Maxsurf allows you to create complex designs using any number of a range of surface types. The two main types that are supported in Maxsurf are:
NURB stands for Non Uniform Rational B-spline. The shape of a NURB surface is calculated mathematically from the following properties only:
§ Control point positions
§ Control point weights
§ Surface stiffness in transverse and longitudinal direction
The principle behind NURB surfaces is explained in chapter 2 in the section The Spline and Spring Analogy on page 6.
Maxsurf is capable of creating complex models using any number of NURB surfaces with up to 25 rows of control points. Maxsurf allows the use of four surface types, which are all members of the NURB family:
B-Spline
This type of NURB surface has all control point weights set to unity. B-spline surfaces are ideal for many applications of ship design. However, they can not precisely represent simple conics such as circular, elliptical or parabolic curves. Maxsurf offers an alternative surface definition (Conic) that overcomes this disadvantage and allows you to combine conic curves with free form surfaces at will.
With this surface type, it is possible to precisely represent conic curves and surfaces. This is achieved by the addition of weights, which may be varied for each control point.
When this surface type is selected, a NURB surface is used and the control point weights are automatically calculated by Maxsurf to produce a precise conic surface. Use this surface only if you require precise conic surfaces, such as for a bow cone.
Note
The control point weights are only computed automatically for 3-point forms. If you are using 4-point forms you will need to make your surfaces NURB surfaces and type in the weights manually.
Developable
A developable surface is one that can be formed from a flat sheet without stretching the material. This material may be aluminium, steel, wood or even paper - the material chosen has no effect on whether or not the surface is developable. Examples of simple developable surfaces are cylinders and cones, as opposed to a sphere which is clearly not developable. A developable surface is created from the edges of a B-spline surface and a set of offset points can be generated at each station, See Generate Markers for Developable Surfaces on page 206. These offsets can then used to manually or automatically fit the B-spline surface to the developable shape. This allows you to create a surface that is developable within practical tolerances. Maxsurf can give you an indication of the level of developability by displaying a Gaussian curvature rendering of the surface. See Curvature Display Using Rendering on page 92.
For the actual plate expansion calculation and export of the expanded plate shape to a CNC cutting machine or other CAD software, you will need Workshop; the structural definition module of the Maxsurf suite.
A TriMesh surface is a linear surface made up of triangular facets. It is not a NURB surface, it does not have control points, and it cannot be trimmed with NURB surfaces.
The TriMesh surface is not designed to be smooth – it follows the input data points exactly and is linear between the data points. The Markers should provide sufficient detail for the purpose to which the model is going to be put such as, for example, hydrostatic and/or seakeeping analysis in Hydromax, Seakeeper and Hullspeed.
It is often the case that line or point data for a vessel exist but a full NURB surface model does not exist. Reverse engineering an accurate NURB surface model from point or line data can be laborious and time consuming. Often the level of smoothness obtained from a NURB surface is not required and a surface made up of linear, triangular facets is sufficient. The aim of the TriMesh surface is to provide a quick route from line or point data to a vessel surface model that may be used for analysis. The accuracy of a TriMesh surface representation of the hullform is dependent on the number of data points used to generate the TriMesh.
Because a TriMesh surface is a linear surface, it is generally not suitable for construction purposes. This means that if for example plate expansions and longitudinal stiffener geometry outputs are expected to be required at a later stage in the design process, the Maxsurf and Prefit tools for fitting NURB surfaces to markers should be used.
See Generating a TriMesh Surface on page 162 for more information.