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North Sails 3Di - The How and Why

by Rob Kothe on 13 Aug 2010
3di in action MIAA
During the lead up to the 32nd Americas Cup, the Swiss Alinghi team hoisted sails on their boat so unique, that it had tongues wagging and people speculating as to just what were those unique, solid black looking sails with no apparent Mylar film.

Fast forward to now, and the development of these sails now known as 3Di by North Sails, have been appearing on the water this year in high end racing fleets in the Northern Hemisphere such as TP52’s and RC44’s, and now into Australian waters.

To find out more, we talked to Bill Pearson, in charge of the materials and research and development for North Sails 3DL and 3Di in Minden NV, and a 20 year North Sails veteran.

‘The development of 3Di started in earnest when we bought the initial research technology from the Swiss Alinghi team after the 32nd Cup America’s Cup, as we believed there was a clear development pathway for it through its 3DL process. It has been a very expensive multimillion dollar R and D process to reach this point.

‘3Di sails are made over our existing three-dimensional 3DL moulds. However, the difference is we are not using round string thread, we are making these sails with pre-Preg composite tapes, in a similar way to building a race boat hull with pre-Preg.

‘Up to now all modern laminated racing sails have three components. Resin or glue, fibre and film, normally a type of Mylar. With these 3Di pre-Preg sails, they are built like any other moulded composite – an airplane wing, a hull of a boat – its just fibre and resin, no film. So we get an immediate weight reduction.

‘The sail is going to stretch less and/or be lighter.

‘If you picture a yarn from any string sail, be it the 3DL sail or other 2D string sails, that yarn looks like a single element. It is actually made up of a bundle of very small filaments about the size of a human hair.

One carbon or one yarn for instance is 3,000 – 6,000 – or 12,000 individual filaments. ‘Our 3Di pre-Preg tape is made of Carbon, Aramid or Spectra yarns, with the filaments spread out on a sheet of resin side by side in strips, 400mm long by 100mm wide.

‘The process of component preparation before the delivery of resins to the mould surface is more complicated, but you still have a yarn in the same properties – it is now in a very thin tape form instead of as a yarn bundle.

‘The tapes are green, they have to be kept in a freezer until they are ready to use. The resin is a thermo plastic not an epoxy but it is very much the same as a boat building pre-Preg resin where it’s live, there is a curing and catalyst stage inside of it.

'The tapes are just tacked to each other so they are very drape-able and formable.

'Now you lay them over the 3DL moulds, they are overlapping, with a true scarf joint because the tapes are positioned in a staggered pattern.

‘Then you vacuum bag it and heat it up just like any other composite part and when you pull it off, it is all one piece. Just like the hull of a boat, you can’t rip apart the tapes once you have activated the resin.

‘The production process is well organised. Our eleven moulds are running 24-hours a day/ 7 days a week. To adjust a mould it is an eight minute process. It’s a push button thing.

So what does the ‘i’ stand for? ‘ Isotropic because it is an isotropic membrane.’

‘The engineering term for the type of a structure where you have fibre orientation in all directions, and therefore in theory you have equal strength throughout the membrane, is isotropic.

‘With a 3Di sail it is a very different equation. Instead of having this load path membrane where all the fibre originates in the corner and fans out into the body of the sail and goes to the next corner, the 3Di tapes are all over the sail going in many, many different orientations.

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‘It is much more like the airplane wing or the boat hull I keep referring to.

‘It’s that sort of a structure, so the fibres stretch out more equally in the sail and it is not just going in a load path direction. Rather it is going in all different directions. So the membrane is very rigid dynamically, as it is that sort of ratio of modulus to tensile strength and not so much in the favour of tensile strength, as it is in the 3DL.

‘Learning about balancing the modulus and the tensile strength in 3Di, is where we are now. It is a learning curve, balancing different types of fibre on different parts of the sail.

‘It is very easy to tailor one of the 3Di sails, to tailor the amount of laminate exactly to what you want because of the way it is built up with these stacks of tapes. However right now we are not focusing on building lighter sails, just stronger ones.

‘At present the sails have so much more modulus or resistance to stretch than the 3DL sail. Ultimately sail designers are going to be able to take the weight down, but now the focus is on rigidity.

‘As far as shape holding ability and aerodynamic rigidity is concerned, 3Di is another very big step up, in the same order of magnitude as when 3DL replaced tri radial panel construction sails.

Then we talked to Michael Coxon, CEO of North Sails Australia, who was tactician on board Neville Crichton’s Reichel Pugh 72, Alfa Romeo III, the winner of the Mini/Maxi division and second overall at the Maxi Yacht Rolex Cup Regatta held at Porto Cervo, last September.

‘When we first put the sails on Alfa Romeo III in pre-event practice, it was my first serious experience with 3Di.

‘For many years we thought 3DL was an incredibly stable sail and certainly compared to any other that we had seen in the market.

‘When we would go over a wave or the pressure would increase, I would have said the 3DL sails didn’t seem to move. There was no ‘panting’ (where the sail stretches under extra load, and shows movement against the rig or boat). I look at the stability of the sail, as to what pants more or less, and the goal is for less pant because you are converting it into usable energy, rather than just stretching the sail.

‘If a sail is panting as the pressure increases, the sail moves out across the deck which is exactly what you don’t want because it is making the sail deeper when you want it flatter.

‘We didn’t even realize the 3DL sails were moving until we put a 3Di sail up.

‘It really did not move at all. What it means is the load strength wind and pressure converts to loads and boat speeds faster - it is a more stable sail.

‘We then went back to the 3DL, watched it as it would pant a little bit as it went through a wave, then we would go back to the 3Di and there was no panting.

‘Panting was absolutely eliminated. The originally designed shapes hold their form and pressure converts to force and hence to speed, so there is no question 3Di will be the future.

‘3Di resists stretch in nearly all directions and so there are minimal halyard, outhaul or Cunningham adjustments to maintain shape. While 3Di material is stiffer than other laminates, it can still be rolled or folded. It pretty much eliminates all the give in a sail so blocks and halyards are being replaced with stronger ones to ensure the extra rigidity does not cause breakages' said Coxon.

3Di sails have now arrived onto Australian shores and North Sails Australia Sales and Marketing Manager Julian Plante explains a number of IRC racers, including Shogun and Ginger are already using 3Di. Other boats that will to be seen with 3Di sails at Audi Hamilton Island Race Week include Loki, Yendys, Terra Firma and Living Doll.

For the latest info on 3Di, check out the North Technology pages at

There they have a great Q&A page plus some short videos and a customer perspective on 3Di.

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