rig trimming conditions and navigational parameters (wind, boatspeed and attitude, and so on). Most types of boats can be modelled using FSI, ranging from monohulls to multihulls, sloops to ketches, boats with rotating masts, boats with deflectors, barber-haulers and inhaulers, etc. Loads on each component are calculated in the modelling, allowing keenly precise presentations informing, among other things, optimal sail design shape, load strain mapping and sail material selection. UK Sailmakers International, with its
worldwide network of over 50 lofts and service centres, is a heavy user of BSG’s FSI software. ‘Many sailmakers use our SailPack program pretty effectively,’ says BSG’s software development manager Laurent Guillaume, ‘but UK’s lead designer Pat Considine is one of the few who really know how to make our FSI product sing. I’ve seen many of the analyses he’s conducted over the years and it’s truly impressive to see how he blends the art of sailmaking with today’s sailmaking technology.’ Considine has been using FSI to lead
UK Sailmakers’ design team for 12 years and has performed countless analyses for customers around the world. ‘The beauty of FSI is that it allows the designer to move beyond “designed shape” and see a sail’s “flying shape”. The difference being that regardless of the sail design, its flying shape is determined by the combination of all the trimming variables applied to it (halyard tension, sheeting position and tightness, inhauling, mast rake, headstay sag, etc),’ says Considine. ‘Achieving the designer’s optimal design shape while under way is virtually impossible. FSI, however, allows me to play with all those variables and replicate a flying shape on my computer screen. I can then work backwards and apply the learning from the modeled flying shape to my design. This ability to translate design concept to on-the-water reality represents a dramatic advancement for sailmaking.’ UK Sailmaker’s president John Bennett describes one of the two main types of analyses the company does with FSI: ‘The first thing we look for from FSI is optimising sail design and shape. This first image is a stress load map, showing graphically the critical load paths of the sail which calculates all manner of loads from individual yarn bundles to battens, to rig tension. One of the things we found in this test was how the load on a square-top main is mainly in the luff vs. being leech- centric on a traditional main. This gave us important new insight into how to trim a square top main... and that is information we share with all customers.’ UK Fremantle’s Geoff Bishop adds:
‘once we have the sail design optimised in FSI, we get to “trim” the sails, assessing how each of our adjustments impacts speed and height. Pat conducted an FSI analysis on a new suit of sails I was making for a client who wanted them
Top: the blue line shows the cross-sectional shape of the quality Dacron sail and the red is the inexpensive cloth. The data shows that the inexpensive Dacron sail stretched asmuch as a thirdmore. This can be seen in the two cross- sectional plots. The top plot shows the shape of the inexpensive dacron and the lower plot shows the draft staying further foreword and not growing as deep
Helping clients make informed choices
Another example of how FSI can help inform a better sail purchasing experience was when a customer wasn’t convinced he needed to pay for premium quality Dacron sails vs. sails made with less expensive, lower quality Dacron. These images compare jibs on a typical 36-footer with exactly the same design specs (panel designs, reinforcements and finishings) with the only difference being the quality of the Dacron sailcloth being used. It is widely accepted that a Dacron
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sail will start to stretch from the first time its set, but how much? In these photos we see the draft of the sail with inferior Dacron indicated by the red
line; the quality Dacron sail’s draft is marked by the blue line. The grey line is the sail’s draft. As you see, once set, the sail began to stretch as Dacron does, but note the considerable difference (+X%) stretch indicated by the red line as the leech opens and the draft moves aft. Yes, there is some movement in the blue line, too, but nowhere near the shift in shape. After seeing this demonstration, this UK customer, and numerous others since, have confidently ordered Dacron sails made with superior sailcloth knowing the sail they are buying will have a much longer peak performance life.
SEAHORSE 61
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