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# How to split surface for patches right way.

Updated: Mar 24, 2019

Practically all modern systems of surface smoothing of ship’s hull are based on NURBS. This allows you to use IGES file as a standard file for exchanging ship hull geometry between different systems. Of course, this gives great advantages, because the surface can be transferred without distortion and exactly the same how it was done in the source system. But there are some drawbacks. Any surface defects that could not be resolved will be visible, for example, in the design system of the hull structures.

Often, I see it in Shape Maker, when I import the preliminary surface for final smoothing. With all the variety of surface smoothing programs, in most cases these models present the same typical errors. If you consider these errors, even the initial surface of the hull can be smoothed much better.

### 1. Modeling of the whole hull with only one surface patch.

I have repeatedly seen on various demonstration videos how the hull surface is modeled by just one piece of surface. Everything modeling just by one patch: flat side and flat bottom surfaces, and the curved surface itself. Of Course, this allows to generate the hull surface very quickly, but it does not always meet the requirements of the designer to the ship's surface. This surface does not have a clearly indicated flat side and flat bottom. They kind of turn out to be themselves. It is not possible To control the shape of the flat side and flat bottom lines. It is Also impossible to control the shape of sections in areas close to the flat bottom line and a flat board. At the same time, these lines Are very important for the technology of modeling of Hull designs. If you look carefully at the shape of the Midship line, it is usually the vertical line of the board, the horizontal line of the bottom, and the arc defining the shape of the bilge area of the vessel. When You specify a line such as a single curve segment, you need to change the weights at the start and end points of the arc. In most cases, which I saw, instead of an arc was used just a curve close to the arc. In other words, this way of setting the surface is very difficult to control the shape in the areas close to the flat bottom and flat side.

Buttocks shape without flat side line.

### 2. Unnecessarily large number of surface patches.

Some surface design systems, the presence of a large number of Bezier patches is justified by the algorithm of building the surface itself. Sometimes a large number of surface's patches is caused by the inability to modeling correctly that, or another, surface's area, or a clear lack of experience with surfaces modeling. In this case, the number of surface areas begins to grow like a snowball. Defects of one surface are closed with new patches. Any manipulations with such surfaces are very complicated. It is almost impossible to control the shape of such surface and, moreover, to withstand the smoothness conditions. It is also very difficult to determine the common boundary line location of two surface patches on the imaginary hull surface. It is necessary to fulfill conditions of joining of these two patches of a surface among themselves. This requires a lot of designer's time. The quality of such a surface cannot be good.

About 250 Bezier patches is used for hull surface definition. Boundary lines has yellow color.
Example of not good splitting of surfaces for patches.

### 3. Gaps between surfaces.

In systems that do not support topological links between patches of the surface gaps between patches can appear. This indicates that the adjacent boundaries of these two patches are defined by different lines that do not match each other. This problem has to occur most often. Maintaining the continuity of the hull surface is an important task for calculating hydrodynamics and hydrostatics of the vessel. Everyone solves it differently. Some build patches of the surface between adjacent borders, others extend the surface patches and build the intersection line. In any case, this requires a lot of time, especially when the surface needs to be changed several times in the initial stages of design

Gaps between surfaces.

In general, the process of subdivision the ship's surface into patches is a non-trivial task. The complexity and quality of the resulting smoothed surface depend on how optimal the surface is divided into patches.

When I make surface smoothing is Shape Maker I try to follow to the next rules:.

### 1. Make separate surface patches for flat bottom and flat side.

This makes it very easy to define the flat shell plates area to produce the vessel's hull documentation. It is also very important to define the geometry of the blocks and sections of the hull. Robotic mounting and welding lines can be used to assemble flat sections.

Examle of fore ship hull surface subdivision. Flat side, flat bottom, horizontal keel and flat bar in CL splitted into different surface patches.

### 2. Try to make curved surfaces in one patch.

As a rule, smoothness on the first derivative is not enough to make the smooth hull surface. The smoothness of the first derivative and the continuity of the second derivative of the surface remain only inside one patch of the surface. When two adjacent surface patches are connected, only the first derivative (tangency) continuity is performed. Therefore to join such patches to achieve natural smoothness of surfaces difficultly.

### 3. Join curved surface patches in the area of the surface without double curvature.

If you are not able to avoid smooth connection of the two curved surfaces, try to dock them in the area of the surface without double curvature. In this case, the acceptable smoothness of the joined surface patches is much easier to achieve.

Examlpe of joining two curved surfaces.

### 4. The surfaces between the knuckle lines must be defined by the separated surface patches.

Shape Maker has topological relationships between elements. Due to this, when changing the shape of the boundary curve of the surface patch, the shape of the surfaces relying on these boundary changes naturally. Another nice feature is that the mathematical apparatus of the surface setting excludes the appearance of gaps at the borders when changing the shape of the boundary curve. It allows forming separate patches of a surface between knuckle lines.

Splitting surface by knuckle lines.