The scientific Finn

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Since autumn -94, WB-Sails has applied a rigorous scientific method to their Finn-sail development. The results are excellent: In less than a year we have come up with a sail/rig combination that has won 7 out of the 11 races sailed in the two pre-olympic regattas at Savannah this summer and fall.

The sail development system consist of:

  • a masthead camera fitted to the top of the mast
  • a computer program to analyse sail shape from photos (ShapePlot)
  • a computer program to predict sail force (MacSail)
  • a computer program to calculate mast bend and its effect on sail shape (TrimSim)
Initially, several sails of different make were photographed, before the first prototype sail was built. The MastCam film is developed onto a CD-ROM, which allows us to read the shapes directly into the computer for an accurate analysis. The analyzed shapes are read into a high-end aerodynamic program MacSail, which predicts the driving force and the heeling moment of the rig very accurately. MacSail can pinpoint flaws in sail shape, such as flow separation or luffing, and allows us to perfect the shape.

Of course, simultaneously with the theoretical work, most valuable practical information about the behavior of the sail was obtained from the two champion sailors, Jali Mäkilä from Finland and Fredrik Lööf from Sweden. The input from the sailor is even more important when it comes to judging the stiffness & bending of the mast.

Finn TrimSim

The Finn class having recently allowed carbon fiber in the rig, it was necessary to develop a new mast. For this purpose we wrote a new program, Finn TrimSim, which calculates the mast bend when material characteristics and mast cross dimensions are given in ten sections at 60 cm intervals. TrimSim allows us to tell accurately how much material needs to be added to achieve a certain mast bend or, on the other hand, how the mast bend effects the sail fullness and shape.

Finn sails MastSim

The TrimSim window

Finn sails TrimSim

Wind tunnel tests

WB-Sails also did a series of wind tunnel tests to study the importance of the mast shape on sail force. The Finn class rule allows a small wing mast - it proved clearly superior to the traditional oval or round spar, so a wing mast was produced. Sailing, it showed definitely faster than the round mast with similar bending characteristics. Part of the advantage comes purely from the area that the mast adds but which is not measured by the rule. Most of the efficiency, though, comes from a smoother airflow on the windward side. On the leeward side the airflow is slightly more disturbed, but this doesn't seem to bother too much. All in all, with a correctly shaped wing mast lift can be increased more than 5% with no effect to the drag. On the downside, the wing mast is slightly heavier, more difficult to achieve the correct bending characteristics with, and much more expensive to produce than a round mast. 

The same scientific development tools - MastCam, TrimSim and MacSail, are applied to the 470- and Europe-class as well. They have also been used in sail development for other one-design classes, such as the Dragon or the H-boat, as well as the Maxi-yacht Nicorette.

The Sail Design Spiral

Finn sails DesignSpiral

  • The creation of a new sail design starts with a MastCam session.
  • MastCam photos are analyzed with ShapePlot, to record the flying shape of existing sails.
  • The flying shape is read into MacSail, to calculate sail forces. Design flaws are corrected and shape is optimized.
  • Sail forces are input to the VPP, to verify performance with the new, altered shape.
  • New sails are designed in the computer ...
  • cut with the Laser and sewn together.
  • ... The spiral starts again: new sails are photographed for computer analysis.

This way, sail shape is improved systematically. By combining computer simulation to the real world via MastCam, development work progresses towards an optimal shape much faster than with the traditional trial-error method.

ShapePlot caption of a 470 main sail

Finn sails ShapePlot

Sail shape is picked up into the computer by mouse-clicking tape markers on the sail scan (above). The program then gives a detailed analysis of the sail (below).