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SAIL DESIGN

North Sails started using its own computer software to design sails in 1977. By 1979 North had developed Computational Fluid Dynamics (Aerodynamic Performance Analysis or CFD) programs to calculate sail efficiency and Finite Element Analysis (Structural Analysis Software or FEA) to calculate flying sail shapes. These programs became the building blocks for today’s North Sails Design Suite... the most advanced software suite in sailmaking. Every single North sail is created using the same design and analysis tools used for America’s Cup and Volvo Ocean Race projects.

The Design Suite brings unprecedented power and flexibility to sail and rig analysis. Each specialized program can interface with others within the suite, allowing North engineers and sail designers to virtually sail and test a boat in an infinite variety of conditions before construction. The benefit is significantly reduced tuning and development time after launch.

1) Desman

Desman develops a complete rig model in a three-dimensional environment. It is the skeleton of North Sails Software Suite. In Desman we can define all rigging attachment points to the deck, mast size, rigging positions, trimming locations, etc. The modeled sail/rig system incorporates the mechanical properties of the rig and the running rigging, as well as moments of inertia, sail and spar area, materials stiffness and resistance to stretch. Using this information, MemBrain (described later) determines deformation under load for the sail and every piece of standing and running rigging, right down to stretch in the sheets and halyards.

2) Spine & Spiral

Spiral initially defines the sails’ three-dimensional shape as a ‘molded’ surface without any wind pressure or load applied. Precise control over all aspects of a sails’ shape allows the entire range of sails to be designed, including mainsails, genoas, jibs and asymmetric spinnakers. In addition to the sail dimensions, the sails’ 3D edges and shape are adjustable both in horizontal and vertical axes.

3) Flow

In Flow 2006, air flows over the sail and produces a pressure map on the sails’ surface. The pressure varies over the surfaces based on the size and shape of the sails and the conditions of the flow. This pressure field is then linked directly to Membrain for finite element analysis. Flow 2006 also reports the force and moments developed by sails. That is why it can also be used to derive the forces and coefficients used by either the North Sails VPP or other VPPs for sail performance analysis.

4) Membrain

Membrain takes the sail shapes from the Desman model and the pressure from Flow and applies pressure to the sail/rig combination, deforming the mast and sail as a single unified structure. As the structure is deformed, the shape of the sail changes, so Membrain links back to Flow to acquire new pressure fields for the deformed surface. This changes the sail shape yet again, and the process is repeated until there is convergence between pressure and rig/sail deformation.

5) VPP and HullVPP

During the last America’s Cup North’s VPP (Velocity Prediction Program) used hydrodynamic data (tank testing and CFD runs) from the different teams to optimize sail shape and sizes for each boat. To apply this technique to other types of boats without tank testing and/or expensive CFD modeling, North developed HullVPP. Created by North Engineer Michael Richelsen. HullVPP calculates hydrodynamic forces for a specific boat using a 3D surface file of the boat’s hull and appendages. North VPP then integrates these hull forces with the sail forces generated by Flow, Membrain and Virtual Wind Tunnel. As a result, sail design can be tailored to a specific boat in specific wind and sea conditions. No other sailmaker can even come close.

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