Search

Computer graphics in hull shape design. Pros and Cons.

Updated: Jan 16, 2020

Look at this wonderful picture of Cecil Beaton. It was made in 1943. But up to now not so many changes happened at loft flor. Only bending templates now cut by cutting machine. It is steel a lot of manual, work at shypyard for parts bending and preparation.



Two men cutting templates in the mould loft, Tyneside Shipyards, 1943
Two men cutting templates in the mould loft, Tyneside Shipyards, 1943

It so happened for a long time that the shape of the hull surface is determined by a set of intersecting orthogonal sections. A set of cross sections says a lot about the shape of the case for an experienced designer. The smoothness of the lines and the change in shape from one curve to another clearly represent the shape of the hull. Only twenty years ago we began to use analytical surfaces to determine the shape of the body. Prior to that, computer models used all the same grids of orthogonal curves as many years ago. For designers and shipbuilders, this choice is quite logical and natural. Hull structural elements are typically a set of frames in orthogonal sections of the hull. The current waterline determines the shape of the hull when the ship is immersed in water. In a word, a theoretical drawing is not only one of the most important drawings of the future vessel, but also an open book for the designer, according to which much can be said about the characteristics of the vessel.

Only hull lines builders use for hull shape definition and checking.
Only hull lines builders use for hull shape definition and checking.

Modern ship surface design programs offer a variety of new forms of shape quality control for shipbuilders. Are all of them suitable for surface quality control?


Modern programs for modeling the hull surface instead of a grid of orthogonal curves use patches of parametric surfaces. This makes it possible to mathematically accurately determine any point on the surface. It would seem that the need for visualization of cross sections disappears. The graphical capabilities of computers allow you to visualize the surface of the hull both in shaded form and in the form of graphs of curvature, zebra and realistic reflection. Are these methods sufficient to control surface shape? Perhaps in other industries, such as jewelry and modeling the surface of the car body, this is enough. In shipbuilding, in addition to the aesthetic presentation of the surface, technological factors must also be taken into account. To assess the manufacturability of the hull surface is unlikely to help, for example, a color representation of the Gaussian curvature of the surface. It is rather a mathematical characteristic of a surface that says little to the designer.


Shaded hull shape for preliminary model.
Shaded hull shape for preliminary model.

Shaded hull shape for finally faired model.
Shaded hull shape for finally faired model. It is not so easy to see difference. Shaders design to hide app shape defects.



Zebra shading for preliminary hull.
Zebra shading for preliminary hull.



Zebra for finally faired hull. It is difficult to undersand hull shape quality.
Zebra for finally faired hull. It is difficult to undersand hull shape quality.


Preliminary surface reflecition.
Preliminary surface reflecition.

Faired hull surface reflecition.
Faired hull surface reflecition.

Mean curvature for faired hull
Mean curvature for faired hull

Same hull, but with differen limits of curvature. What kind of information we can get from here?
Same hull, but with differen limits of curvature. What kind of information we can get from here?

For the designer, the shape of the lines of frames, waterlines and buttocks is still the most informative. It is these lines that are most often used to determine the geometry of hull structures adjacent to the outer skin. On these lines, plate and profiles hull parts are built. The shell plates of the outer shell bend along the same lines. If, when modeling body parts, a line is used that exactly repeats the contour, with all its defects, then the shell plates in this area will take a natural shape close to the shape of the spline. This leads to gaps between the shell plates and the internal structures. As a result of poor-quality smoothing of the ship surface, there is a need to fill the gaps with welding, overheating of the metal and deformation of the structure. This, in turn, creates internal stresses leading to the development of fatigue cracks in body structures. An opinion on the quality of the ship surface can best be obtained from shipbuilders at the shipyard. I myself saw a team of 5 people using a crane tried for four hours to install one sheet of skin on the body. The reason is an incorrectly smoothed surface in this area.


A poorly smoothed hull surface may not be optimal from the point of view of hydrodynamics and this makes the vessel more expensive to operate. According to data from various sources, for example, an average fishing vessel saves about 500 thousand dollars a year with a 10% reduction in hull resistance. In addition, a poorly welded and assembled body rusts faster and requires additional costs for cleaning and painting and eliminating fatigue cracks in the body.