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station line and the waterline. Both the B-lines and S-lines are used for both fairing and forming the
mesh of the X-surface. The R-line is only used for reference purpose and does not form the mesh of
the X-surface.

2.2 Non-manifold Data Structure

In EzHULL, the hull surface data is represented as NURBS (Non-Uniform Rational B-Spline) surface
patches. To improve the usability of the hull surface data, it is necessary to save the topological
information that represents the relationships between surface patches as well as geometric information
of them. To do this, EzHULL uses a non-manifold data structure that can store both the topological and
geometric data of the hull surface without thickness [2]. Since the non-manifold data structure can deal
with not only manifold data, such as, solid objects but also with non-manifold data, such as lines and
surfaces, many commercial CAD systems have recently adopted non-manifold data structure. So,
representing the hull surface data in the form of non-manifold data structure facilitates the transfer of
surface data to the next process.
In EzHULL, geometric information, which is stored in the non-manifold data structure, is mainly used
for verifying surface fairness by surface shading. The topological information is used for points
ordering to generate a new hull line of the wireframe model. The non-manifold data structure is also
used in the solid model-based compartment arrangement and the hydrostatics calculation system,
which are currently being developed.
Fig. 1 shows an example of an afterbody wireframe model and the corresponding surface model. More
detailed process which converts the ‘X-topology’ of the wireframe model into the non-manifold data
structure of surface model, is described in Section 4.1

X-Towloa~ Structure Non-manifold data Structure

Geomebv data
Figure 1 : Example of an afterbody wireframe model using the X-topology and corresponding surface
model represented by non-manifold data structure

3 ASSOCIATION BASED CROSS FARING

3.1 Priority Order of the Association
In EzHULL, association based cross fairing is developed using the X-topology data structure to