Excerpted from "What's Up" by Matthew Sheahan
January 14, 2003
Source: www.lvcup.com

While keels, rudders and appendages are being discussed, other major developments have been on display for a while.

. . . it’s Team New Zealand’s four spreader Millennium rigs that are becoming a focus of attention.

Last time around the home team caught the rest of the America’s Cup world napping when it stepped this unusual configuration. Rather than the four spreader rigs that were the norm, three spreader rigs with a complex array of double diagonals seemed to be de-riguer aboard the black boats.

During the 2002/3 Cup build up, several teams decided this was an innovation worth copying and duly fitted the innovative spars. But instead of turbo charging their boats’ performance, the new configuration provided a nasty shock for three teams in particular and a few near misses for a several others. The Millennium rig was not as straightforward as it looked and the brand new spars aboard Oracle, Stars & Stripes and OneWorld came tumbling down in the first few days of sailing trials.

Meanwhile, Team New Zealand, having proved to themselves that the concept stood up to examination, was busy preparing for the next stage of development, a return to a four spreader mast, but with the same double diagonal concept. Given the time that the team had spent sailing with this configuration and the fact that both it’s boats are fitted with these spars, there appears to be good reason to believe that the team thinks it’s onto something good. But why?

As is so often the case with ACC boats it’s the rule that is at the heart of this development. Once again Team New Zealand are squeezing all they can out of the small print that restricts these boats’ development.

The idea starts with the basic fact that the class rule on ACC rigs states a minimum weight and centre of gravity, 820Kg and 12.25m above the deck respectively. It’s a pretty fair guess that most of the syndicates have found that it’s possible to build down to these figures, thanks to modern materials and build techniques. This in turn means that there’s some scope left for rig designers to play with.

Broadly speaking, most designers will start by designing the standing rigging set up first, and from that work out what the total weight of the standing rigging will be. They then subtract this figure from the minimum weight under the class rule which leaves them with a material weight for the spar. Given that this allows more material than you really need to make a Cup mast stand up, there are several options.

Initially, designers took the view that a very small mast section size and a thick wall would minimise the windage of the spar. The trouble was that a small section spar was difficult to control in the fore and aft plane as there are only the forestay and three sets of backstays to control this lanky mast.

Following research from the last Cup, Team New Zealand and possibly others, discovered that there was a big gain in making a spar section long in the fore and aft plane as this made the mast stiff in this direction too. By jiggling around with the section profile, the mast could be treated like the leading edge of a wing and helped improve the airflow over the mainsail, while also making it easier to control. On top of this, the windage produced by the mast tube reduced.

Meanwhile, the Millennium rig concept also proved that the double diagonals had the effect of reducing the effective panel length between spreaders. In other words, the mast thought it was being supported by six spreaders instead of three. This in turn made life much easier when it came to controlling the sideways deflection, or fall off, of the mast. Reducing the amount that the mast falls away to leeward also helps to control it in the fore and aft plane. The result was a mast that was less flexible in both directions and a mainsail and genoa that were easier to trim.

In building a mast with four sets of spreaders, as Team New Zealand have done this time around, the sideways control is even better allowing the mast to get skinnier in this plane. Increasing the fore and aft dimensions of the tube have helped to further reduce the windage.

Having said that, there is an increase in windage from the extra rigging, but it would appear that even with this extra drag, the net drag of the complete rig is still less than for a three spreader rig.

As mainsail shapes become ever more refined and slender in section, the turbulence created by the mast has become even more critical as trimmers try to minimise the amount of backwinding on the mainsail. According to ex Team New Zealand member, Alinghi strategist and rig expert Murray Jones, flogging the mainsail just once can cost 1/3 of a boat length.

A slender mast section has never been more important aboard these boats but there are limitations as to how extreme a designer can go. The rules limit the mast tube dimensions to between 430-300mm x 150mm near the deck, 364-260mm x 150mm at the hounds and 210-150mm x 130mm at the top, but these proportions are pretty generous and to Team New Zealand’s mind at least, there was scope for improvement.

“There’s still a long way to go with rigs, the induced drag from the sails remains an enormous part of the total drag,” said Team New Zealand’s Tom Schnackenberg.

“Under the class rules you’re limited in how much you can increase your span. Without being able to change this, induced drag comes down to sail shape and rigs are a part of that. So it’s probably still one of the areas that has the biggest possible opportunities.”

There are also further advantages in going for a slender mast. A narrow mast section also helps to allow the mast tube to twist off higher up the mast. The apparent wind angle on the mast higher up can be twisted as much as 30 degrees off the centreline of the boat. Any twist in the tube can allow the mast to be better aligned to the wind flow with the result that drag is reduced further.

Again, there are rules that limit how much a mast can be induced to do this, but one of the tricks is to attach the rigging that’s connected to the deck, (lower diagonals and the cap shrouds), towards the leading edge of the mast. Alinghi for one have adopted this configuration.

Elsewhere there are other features that are designed to minimise drag and improve sheeting angles. Spinnaker pole tracks that are integral to the mast reduce drag, as do the ‘cigar’ spreader end fittings for the rigging. Jumper struts that can be swung fore and aft independently clear the leeward jumper stay from the leach of the genoa.

The result is that while recent months have produced stacks of speculation about what’s going on down under, there’s plenty going up top. And so far it’s stayed there, with no complete dismastings. Further proof perhaps that the real development has been going on in front of our eyes for some time.