Three dimensional modeling, pressure distribution, stress strain pictorials, laser cutting, wind tunnel testing, and much more all figure into making a modern sail for your boat. The following outline will bring you through some of the steps used in designing sails in the 90's and beyond.
The first step in any sail design process is to decide on the geometry of the sail needed to fit the boat. All the outside dimensions need to be defined. In most cases this is an easy operation and can be accomplished by directly measuring the boat with a tape measure or consulting a sail plan which is a scale drawing of the boat. Once the size of the sail is confirmed the interesting analysis starts.
The outside dimensions of the sail are entered into a sail design program where a 3 dimensional shape is described. In most cases 3-dimensional shapes are described by horizontal and vertical cross sections of the surface. The key factors in describing these sections are chord depth %, maximum draft position, entry and exit angle, and twist (the amount of deflection the leech has from a straight line between the clew and head, generally described in degrees). For most boats the optimal sail shape cannot be applied to the sails because of several obstacles like genoa track locations, spreader lengths and position, headstay sag and mast bend.
For example, if a sailmaker designs a genoa for a boat that has long spreaders and does not consider the spreader length when designing the sail it can not achieve the designed 3-dimensional sail shape the genoa must be trimmed through the top spreader to do so. Since the spreader cannot move the sail cannot be trimmed in, and thus the sail sets with more twist than designed. This one little design flaw causes the sail to set with the following problems, flatter camber than designed, fuller entry to the sail in the upper portion thus, the sail luffs earlier and, wider exit angle decreases the power in the sail.
Once the 3-dimensional sail shape is described the sail is ready for wind tunnel testing. The goal of wind tunnel testing is to optimize the lift and drag ratios for the boat. These coefficients change with the change in 3-dimensional shape. Therefore several models may be made to hone in on exact specifications.
Once an optimum shape is settled upon the sail now enters a purely analytical aspect of the sail design process, finite element analysis and pressure distributions. To get meaningful information from finite element analysis we must consider the properties of the material used to make the sail in addition to the loading on the sail. Since sailcloth is non-isotropic in it stretch resistance we utilize a polar plot of the materials resistance to stretch, and the placement or alignment of this material in the sail. Since sails are dynamic in their use and do not have one load path that handle all conditions and trim. The placement and aligment of the material utilized is an important aspect of the optimization process. Therefore the material selected must have ample strength in all directions to handle the dynamic loading. To help us understand these types of loads we generate stress and strain graphs of the sails. These graphs analyze the sail and allow adjustments to the design or shape of the sail, these adjustments are made to bring the sail into predetermined stress/ strain limits. The output from this process is a loaded 3-D sail shape taking into account initial 3-D shape, material properties and alignment, wind speed and direction. This deformed 3-D shape is then brought back into the design program.
Once all the analytical work is completed and the 3-D mold of the sail is decided upon and the exact material is selected, the sail can now be broken down into small 2-D pieces and cut out. This is accomplished mathematically with geodesics. A geodesic is the shortest distance between two points on a curve surface. Similar to a great circle route when crossing and ocean, the same principals apply to the sail surface. The term mold is used in several sail companies. Currently all sails, if designed utilizing a 3-D design program, are mold shapes. No sail material is molded. All sails today are all constructed with flat plates that are shaped to simulate the 3-D shape. The image below illustrates one such panel, the yellow line shows a magnifaction of the curve used on a panel in a sail.
- Stress: F/A, or the load divided by cross sectional area.
- Strain: (Li-Lo)/Lo, or Deformation in length divided by original length.
- Modulus "E" = Stress / Strain, is the slope of the straight line portion of the stress strain diagram.
Courtesy of Doyle Sailmakers