Sail-World.com : We were all 18 once…
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'Mark Foy Trophy in Carnac (littoral-ouest-photos.com) - Mark Foy Trophy in Carnac (France)'
Franck Gicquiaud ©
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What can 18ft skiff sailors expect from a new-look North Sails wardrobe? Designer Gautier Sergent explains some of the key changes that have been engineered following a period of intensive focus on the class.
18ft Downwind Sails
Shape
We started to refine our existing designs earlier this year, and had three concepts going from very flat to 'on the full' side. We were always looking for more ease of trim and forgiveness improving range and average speed rather than warp speed in one condition. These boats cover a lot of ground in a race and go from one breeze to another very quickly so you don’t want to get stuck in the 'wrong / slow' conditions and pay dearly. It is all about passing lanes as well with the fleet getting bigger - this was highlighted during the latest Marc Foy trophy in Carnac (France).
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Therefore, we are trying to maximise luff projection while retaining ease of trim. There are a couple of ways to achieve this, including broadseam or luff curve, so we tried mainly three combinations and drew some conclusions both on 1 and 2 rigs. We have reached a balance which we think will achieve our initial goal. Of course this is always open to discussion with each customer to suit their needs and come up with something tailor made. We now know we have a good base for development and we know the key parameters in terms of speed, height and trade offs. We are still convinced we can improve but we are also convinced it is a clear improvement. We have an idea where to go but more importantly we have a clear idea of where not to go.
Material
Polyester was always a cloth of choice in this class because of its high strength to weight ratio and low water absorption properties. However, polyester was getting very hard to source as suppliers moved away from this cloth and discontinued most lines. The main problem with this cloth was a lower tear resistance. During the last Volvo things changed with the need for a material that could cope with the flat shapes (fast boats = small AWA=flat spinnakers) but still retain the light weight and 'soft hand' of the nylon. Cloth suppliers did a lot of development on polyester woven fabrics. Ericsson and Delta Lloyd (and maybe others) both used polyester spinnakers very successfully. There is now a whole range of polyester fabric available right down from very lightweight to really heavy so we can optimise our sail layouts. The tear resistance has been greatly improved, and is no problem on the Volvo boats even with a fractional heavy air spinnaker.
I was convinced of the quality of the product having made a sail for Delta Lloyd out of it and felt confident this was the right way to go for the 18ft skiff. To date we would have made at least five18ft kites out of this new fabric and directly noticed the benefit of using it against the traditional nylon for this type of application. We built identical sails out of both nylon and polyester and the shape holding ability and low water absorption was a clear winner while the greatly improved tear resistance has not been an issue. It is also readily available in NZ. We also built a lot of sails for the Farr 80 Beau Geste out of this woven polyester.
Upwind Sails
When it came to upwind sails, we started from a blank sheet of paper, working in close collaboration with the rig designers (C Tech and Southern Spars) as well as the sailors. We got the information on the rig set up and stiffness from the rig designers to model the rig with our Membrain and Flow programs. Starting with the existing rigs, we first duplicated a docktune (loads on rigging at dock) on our computer model and applied a giving load at the top of the rig down to the end of the boom as the guys do on land to check mast bend and other factors. Once we made sure model and reality matched we proceeded to putting sails on the rig and blowing some 'virtual' wind across it.
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This gave us a base for discussion with sailors and rig designers and some direction to take and things to experiment based on rig designer suggestions and sailors’ feed back. We tested a wide range of solutions in terms of geometry, mast stiffness and sail shape and structure. The trick on these boats is that you have no control over your rig shape other than Cunningham and Vang (no aft rigging) and you need to cover a massive wind range, from 0 to 15 knots with 1 rig with one rig / sail configuration. So we modelled under powered conditions with the skipper off the wire then nicely powered up conditions with all 3 guys on the wire and finally over powered conditions with 3 guys on the wire trying to keep the boat on its feet. Each time we made sure crew position was adjusted according to reality (forward in light and then moving aft with wind increasing).
This has a big impact on the rig shape and forestay tension. We validated the wind range for each 'trim / configuration' based on righting moment. We had a lot of input from the sailors on the actual onset flow (AWA, TWS, boat speed, leeway etc ...). We made sure that the sails were as powerful as they could be in the light without affecting the twist profile and overall shape distribution. Then they need to gradually flatten and twist off as the breeze increase without running out of luff curve or distorting completely.
We worked on sails and rig simultaneously to achieve this without any constraint but keeping an eye on the rig load and mast compression in order to keep the rig light and not apply too much load on the hull which is quite bendy. What we achieved was a sail and rig package which works in harmony throughout the range. So it is a whole new rig design and sail design as well as a brand new 3DL layout to achieve high strength to weight ratio. We did this for 1 and 2 rigs. We are thankful for the rig designers to have responded very openly to our initiative and to the sailors for sharing their precious feed back.
12ft Upwind and Downwind sails
Thanks to the entry of former Olympic Finn sailor Clifton Webb into this fast paced class, and his continued feedback from the water, mainsails have changed dramatically, with massive heads. Modifications developed during 18ft skiff modelling have been applied to the 12ft skiff – despite being a very different boat, a lot can be taken from load and shape distribution, especially for the upwind sails. The same polyester woven fabric now in use for the 18ft skiff is also being used on gennakers, for the same reason. Currently, two prototypes are on the go for downwind sails, and North Sails has two staffmembers – Ben Defluiter and Rod Chave – active in the class and willing to try things.
The first set of 18ft sails are currently being finished in the One Design loft.
by Zoe Hawkins Share
8:21 AM Thu 1 Oct 2009 GMT
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