Often a Naval Architect has to produce hydrostatic or other data from an existing design. For example: a modification to the hullshape has been made and a stability booklet has to be re-submitted; or a similar vessel is to be built from existing plans. The existing design may be in various formats: paper lines plan, a CAD drawing, or an offsets file. Depending on the type of output required and the design data available there are different tools available and they are used in different orders.

1. Data Representation of Shapes

You can represent a shape in space in the following ways:

o        Points

o        Lines

o        Surface(s)

A cloud of points can describe a shape in 3D, but it is very difficult to see whether all points are exactly on that shape. Or: whether the shape that runs through the cloud of points is fair.

By connecting the points with fair lines, the shape can be better visualised. The problem with this method is that there is no explicit relationship between the individual lines thus making changes to the 3D shape is time consuming because changing one line means that all other lines must be updated so that the lines are all consistent with one another.

The highest order to represent a 3D shape is using a 3D surface. The surface can have different levels of stiffness which can be seen as the stiffness of the traditional fairing battens. Any required line (e.g. section, waterline, buttock) is taken directly from the surface ensuring that all lines are consistent. This is what Maxsurf NURB surface modelling does. For more information, see the Maxsurf manual chapter 2.

The greatest advantage of using a surface to represent a shape is that you can derive points and lines from the surface instantaneously and this allows you to produce lines plans on the fly and make changes easily. The Maxsurf suite takes this concept one step further and derives the input required for stability, resistance and seakeeping analysis as well as production information directly from the surface shape. This enables a naval architect to make last minute changes to the hullshape as the design iteration cycle progresses.

2. Fully automatic fitting

Before you continue reading this document, it is important to establish that a fully automatic fitting tool that fits a NURB surface to a set of points or lines does not exist. Human interaction to judge the input, the hullshape and decide on the output information required from the surface model is necessary.

Having said that, there are some cases where fully automated surface fitting is possible: a round bilge simple hullshape where only Hydrostatic output information is required can be fitted automatically.

3. Maxsurf Tools

Maxsurf has two options to assist with Naval Architecture tasks that start with existing design data in any format:

o        NURBS Surface

o        TriMesh Surface

Which one to choose and how to get from a specific type of input data to a specific output is described in the next step-by-step sections.

Step 1: Think Ahead

There are many different considerations you need to make before starting on a surface fitting process.

1a. Types of input

Typically you can have the following types of input:

o        Pictures of the vessel

o        Paper lines drawings

o        Table of offsets

o        2D DXF lines plan

o        3D DXF lines plan

o        NURB surface description in the form of an IGES (or similar) file

The type of input you have does not really determine whether the TriMesh or the NURB surface method is suitable, but it does determine how much work is required to produce a hull model by each method.

1b. Hull Shape

The hull shape of the existing design is a very important factor when determining how to create a surface model. Some hull shapes like catamarans and trimarans can be difficult to achieve with TriMesh surfaces. These can then best be done using NURB surfaces.

Whenever you use NURB surfaces to reproduce an existing design you will have to determine the required topology and how best to lay out the surfaces at an early stage. To some extent this is also dependent on Step 3: what you want to do with the surface model. For example: in case you want to use the surface geometry to develop plates, you may need to use a different topology than when you are producing a hydrostatic model.

Basically, when you are talking about surface fitting, you can divide hullshapes into the following categories:

o        Simple: Round bilge

o        Complex: Chined hulls, stern bulbs, bulbous bows, specific features in the design (bilge radius, flat of sides, bow cones), multihulls

Interpreting the hullshape and deciding how to arrange your surface model is the most difficult part of surface modelling and requires some experience. Some general guidelines to surface topologies (surface lay outs or arrangements) are given in Determining Surface Topology on page 155.

Note: when you fit surfaces to an existing design, always try to break down the hullshape into the simplest shape. Do not include keels or other appendages at this stage and also ignore steps in the deck or specific transom features. These can be modelled later with separate surfaces and possibly using surface trimming (in, for example, the case of producing a vessel with a stepped deck).

1c. Types of output

What do you want to do with the surface model? Do you want to do some hydrostatic calculations, resistance analysis etc.; or will you be delivering production information from the model as well?

Basically there are two types of output data that can be derived from a surface model:

o        Hydrostatic calculations, resistance analyses, seakeeping analyses.

o        Production information

This determines the required model fairness and accuracy and thus also the tools you will have to use to create it. A Hydrostatic model generally requires less fairness and accuracy then a production model.

1d. Ability to make changes

Another important question that you have to ask yourself before producing a surface model is whether you will need to be able to make changes in the future. Also the type of change that is required later on may be relevant. For example, if the existing design is going to be lengthened by inserting a parallel midbody, you will want to split your design in to a fore and aft part and bond them in the middle.

Step 2: Choice of tools

The process that you are about to start is called “surface fitting”; you will be fitting a 3D surface to a set of lines or points. The Maxsurf range offers different tools:

Next: see Surface Fitting – Procedures on page 154.