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Rig Development in the 2007 America’s Cup Racing
Southern Spars Rig Designers, Martin McElwee currently working for Emirates Team New Zealand and Steve Wilson of BMW ORACLE Racing, discuss the latest developments in AC rig technology.
The America’s Cup has always been the most active breeding ground for new ideas in sailing and yachting equipment. The various class rules that the cup has been sailed under have allowed a large enough degree of development to encourage most of the worlds top yachting brains to have involvement at some stage in their careers.
Carbon Fibre was first used for America's Cup masts in 1988 when both New Zealand’s ‘Big Boat’ and Denis Connor’s catamaran used rigs and boats made mostly of carbon fibre.
The result of this challenge was a change in Class for cup racing and in 1991 the first carbon mast was built for the new America’s Cup Class (ACC).
Today, the boats are far narrower and certainly the developments in rig and sail technology are immense when comparing to those initial designs, with narrower shroud bases and better construction techniques resulting in very different rigs than those first built in 1991.
Conditions in Valencia have been invariably light to moderate winds around 10-14 knots and lumpy seas. Sailing in these conditions, it has been imperative that the rig has good stability in order to hold sail shapes and keep the boat fully powered up.
Manufacture The majority of America’s Cup masts are built in female moulds as were the winners of the last four America’s Cups. Female moulded masts enable the installation and accurate placement of tricky hardware prior to joining the sections.
Each half of the mast is laminated using the rule maximum allowable modulus pre-preg carbon fibre. This is then cured under high pressure of 5 Bar in an autoclave at up to 130oc.
Design Various design tools are used to ascertain the optimum stiffness and structure of the mast tube. This involves complex engineering programs including finite element stress analysis codes.
Southern Spars use their own in house developed ‘Rig Calc’ program to quantify the loads and optimise rigging configurations. This is then further refined using North Sails' MemBrain finite element suite of software. This begins by predicting the exact sailing forces generated by a yacht through a flow code. These forces are then applied to the rig and the deformed structure is studied. MemBrain allows the designers to study and analyse the mast with flying sails attached in a full range of sailing conditions.
As with all high performance rigs, optimising the design of an America’s cup rig involves achieving the desired stiffness while achieving the Rule weight and CG and keeping windage to a minimum.
Stiffness is still the #1 consideration Class rules provide limits within which the parameters can be varied. In the latest version of the ACC rule (V5) there have been a number of changes that have given rig designers new considerations and boundaries to work within.
The rig loads have always varied from boat to boat depending on where the boat fitted within the rule. Changes in Version 5 have affected the loads engineers work with when designing the rig.
Draft has increased 100mm, downwind sail area has increased 50m2 and maximum displacement has reduced by 1000kg. Added to this the rig is now allowed to be 70kg lighter and the carbon modulus has been increased (from 310mpa to 385mpa).
The effects of these changes isn’t seen by the casual observer but has resulted in the designers spending many hours engineering the exact numbers of layers of pre-preg carbon fibre material to meet the newly calculated loads and achieve the sailing stiffness that the crew has requested. Small pieces and strips of carbon are placed exactly as designed inside the mast shell to meet the predicted loads. Areas identified with extra stresses have extra layers compared to the lower stress areas.
Before joining, the inside of the mast tube looks like a complex topographical map in some areas.
Martin McElwee, Rig Co-ordinator for ETNZ comments that “one of the critical aspects is looking for stiffness of the mast to support the sail shapes required. With the advent of the bigger tops in the mainsails, the masts have to get stiffer and stiffer.”
The extreme heads on the mainsails can extend a number of meters aft horizontally from the masthead creating a huge increase in the loads these masts now handle.
The allowed increase in carbon modulus means that teams can achieve stiffer rigs for the same weight or lighter rigs for the same stiffness. Again, this is another option for the design team to play with in achieving the best overall package.
Aerodynamics gains metres. Aerodynamics and specifically reduction in drag is an area that attracts a lot of attention and of which the results are often a lot more obvious to the naked eye.
Some things are obvious improvements and all boats adopt them once they become public. Others are being tested and can be seen clearly when one boat is configured differently to its stable mate.
In the latest version of the ACC rule (V5) the restriction on mast twist at the butt has been removed. This has added fuel to the quest of rig designers to develop a rig that twists to varying degrees up its length to try and match the angle of the apparent wind as it passes the mast. This angle is greatly affected below the hounds by the influence that the genoa has in redirecting the wind flow. However above the hounds there is a sharp change in direction which ideally requires the mast to twist quite quickly in this area. Some obvious things that all teams are trying are more aerodynamic spreaders, both in sectional shape and plan form. A lot of attention goes into trying to reduce high drag areas like at the junctions of components where the spreaders meet the mast tube or at the outboard end where the shrouds are attached. However, even the smallest gains are chased so you will see some differences in the plan shapes of the spreaders and jumpers as each team looks for their best solution to eliminate the windage these items cause.
Options are first tested and studied using CFD (computational fluid dynamics) software to ascertain the theoretical drag savings. These are further tested in wind tunnels for confirmation and more accurate results on the more complex ‘junctions’ areas. From this the designers can put the theoretical results into their VPPs and tell the crew how many seconds around the course they will gain with these changes.
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In nearly all of these cases the teams will never see a test result on the water, however they know that extra 3 seconds around the course will add up with another 3 seconds and eventually mean they can cross the opposition on the final downwind leg.
Some teams have been testing jumper-less rigs. As well as reducing the windage from rigging, it also eliminates the need for the crew to adjust the jumpers while sailing. However the rig needs a lot more structural strength in the tube to handle the loads previously taken by the jumpers and hence it is normally larger in section. The rig behaves differently in this new configuration and a new trim style needs to be developed alongside the conventional rigs before deciding what the best racing option is.
Jumper-less Rig
Steve Wilson, rig co-ordinator/designer for BMW Oracle comments, “The design and build of the mast, especially that of the top mast, has to be very accurate to support the mainsail leech correctly. If there’s not enough stiffness in the top of the mast, the mainsail will de-power too early.
Rigs without backstays are also becoming common as teams learn how to handle the loads traditionally taken by the backstay by engineering the carbon laminate accordingly. Of course when you remove the back stay or jumpers, you also remove the sail trim options that these items offered to mainsail trimmers in the past, so it is not a simple engineering problem, but a sailing team issue as well.
Attention to detail saves Weight Most teams took advantage of new rigging products on the market to take significant weight from their forestays and other standing rigging components. EC6 carbon rigging and PBO have been used for backstays, jumpers and headstays. Every kilogram eliminated from the rigging is another kilogram of carbon that can be added to the mast tube to improve its performance where required.
McElwee comments “Everyone is aiming for a minimum weight rig. You have this weight to spend – you can spend it on the tube or on the rigging or a combination. You could put really heavy rigging on and less carbon in the tube or light rigging and more carbon in the tube. It’s a trade off between these options.”
All teams are using a cross rigging configuration, more commonly referred to as the millennium rig. Cross rigging supports the mast transversely better than conventionally rigged yachts. This rigging concept was first introduced into the AC class by TNZ in 2000.
Another area that is hard to see is the huge engineering effort that goes into the detailing of the fittings to make them stronger, lighter and smarter. Many of these are meticulously fitted to the inside of the mast tube before the 2 halves are joined and form part of the complex internal workings of these masts.
New Fittings now commonplace
Booms Some teams opted to use the ‘Lattice’ style boom, which was introduced by TNZ in 2003. A lattice boom is constructed from a series of lightweight carbon tubes arranged like a set of scaffolding. The advantage of the Lattice boom is that it places the structural members as far away from the neutral axis as possible to give the greatest support. The result is a boom that is significantly lighter for the same strength.
Making a boat go faster is just one of the key objectives, improving manoeuvrability and acceleration can be just as important when it comes to gaining the upper hand.
Many of the major developments have happened above the waterline where rigs and sail plans have seen some of the biggest changes in the style of modern Cup boats. The fact that the development has inspired such a broad range of approaches and styles makes the new generation of boats all the more interesting.
More on Southern Spars involvement with AC rigs: In the 32nd America’s Cup four teams have used rigs manufactured by Southern. These syndicates are: Emirates Team New Zealand (winner of the 2007 Louis Vuitton Cup), BMW Oracle Racing, Areva Challenge and Desafio Espanol. Notably, three of these four teams were semi finalists.
Southern Spars has a long history of producing rigs for winning teams and luxurious super-yachts. The company built its first carbon fibre spar in 1991 and in 1992 embarked on its first America’s Cup projects for New Zealand Challenge and Nippon.
Since then, Southern has worked with America’s Cup defenders and challengers in the last four America’s Cup regattas, including Team New Zealand’s successful bid in 1995 and again in 2000 with Team New Zealand’s victorious defence campaign. In the 2002 – 2003 America’s Cup, eight out of 10 syndicates used a Southern Spars rig.
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One of the most obvious features of recent rigs are the ‘bat wings’ or ‘flippers’. These are attached to the top spreaders and used to hold the genoa leech out beyond the spreader end. This has a number of positive benefits for a boats performance. Firstly, the genoa leech is not wrapped around the top spreader, deforming the leech of the genoa in that area. The flipper is also used to extend the sail leech further aft, with a similar effect on the sail as a leech batten would have in that area so the genoas can now be bigger. Once this is achieved it is easy to use the flipper to influence the exit angle of the leech to achieve the best forward drive from interaction of main & genoa.| Latest News :This news ticker only works in Internet Explorer 5 / Firefox 5 or later. |