FSI Software Game Changed for One Design Sails

By Live Sail Die | July 10th, 2019

SailPack is the typical tool Doyle Sails designers use for daily sail design. The software is very capable, allowing designers to create the sail profile, curvature, panel layouts and fibre layouts for each project. More impressively, these tools can set up the rig geometry to see how the sail mold will actually look on the rig.

However, SailPack does not allow us to accurately predict how the sails will fly in real life. But when coupled with Aravanti FSI tools, we have the added ability to build a full computer model of the rig and put the sails on it. Not only does this help with the geometry, it can indicate the deformed rig and sail shape geometry through a range of environmental conditions. We caught up with Doyle Sail Designer Andrew Lechte to explain how it works and why it’s a game changer for one design classes.

SailPack rig mode (left) and on-water testing of new sail designs (right)

SailPack rig mode (left) and on-water testing of new sail designs (right)

FSI is a Finite Element Analysis tool which is coupled to a Computational Fluid Dynamics solver. The FEA models the structural elements of the rig and sails while the FEA models the airflow and pressures. These 2 tools run iteratively to reach equilibrium in load and forces. When the 2 processes converge, the resulting deformed geometry closely matches the real-life sail and rig shapes and loads.

The information gleaned from the FSI results allows designers to look at how the structure of the sails and rig handles the loads and can point to areas in the design where materials and structure can be improved or optimised for the particular project.

It is also possible to optimise drive force through alterations to the sail design or through trim changes. Generally, FSI is used mostly on big projects where rig and sail loads are large and the additional time that goes into analysing all the rig and sail analysis covers the cost of the licensing and analysis time. However, using FSI on any yacht project will result in a better understanding of the yacht and sail design/trimming so ideally, we would use this design software for everything possible. But for small projects, the cost of running it can be difficult to justify simply because the margins are very tight and will not usually cover the time needed to research the rigging specifications, tuning, model development and reporting that is needed for each project.

But when I have spare time during or after work, I have used it to prepare small boat models in popular one-design classes. It is the best way to learn enough about a particular class with limited time actually sailing the yachts.

VX-One rig analysis: Pressure delta (left) and VX-One fibre layout and rig geometry (right)

VX-One rig analysis: Pressure delta (left) and VX-One fibre layout and rig geometry (right)

The computer model allows the designer to change more than the rig-tune and sail trim. We can change batten specifications, fibre distribution, surface films, mast inertias and sail design parameters to see what works and what doesn’t. All without getting wet!

We are currently doing this with a 420 project, we can windward-sheet the jib, for instance to understand how the windward sheet will affect foot depth and leech twist. Something that cannot be done without the FEA/CFD components. It is exciting for me coming from a North Sails background where to be honest, I believed their sail analysis software to be the best. The FSI tools are actually far more capable than I am accustomed to so modelling such customised rigs like the Moth class, sports boats with inverted strut vangs, gaff-rigged boats and even the Middle eastern Dhow rigs is possible.

The experience of building the smaller dinghy and sports boats rig models improves the efficiency of preparing the rig model so there is more likelihood that Doyle Sails will continue to develop models and run them with FSI in the future.

Recently, FSI has been used to refine sails for the International Cadet class. The International Cadets have a world championship in Melbourne next year so there is quite a bit of interest in ensuring the kids sailing have the best chance of performing well in the conditions expected at Williamstown in Port Philip Bay.

International Cadet rig and sail geometry (left) and Flying 11 rig and sail model set up for FSI analysis (right)

International Cadet rig and sail geometry (left) and Flying 11 rig and sail model set up for FSI analysis (right)

The International Cadet is another class with limited jib sheeting options where the windward jib sheet is used to bring the clew closer to the centreline of the boat. This helps with the ability to point higher. We have also collaborated on developing a mast/sail combination with C-Tech Spars for the Flying 11 class. The mast section is quite small for the size of the rig so running FSI with varying mast specifications and at a range of wind speeds enables us to find the best mainsail shape to get the boat through a range of conditions with optimum performance. We have also worked closely with the boat- builder to optimise the jib sheeting locations to suit the sails.

I am looking forward to getting more dinghy rig and sail models prepared for FSI analysis and learning all there is to know about each class.

Andrew Lechte
Doyle Sails Designer

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