Sure, your sailmaker may know how to use a computer to design a sail, but does your sailmaker know how to make that design software perform at a level comparable with winning that F-1 race, making a living playing backgammon, or being the foiling Moth World Champ? Probably not.
Pat Considine has been working for UK Sailmakers leading its Chicago loft for 30 years. Over the past decade, his design work has blended his intuitive artistry as a traditionally trained sailmaker with BSG Developments’ leading edge sail design software applications.
BSG Developments is the leader in sail design software providing a suite of related products. Perhaps their most widely used product is SailPack, the core design software that allows the sailmaker to configure a “mould” on which to “attach” the sail panel layouts after which reinforcements and other finishings are added. The SailPack output is a three-dimensional model of the boat and its sail plan that can be viewed from any perspective. This allows the sailmaker – and the customer – to visualise the sail before the first panel is cut. When loading in the SailPack data, a vast range of data points are input ranging from sailcloth being used, mast/spreader/shroud specifications for optimised rig setup to yarn thread paths, deck layouts indicating where sails will be trimmed to hull, batten specifications, and hull/ballast characteristics.
SailPack is an excellent platform for primary sail assessments, but if the designer wants to further ensure performance levels, a fluid structure interaction (FSI) is required. BSG’s SailPack FSI is made in collaboration with K-Epsilon. This is BSG’s most advanced and accurate solution for FSI calculations, adding layers and layers of detail and understanding well beyond what the core SailPack delivers.
Ideally, UK’s SailPack and eventual FSI modeling starts by importing the boat designer’s 3D composite images of the boat, including the range of rigging, systems and nautical specifications that went into creating the yacht. Sometimes, the designers don’t like giving up those files, so UK have to create the boat/rig model themselves from the sailplan and line drawing of the rig. Then they need to define everything they’re going to study, including the characteristics of the mast so we know what to expect when we put on more backstay in different amounts of breeze. They go on to define the angle of the spreaders, the composition of the shrouds, the kinds of battens being used, etc. Polars of speed and angles also come into play. They designate where the jib tracks, inhaulers and travellers are, too.
The SailPack design starts with a standard sail configuration. UK Sailmakers creates the “design shape” of the sail vs the “flying shape” of the sail – when flying, a sail can be trimmed in an infinite number of combinations that can’t always be anticipated by the designer. Trim, leads, inhaulers, halyard tension are all variables. By starting with the design shape, adjustments can be made to approximate various flying shapes using FSI. An example of flawed logic is that many people have tried to design a sail’s design shape that will twist around the spreaders but can never achieve that in the flying shape.
FSI modeling starts with the detailed data originally input for SailPack: models of sails and rigs, mechanical properties, 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.
"Pat Considine is one of the few who really know how to make our FSI product sing"
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 underway 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 leechcentric 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 delivered in the Mediterranean. We were able to give him a plan for setting the sails in a range of wind strengths (6, 10, 15, 20+) including detailed guidelines on mast bend, headstay sag, jib car/traveller positions, in-hauler setting, and more. When he set the sails for the first time, he knew exactly how to set up the boat and sailed away having benefited from our indepth look at the flying shape for the sails we created using FSI. I’m pleased to say this customer was ecstatic with these guidelines...and the sails.’
In this study for a MAT 1180, inhauling gives a slight increase in the coefficient of lift. Even though the drag increases, in light to medium winds the boat will sail faster with the jib inhauled
It should be noted that UK Sailmakers has applied the FSI technology in ways that will positively impact customers who aren’t buying premium-priced sails or looking to pay for FSI input for their new sails. One example of this is Considine’s work with the Chicago-based Tartan-Ten fleet. The Tartan-Ten is a 1978 33ft one design with a 7/8 fractional rig and a non-overlapping jib. A popular class in Chicago, there had been one jib design that was winning all the races... but wasn’t able to point as high as desired. The specifications of the original jib were input into SailPack FSI and a baseline assessment was made using widely accepted inhauled settings. UK Sailmakers then created a number of new Tartan-Ten jibs, each with the objective of matching the current jib’s speed but pointing higher. The key was that the inhauler settings were evaluated for the old jib and the new prototypes. By varying the designed-in leech twist in the sail, a new jib was arrived at, meeting both the speed and height objectives. This new UK jib has revolutionised the Tartan-Ten fleet in Chicago and the benefit of this this “twist” logic has trickled down throughout UK Sailmaker’s one-design and non-overlapping jib designs ever since... bring the benefits of FSI to all their customers.
In the 16 knot image the runner tension is increased which adds mast bend, flattening the main, increasing twist and depowering it. The jib is depowered by moving the jib car aft, which adds twist in the sail
Importantly, UK’s use of FSI isn’t an empirical exercise where once the design is out the door, the designer moves on to the next project. Definitely not! In fact, there is as much information flowing back up to Considine and his design team as flowing down. UK’s sailmakers report back on the real-world, on-the-water performance results from sails output from FSI. Here is art informing technology both before and afterwards.
Considine explains that in changing FSI model inputs, it’s not as if you can see the sail shifting shape in real time. ‘The minute shifts in performance data as loads change positions, leads and trim are adjusted, and conditions change can only be realised from the data... making this part of the process more technology than art. Look at the data points in these exhibits and you will get a feel for how precise and subtle the improvements are projected by FSI. Subtle changes in where the JPK owner set the leads for his new sails, subtle changes in the shape and trim of today’s winning Tartan-Ten jibs, subtle changes in the properties of the materials used to build a sail. This is where the technology is required to access and evaluate these often imperceptible differences,’ he says.
Without that technology, we would not have the sails we have today. However, without the artistry of a sailmaker to create the original design concepts, the technology alone would be lacking. That’s why it takes people like UK Sailmakers’ Pat Considine, Barry Hayes, Stuart Dahlgren, Pedro Gianotti and others to provide today’s needed blend of art and science.
Helping Clients to Make Informed Choices
(Above) 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 as much as a third more. This can be seen in the two crosssectional plots. The plot just shows the shape of the inexpensive dacron and the bottom plot shows the draft staying further foreword and not growing as deep
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 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.
