Opposite: London 2012 silver medallist Luke Patience and Rio 2016 crew Chris Grube training in Palma earlier this year. Left: the powerful class-specific software developed by 470 sail designer Masanobu ‘Nobu’ Katori allows images of sails in use out on the water to then be swiftly digitised so that dynamic loadings and resultant movements in sail shape can be accurately recorded for detailed analysis
the spars – is less clear in Olympic class boats due to the greater variations in dimensional stability and stiffness, key to the aero-elastic coupling process. ‘A custom-built all-carbon TP52 with a carbon spar is much easier to describe for our design tools than the boat and mast for a 470,’ he says. ‘Therefore we need to do a lot more measurement and testing of the smaller boats and spars to allow our platform to work as intended.’
Guillaume Florent, with each using slightly different designs optimised for their own different weights and sailing techniques. ‘On to London 2012 and Weymouth,
where conditions were expected to be any- thing and everything. This is where Ben became Sir Ben after claiming his fourth Olympic gold medal after being pushed hard by Dane Jonas Høgh-Christensen who, as a bigger sailor, was using a differ- ent North design tailored to the unusually stiff mast that he had built for the regatta.’
The process
The tools and the process used to develop these medal-winning designs parallel those used for developing any race-winning North sail, with variations made based on the unique characteristics of the boat, its spar and other parameters. The process is actually an iterative feedback loop involv- ing three primary elements – sail design, evaluation and performance measurement. Unlike in Lowell’s day, when the process started manually with deconstructing a pre-existing design that was proven to be fast, the current process is a digital one that starts in the computer to create an initial mould design that will hang on a digital model of the spars and boat.
This initial three-dimensional shaping of the sails is then assigned a structure using North’s 3D Layout software, where cloth type and panel layout are assigned to the full 3D shape.
With initial mould shape and structure now in place, North’s CFD-based Flow program is used to show pressure differ- ences in the sail surfaces and how the aero- dynamic loading acts to deform the shape. The next step is to apply this pressure map to the moulded shape using North’s Mem- brain program. Membrain considers the specific sail fabric and panel layout, adds
in wind pressure and then shows how the sails and rig will deform when loaded. The last step is in fine-tuning the shape, where the smallest differences may distin- guish between a sail that is merely fast and one that is medal material. In the Finn, for example, this is why so many excellent but barely used sails are steadily passed on into the wider fleet; what is good enough to win a national championship may not be quite good enough for Giles Scott. Once the sail’s flying shape is tuned and trimmed in Membrain, the deformed (pressurised) shape is then sent back as an input to the Flow software. This shape will be different from the non-pressurised base shape that was created in North’s Spiral program, so Flow recalculates a new pressure map that in turn goes back to Membrain and is reapplied to the sails to get the next deformed shape.
In simple terms, an iterative calculation process between Membrain and Flow called aero-elastic coupling is used until there is no longer any change in Mem- brain’s ideal sail shape. The ability to accurately and quickly undertake aero- elastic coupling makes for a powerful tool, especially for developing fast shapes in new classes where the time in the boats for analytic methods has not been available in time to meet initial demand.
Refinement
As Hobson mentioned earlier, North’s Juan Garay has been a successful designer in many Olympic classes, including the Tornado, Star and Finn, but he has also recently found success at an entirely differ- ent scale with the Azzurra TP52 pro- gramme. Juan says the Olympic develop- ment process is actually more difficult than for big boats like the TP52, because the platform the sails hang from – the boat and
Juan says that for the Finn it’s different again because mast bend characteristics are customised to suit each sailor, while the sail material can also be varied to suit. As in any other laminate sail, the tools allow fibre layout to be readily customised to accommodate load paths for better shape control (and retention). For the best sailors this leads to a level of optimisation that can translate into a winning result. This optimisation process becomes more tricky when there is less control over spar, boat and cloth variations, such as is seen with the more dynamic 470 platform. As Masanobu Katori (‘Nobu’) explains, ‘these profound variations strongly influ- ence our sail refinement processes. ‘There are limits to our shaping options: a 470 mainsail is limited to 18 panels and use of a minimum-weight polyester sail- cloth, so we use a tri-radial layout to allow more control on shape.’ And on a mainsail luff length of only 6m the accuracy in panel shaping needs to be ‘within one milli metre’ to ensure the desired shape. Nobu adds, ‘So much of what we analyse and think about in sail shape is driven by the control of mast bend, and this is always variable.’ Nobu still uses the same analysis methods he learned in two Nippon Challenge AC programmes, as well as during time later spent with Emirates Team New Zealand in 2007, to evaluate shapes so that proven fast designs can be recorded and later replicated. With 470 cloth constrained by class rules, so North has established a good relationship with cloth producers Dimension Polyant and Taijin to supply the cloth they want consistently. With cloth variations under control, the design process can then focus on variations specific to the crew and the regatta venue. For example, Nobu says the women 470 sailors generally prefer softer spars than those used by the men, so differ- ent shapes are also required in their sails. Finally, the 470 planes more readily than the heavier Finn, so there are bigger variations in how each team achieves its different sailing modes – this too all has to be accounted for by the sailmaker.
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