Use of Offset Data to form a Surface

This facility allows hull data, stored in BRITFAIR file format, to be used within surface.

This data is used to form a rough surface by creating triangular planar faces between the points in two consecutive curves.

The following outlines the best approach to take when using this facility. Firstly, some aspects of the data are explained and then the specifics of how to obtain certain features of a hull form.

Point Types

For the purpose of the Britfair sparse input data, at present there are only sections and these sections are made up of data points. These data points are either ordinary points or knuckle points.

Multiple points

Any two points found to be less than a tenth of a millimeter apart will be treat as the same point. The point, which appears later in the file, will be ignored.

Planar faces lying on the Y=0 plane

Any planar faces which lie entirely on the centre plane (that means, Y=0) are deleted. This prevents unnecessary faces being created in such regions as that above a bulb. It is useful at times to define an entire section that lies on the Y=0, this is to ensure that some faces will be deleted. This will be dealt with below.

Reducing the number of edges

By default any unnecessary edges on the hull form will be removed. This typically occurs between two adjacent faces, which lie in the same plane. This feature can be switched off.

Multiple Sections

Within the Britfair sparse data representation, it is now possible to define more than one section at the same X value. This has a number of uses: defining a step in a deck; defining the transition between the number of knuckles running along the hull form; and dealing with problematic pairs of sections. These issues will be dealt with below.

Knuckles in a surface and knuckle points

The knuckle points are necessary to maintain required features in the final hull form such as deck edges, transoms, chines When a surface is created from Britfair sparse data, that surface is constructed from individual faces. These faces are bound by straight edges where a pair of points define each edge, one at either end. If we wish to model a feature, which runs from one section to the next, we will want a particular point on one section to be connected to a particular point on an adjacent section, by an edge. The first step in ensuring this is to make both points knuckle points. The next is to make sure that the sections containing these two points contain the same number of knuckles. If two sections have the same number of knuckle points then the corresponding knuckles will be joined by an edge. That is to say, the first knuckle on one section will be connected to the first in the next section and so on with second and third Therefore, to model particular features each section must have the same number of knuckles. This is not such a restriction as it may first appear. If we wish to create a transition from one number of knuckles in a section to another number of knuckles, we can simply create two sections with identical points (that means, identical in position) but with different points flagged as knuckles. This is very useful when creating complicated features such as rudders. An example of maintaining features via the use of knuckles can be seen in the example offset design FeatExam1. Here we see the construction of a rudder by the selective use of knuckle points to ensure that certain parts of the surface will be created on the Y=0 plane and therefore deleted.

Result from Offset Data FeatExam1

Creating a bulbous bow

In creating a bulbous bow it is useful to use knuckles to define the stem profile. This can be achieved by defining a pair of knuckles, which start at the point of inflection on the stem profile just above the base of the bulb. The lower knuckle defines the top of the bulb while the upper knuckle defines the prow. The first points of each section in that region of the bulb may define the underside of the bulb. The lower knuckle and the first points in the sections will come together at the front of the bulb. At the front of the bulb the first point and the lower knuckle point can be placed on the Y=0 plane and should be placed 1mm apart so they are not identified as the same point. An example of this can be seen in the example offset design BulbExam1. It is also possible to create a vessel, which has a bulb that extends beyond the prow or vice versa. In the case of a long bulb this is achieved simply by allowing the Y values of the data points to attenuate to zero at the level of the prow, while still giving definition to the bulb itself. An example of this can be seen in the example offset design BulbExam2.

Result from Offset Data BulbExam1

Stepped deck

Creating two sections having the same X value can be used to form a step in a deck. In most cases of this type, the sections coincide for the initial part of the section (starting at the keel) then at some point they separate. Up to the position the two sections separate, the data points must coincide and once the sections have separated they must only cross one another at a given data point which appears on both sections. The sections may reunite at any point but again their data points must coincide.

Result from Offset Data StepExam1

Difficult sections

Between any two adjacent sections constructed from data points, there are potentially many different surfaces which could be formed. Occasionally the surface produced between two adjacent sections does not match our concept of the optimal surface. This is more likely to occur with sections that are very close to one another. Converting ordinary points, to knuckle points can rectify the problem. Each section must have the same number of knuckle points, these knuckles are paired off and when the surface is constructed it is ensured that an edge is constructed between these pairs.

Sometimes there may be features in the hull form which run across one of these problem areas and these features must be maintained. One way of avoiding disrupting these features is to duplicate the two offending sections and carry out the above procedure on the two duplicates. It is important to remember that the two duplicated sections must come between the original two sections in the Britfair file. In the offset design DiffExam1, we see that the surface between sections with X values -2.14 and -2.10 may not be what is required. Now if we examine the model in design FeatExam1, we can see that this problem has been rectified by the inclusion of two extra sections at X values -2.14 and -2.10.

Transom stern

An example of a transom stern is given in offset design SternExam1. Note that the first section is almost a straight line but not quite. It is in fact just a very shallow section that means, with difference in Z values of 1 mm.

Result from Offset Data SternExam1

Occasionally there is some ambiguity in creation of the triangles so the surface produced is not what was desired. These problems can be fixed with the TFLIP tool/ command. This allows any two adjacent triangles to be flipped to form two new triangles in place of the original two. When using this tool it is important that several of its properties should be understood.

• Not all the facets which lie on a hull will be visible to the user. The routine which facets the hull may give rise to triangles which are very small or even zero width. These triangle will generally be invisible to the user as they mostly lie coincident with the side of a second triangle. This impacts in two ways on the user.

• 1. Sometimes it will occur that a triangle of zero width lies between two triangles which are chosen to be flipped, this will cause an error as there are no adjacent edges between the two triangles. At the present time this is a limitation which has not been resolved.

  Sometimes when flipping triangles one of the flipped triangles will appear to vanish, this is because one of the new triangles generated has zero width and so is not visible. As the new zero width triangle cannot be flipped, the flipping process becomes irreversible after such an operation and can only be undone using the rollback.

• The routine used to flip triangles checks whether the two triangles selected share a common edge, if this is not the case the routine will not allow the triangles to be flipped. Note two triangles which share only a common vertex do not satisfy this condition.

• Triangle flipping is not an exact tool and should not be viewed as a tool for hull construction, it exists solely for the purpose of tidying up hulls imported as sparse data. It is also important to state explicitly that this tool works only on triangular faces.

It has been found that in some situations it is better to replace two adjacent triangles with a single quadrilateral surface. It must be noted that this quadrilateral surface may not necessarily be planar. Quadrilateral faces will occur when there exists two adjacent sections that have the same number of knuckle points in them and a section contains two adjacent knuckle points with a corresponding pair of knuckle points in the other section. The creation of quadrilaterals is a default setting, which can be over ridden in the dialog box.

Typically, offset data only represent the port or starboard half of the ship so the default reflect option can be used to mirror the partial hull form about the centreline.

The use of offset data in surface can be initiated via the FILE \ USE \ OFFSET DATA... menu or from the command line using the syntax. There are two possible methods by which this data can be turned into a new hull, on the Use Design dialog these are referred to as Sparse (uses triangular planes) and Skin (uses curved surfaces). From the command line the Sparse option can be activated using the command

IMPORT BRITFAIR /SPARSE $F

If the user requires a closed hull, the optional keyword /REFLECT can be used, while if the user requires a hull containing quadrilaterals the optional keyword /QUADS is available. There is also an option to reduce unnecessary edges between faces, for this the /REDUCE key word is applied.

For the import of files of triangular facets only a command line option exists, this is simply

IMPORT FACETS $F

If the command is issued from the FILE menu, these options are available on the Use Design dialog.