Form Topology

What is Form Topology?

Form Topology is a characteristic of hull forms which define the style and the arrangement of shapes within the surface itself. Different types of hull form have different arrangements of shapes and because it is necessary to have smooth transitions between those shapes certain features will always appear next to each other. This leads to common types of shape and style arrangements. While all hull forms will have Form Topology, some hull forms will have more information than others. In particular, ship hull forms have a wide range of different shapes while yachts and other hull forms where the shape can only be described as “curved” have minimal amounts of specific shapes within the hull surface. Unfortunetly, for these types of vessel, Form Topology is of little use.

Form Topology is a useful concept because it breaks shapes down in the hull surface into separate regions. A conventional monohull ship has a parallel region (Parallel Middle Body) in the midships region with sharpened ends in the bow and stern. This arrangement can be broken down further, there being a flat planar region on the bottom (Flat of Bottom) and on the side (Flat of Side) of the parallel middle body which may extend into the bow and into the stern regions. Within the parallel middle body, the Flat of Side and Flat of Bottom will be connected by a region of surface which is cylindrical in nature and is tangential to the Flat of Side and Flat of Bottom. The curved portions of surface in the bow and stern are most likely to be tangential to the Flat of Side and Flat of Bottom regions although in some cases the transition may be a knuckle (a corner). The curved regions in the bow or stern may also be further subdivided by knuckle curve or a line where the angle or tangency has been controlled.

These vessels have a distinct knuckle which fades into the flat of side. The transition between the forward curved area and the flat of side is quite faint. The flat of side at the stern transtions into a fairly clear cylindrical reqion. This type of hull surface would probably benefit from working with waterline in the bow and buttocks in the stern to derive the shape. The bow has a well define stem surface which dissapear half way up the stem from the bulb.
This vessel has a fair well defined flat of side. This has mean that the ends of the vessel have become relatively short leading to significant concavity as the bulb transition to the hull.
On this vessel the flat of side is again well defined but the flat of bottom looks as though it has signicant rise of floor, potentially not planar. Notice how the light transitions quickly from light to dark from the flat of side to the curved area of the bow indicating that there is fairly tight curvature as the parallel middle body transitions into the bow. This will create flattish plates in the central region of the bow surface perhaps making fabrication easier. At the stem, above the waterline there is a reasonably large radius which tightens up quite sharply at the leading edge of the bulb.
This vessel also exhibits large flat of side. But to complicate matters there is a flat portion on the bow and at the forward quarters. Below the forward knuckle, it looks like the forward flat region continues down to the top of the bulb. Developing a hull surface such as this takes some planning as the designer may choose to blend the radius from the transition between the flat areas above into the hull below. Alternatively, it is possible to achieve a good surface by using procedural fairing but this may not provide as much control over surface curvature and its affect on the shell plating.

Although this description of Form Topology is entirely qualitative, it provides use with knowledge and the basis of a procedure in which we can construct the surface definition. It allows the problem of generating the surface definition to be broken down into smaller pieces. For example, again for a conventional monohull ship, we may start by defining the shape of the midship section, the stem profile and the stern profile. Then we start to subdivide the shape addressing the deck, and then setting up the extents of the parallel middle body. Now the specific details of the design can be focus upon, for example the shape of the entrance or the run without having to worry about shapes in other regions of the hull surface.

The original Form Topology sketch which highlights the different regions of the hull and the types of shape that would be expected.

The arrangement of the Form Topology described previously is fairly typical for conventional monohull ship hull forms, but not all of these features may be present. Some may be missing or a different structure of shapes may be present. Form topology is also present in other types of hull forms. The structure of high speed or planning vessels hull forms is often dominated by knuckles to create flat planar area of the surface to induce dynamic lift and cause separation in the water flow. Knuckles can also be employed minimise fabrication effort in hull forms made of plate material. By reducing the amount of curvature in the plates, the material is easier to handle and will not require specific machinery to form.

By considering how different shapes in the hull surface is laid out before the definition process is started it is possible develop a design process and, in hull design software which allow relational geometry, develop a hierarchy of connections that is going to make the task of defining the surface geometry much easier. It may help even to sketch the hull form out as sketches tend to accurately capture the form topology layout.

IntelliHull: The origin of Form Topology in IntelliHull

Form Topology was first defined as a concept in regards to IntelliHull [1]. However, while Intellihull used Form Topology to generate the hull form surface but the technique only allowed too limited a range of Form Topology variations to be of any real use. The IntelliHull surface definition is based on a set of transverse control curves through which longitudinal curves are intersected to blend the shape of the bow, parallel middle body and stern together. Using Form Topology, the features of the hull form definition could be identified and measured providing feedback on the main particulars of the hull form, hydrostatics and dimensions of certain feature. The user can then change the value of these dimensions invoking a transformation to change the relevant portion of the hull definition geometry as identified by the Form Topology. This characteristic of the technique was successful providing the designer with the power of parametric hull generation but the designer still has control over the style of the vessel and has the opportunity to modify the geometric definition of the hull surface using the mouse. However, as the definition curves are primarily orientated transversely it was difficult to control features such as the shape of the flat of bottom. To take full advantage of the Form Topology approach it is necessary to use a network of intersecting curves and design the hull surface using the Cross Sectional Design approach.

A example of an IntelliHull hull form defined with six "transverse" curves.

Form Topology in X-Topology

In order to exploit the full benefits of Form Topology, a cross sectional design approach using a network of curves was necessary. Once a network of curves is available a surface can be generated by blending surface patches between the edges of each face accounting for the shape of each edge and tangent ribbon information interpolated along each curve. While this technique is generally used by the larger and more expensive ship design tools there is limited documentation on the approach. It took several years to develop a working version, the X-Topology Surface, and as long again to optimise the performance of the software and the design procedure.

Form Topology can now be used to generate detailed hull form surfaces using cross sectional design in PolyCAD. The designer can build up the X-Topology curve network in a hierarchy, relating new curves to pre-existing curves in the model and allow changes to propagate through the definition. Coupled with a user interface designer to support this design activity, fairly representative ship surfaces can be generated in a matter of minutes and fair ship surfaces generated with an hour.

A interpretation of the fourth ship shown in the previous images represented as an X-Topology Surface. Form Topology curves manifest themselve as those with surface attributes, i.e. tangent information, and are highlighted in the image as thick white lines with dashes indicating the direction of the tangent crossing the curve.

However, the original objective behind IntelliHull was to develop a hull design tool which would allow the user to interact with the definition geometry to control the shape and style and to interact with parameters representing the dimensions and properties of the hull surface to control its size. The X-Topology surface is too generic as an entity to provide this capability as it is just a representation of a surface. It was hoped that the hierarchy generate as a result of the curve connections would be enough to allow geometric changes of individual points and areas of the hull to be transmitted to the rest of the geometry. However, it was found that often a designer could generate a curve hierarchy which went against the direction of flow required for changes to update the surface. A solution was found, again with the help of the Form Topology [2]. Rather than updating curves in respect to their connectivity and relationships in the surface definition, they can be updated relative to their connectivity in the Form Topology. Treating the Form Topology as a graph, where intersections between curves representing the Form Topology are treated as vertices, the curves representing the Form Topology as Edges and the “shape” regions of the Form Topology, in between, as faces. To effect a change, a vertex can be moved and the changes propagated through the graph, updating the curves representing the Form Topology before the shape of the surface definition, i.e. the remaining curves, are updated. Parametric control of the hull surface can be obtained by again using Form Topology to identify the role of curves in the definition of the surface and identify the important connections between curves, i.e. the vertices, relevant to different dimensions of the vessel. Consequently, when the value of a parameter is changed, the hull surface is transformed by moving the appropriate vertex in the Form Topology graph.

For a more detailed discussion of Form Topology transformations, the reader is directed to [2]


  1. Integrating Parametric Hull Generation into Early Stage Design, M. Bole, COMPIT '05, Hamburg, Germany, 8-11 May 2005
  2. Interactive Hull Form Transformations using Curve Network Deformation, M.Bole, COMPIT 2010, Gubbio, Italy, 12-14 April 2010.