used in IntelliHull be applied to X-Topology?

A development objective behind X-Topology was the idea that a good user interface could allow ship hull surfaces to be developed so easily that traditional "Black Box" Parametric Hull Generation would become unnecessary. While this is achieved in many respects, the idea of being able to parametrically change the hull form is strong solution, as demonstrated by IntelliHull. The curve network used to define the surface offers some interesting opportunities as a means to deform shape. The resulting transformation solution [3] uses Form Topology to expose the principle parameters that can be identified in the network represented by only the boundary and feature curves. Changes to vertices in this network deform the surrounding geometry subject to the design constraints applied within the X-Topology Curve definition. Vertices can be moved either by parametric changes or by selecting and moving the vertices graphically using the mouse.

X-Topology Hull Surface being interactively transformed by dragging a node using the mouse.

While X-Topology introduces an easy to use user interface and parametric hull form changes, there is reluctance for ship designers and consultants to embrace the opportunity to experience how easy hull surface design can be. Certainly it is challenging to learn a new things and it is impossible to eliminate the basic skills needed to create a good hull surface definition. This suggests that quick methods of producing a hull surface definition are still desired. The Initial Sizing and Hull Generation tools presently being developed in within the X-Topology toolset follow up on this requirement. This solution can be found in the X-Topology section of PolyCAD. It isn't a completed solution but it does show some interesting possibilities.

X-Topology Hull form generated using the Initial Sizing function.

References

  1. Parametric Generation of Yacht Hulls, Bole, M., Final Year Project, University of Strathclyde, Glasgow, 1997.
  2. A Hull Surface Generation Technique Based on a Form Topology and Geometric Constraint Approach, Bole, M., PhD Thesis, University of Strathclyde, Glasgow, 2002.
  3. Interactive Hull Form Transformations using Curve Network Deformation, M.Bole, COMPIT 2010, Gubbio, Italy, 12-14 April 2010.