Belly (sail)
In order to develop the best possible aerodynamic properties, a sail needs a special profile. The shape or "bulge" in the sail surface that is created by the wind is called the belly .
Regardless of whether a sail is driven by its aerodynamic drag - the wind blows into the sail area from behind - or the propulsion is generated by dynamic lift (i.e. through the formation of an airfoil profile), it makes sense to have a shape in which a the wind can form a curved, smooth surface that is not disturbed by folds or irregularities.
If a flat surface is to be arched in this way , its surface is inevitably enlarged. The sail would have to form the bulge by stretching the canvas; However, this would hardly have the optimal shape, would be irregular and also limited by the material properties.
Assemble a three-dimensional shape from flat surfaces
In order to obtain a profile in the desired shape, the required belly is worked into the sail - the sail is not designed as a two-dimensional, flat surface, but as a three-dimensional object. In order to obtain such a shape, it is necessary that individual panels of sailcloth receive a material allowance so that the desired three-dimensional shape can later be formed as optimally as possible.
A simple approach, especially used earlier, is that the sail is given a correspondingly curved cut at the leeches , so the material required for the formation of the abdomen is "supplied" from the side areas of the sail:
It is much more effective, however, to assemble the sail from several individual parts, each of which has a specially pre-calculated cut, which then results in the desired three-dimensional shape. The surface of a curved body - for example a sphere - can be roughly described by breaking it down into several partial areas (whereby the actual curvature of the surface does not cause any problems, so that a spherical surface can also be composed of individual "strips"):
Although a sail can only be compared to a spherical surface to a limited extent (whereby a spinnaker at least comes close to this shape), comparable “cutting patterns” are definitely used for modern sails. However, have, depending on the type of sail claim and also far more complicated sail sections result. Since the fabric of a sail that is “three-dimensionally cut” in this way still stretches , which means that deformations of the surface also occur, the expected material expansion must also be included in the sail cut.
In the past, the sail cuts were primarily determined by the experience of the sailmaker , patterns of particularly successful sails or special templates, today they are usually calculated by special computer programs.
