Sail Design Process
Three dimensional modeling, pressure distribution, CFD and FEA runs, cutting edge fiber distribution and much more all figure into making a modern sail for your boat. The following outline will bring you through some of the industry leading resources Doyle Sailmakers' design team utilizes in designing sails for your boat.
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. Depending on the information available, this can either involve physically measuring the boat or utilizing a CAD file that will specify all of the hull and deck details, the rig layout and the running rigging layout. 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 model of the boat can be produced. 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).
Creating the 3D model, or at least considering all of the parameters of the boat, is essential to the sail flying as intended. As an 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 as a result 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 and fuller entry to the sail in the upper portion. The sail luffs earlier and a wider exit angle decreases the power in the sail.
Once the 3-dimensional sail shape is determined, the engineering team can begin to look at the sail in a virtual testing environment. The goal of CFD analysis is to optimize the lift and drag ratios for the sails, and by modeling the sail with all of its associated rigging, the team can see exactly how the sail will fly. Utilizing a virtual platform saves time and produces results that can be very easily recorded and analyzed. In comparison with traditional Wind Tunnel testing which can only look at certain specific points, CFD runs allow designers and engineers to see lift and drag coefficients at any point in the sail and then easily optimize the sail design. By trimming the sails in a virtual environment, the team can also see the interaction of various sails within the inventory and ensure that not only is a singular sail shape good, but that they will work well in tandem with the other sails that will be flying at that time.
Furthermore, CFD analysis ensures that the rigging and other hardware is accurately accounted for. Not only is the flying shape of the sail investigated, but its interaction with the mast can also be seen. For new project developments, this can also mean that at this point, the shape of the actual rig or deck hardware can be modified for optimal flow.
Once an optimum shape is settled upon the sail now enters a purely analytical aspect of the sail design process - pressure distributions and then finite element analysis (FEA). 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 its stretch resistance, we utilize a polar plot of the materials resistance to stretch and the placement or alignment of this material in the sail. The placement and alignment 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 allow for the sail to hold its shape in the expected conditions, Stress-Load analysis is performed using FEA. This layout of the sail shows the expected load in the sail for the given conditions, which is then utilized in determining the final layout of cloth or fiber.
Once all the analytical work is completed, 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 go into production. 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 an 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 result of this process is sails that will fly as intended right out of the bag and the careful analysis of the loads in the sails ensure that the sails will hold their intended shape over their life. While sailing, Doyle's design software allows them to measure the sail in actual sailing conditions and compare them to the intended flying shape, ensuring accuracy in the system.