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2 December 2002, 12:19 pm
What are the Stripes on the Sail for?
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©Carlo Borlenghi/Sea&SEE

America's Cup

Deciding what to wear on the day might look like a taxing decision when you have 150 sails to choose from. But for ACC teams, knowing their wardrobe inside out is essential.
Testing and evaluating sails consumes a great deal of time both in the build up to an America's Cup as well as during the spare moments in the event itself.

While velocity prediction programmes (VPP) may provide important information, there's still no substitution for seeing the results at full scale. In this respect, two boat testing has become the most popular way of assessing performance changes. But first teams have to be able to ensure that their datum is fixed and that they can sail the boats to the numbers on demand.

Today, every America's Cup programme has a sail scanning system that can record precise details of a sail profile in a given set of wind conditions. One of the first decisions in sail scanning is whether to scan from the top down or bottom up. Scanning from the bottom is easier and cheaper as the cameras are mounted in the deck and shoot skywards, but the results are less accurate.

The reason is partly to do with the perspective and field of view when looking down from the head of the sail, where the often brightly coloured camber lines are better proportioned and easier to plot. There are also advantages when the boom is cracked off, as well as being more tolerant of mast fall off and bend.

The giveaway as to who's shooting what from where, can be seen in the camber stripes on the mainsail. Wide stripes at the top and thin at the bottom indicate deck mounted cameras with the opposite being true for rigs that shoot from top to bottom. Also, boats using mast mounted cameras have large brightly coloured dots at the bow and the stern that act as a reference point for the cameras, another clue.

While shooting from the top may require small, light cameras and provide better results, to get the best requires a system that can constantly scan both the mainsail and the jib and will monitor the precise shape of the sail automatically.

Here, a screen in the cockpit will show the trimmer the precise shape of the sail profile and in turn be capable of feeding a database on the settings.

Whether you shoot from the top or the bottom, the overall goal is the same, to produce a detailed database from which further analysis will then highlight strengths and weaknesses of a particular profile and configuration. Gradually, with repeated testing throughout a range of conditions, the team can gain confidence in the reliability of their target figures allowing them to make more accurate assessments as to what works and what doesn't.

But some teams take sail scanning to another level and use the system to assess small changes in boat, rig or sail trim.

As well as the sophisticated sail scanning systems, some teams such as Alinghi have telemetry systems to allow each trimmer to toggle views of the other boats' sails, while sailing.

Once the boats are under sail, the trimmers can toggle or overlay the images of both the mainsails and jibs to make sure that the sails are trimmed exactly the same. When they are, a small change is made to one of the boats and the data is logged.

Yet even with such sophisticated technology it takes thousands of hours to build accurate performance models for a full range of sail profiles over a range of conditions. Building on this bedrock of data, little by little develops an impressively detailed database. Over time, a full and accurate picture appears of the boats' full performance capabilities often revealing gaps or hollows in performance. The next stage is to decide which type of sails to build and how many of them.

But despite the sophistication of the design and evaluation systems, many of the clues for future developments come from live racing situations.

"More things come out in the wash during racing than they do in two boat testing, especially when you're downwind sailing," said Alinghi's Murray Jones. "Downwind testing is particularly difficult to assess as you stay in the puffs for longer, which makes it very difficult to match boat speeds."

"Downwind sails are also much more difficult to assess, compared to the upwind sails."

This time around and with most teams operating two boat campaigns, there has still been a move towards racing than straight two boat testing. Which in many ways is encouraging for the rest of us watching. Computers, scanners and software may have allowed some of the experts to look at performance in more detail, but at the end of the day, even they resort to seat of the pants sailing. There's hope for the rest of us yet.
Matthew Sheahan
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