Inflatable battens

Imagine being able to hold up a big roach on a headsail without having to worry about battens breaking...

Thursday June 28th 2007, Author: James Boyd, Location: none selected
While flippers, we hope, are a development we may only ever see on Cup boats, another new develop for the 2007 Cup with applications in other markets are inflatable battens.

As mentioned in yesterday's article about AC sail development, having up to four battens to support the giant headsail roach has only been allowed in Version 5 of the AC class rule following 2003 when teams had been pushing the limit of what a 'spreader patch' was. While mainsails have a relatively small degree of movement, their battens in compression, demands on headsail battens are quite different - they must be rigid and yet must be able to withstand being thwacked repeatedly around the front of the mast, abused in pre-starts, etc a constant cycling that makes solid battens not ideal.

It was no shock when the first inflatable battens were used in 2005 as the rule mentioned from the outset that they were permissible. The question was how to make them?

"We’d designed two sails with roach and we were sitting there with three weeks to go trying to figure out where we were going and we had a bit of fire hose with a plug in one end and a valve in the other - that’s where we started!" says Emirates Team NZ's Burns Fallow. "We’ve come quite a long way from there, even in that three week period!"

At Alinghi Mike Schreiber says they did a lot of work in house on them in the early stages and like Emirates Team NZ they have kept their R&D on them strictly in house. Today they use one manufacturer - a company called RBS based at Hood River in Oregon, whereas the Kiwis make their own. "They started out building windsurfer battens where you need something which can be folded over, or really survive a beating," saays Schrieber of their supplier. "So they became specialists in making battens that are really forgiving and take a lot of beating and high deflection without breaking."

The arena of inflatable battens has been very much the wild west for this America's Cup with teams like Alinghi and the Kiwis doing their own thing. Luna Rossa are believed to have developed some small diameter high pressure inflatable battens in conjunction, of course, with Pirelli. Some teams used solid battens, others were using a hybrid arrangement with a solid batten at the top and inflatables beneath - or even the other way around...

Some lower-tech teams were merely getting an inflatable bladder (like a bicycle inner tube, cut in half with a knot tied in both the ends!) and covering it with sail cloth. However in New Zealand Alex Vallings' company C-Tech spent much time and effort developing their own inflatable battens. C-Tech supply mainsail battens to all the Cup teams and so it was a natural development to extend their product range... Their final product only became available towards the end of last summer.

The final material they have come up is effectively like that of a brown, extremely thick, ribbed condom. However it is made of a very special rubber-like polymer, developed by another company in NZ, rather than rubber - according to Vallings the density of rubber is too high and it can be awkward to work with. The C-Tech inflatable batten has no internal bladder but uses Vectran reinforcement within the polymer - all sorts of yarns and fibre orientations were tried.

During the R&D of the C-Tech battens numerous other aspects were investigated such as how much the battens taper and how much they are pressurised. "Some of the teams have developed battens that are under really high pressure and what the high pressure does is that it allows you to go to a smaller diameter," says Vallings. "However the longevity is lower and the load on the materials is a lot more. You need special equipment to blow it up and it is not safe as well."

Typically the C-Tech battens come in two diameters 25 and 32mm and are blown up to 80-150psi, around two to three times how much you would inflate a bicycle tyre. "It is interesting because when you curve the batten the only thing keeping it straight is the pressure. As soon as you are putting the inside edge into compression you only have the polymer working against that compression. Effectively it will curve quite a long way before it kinks. The stiffer the material you use for the matrix of the polymer, the less it will bend before it kinks. And that is an important thing because of the curvature in the top of the AC jibs." The C-Tech batten will bend through 90degs over a length of just under a metre before it kinks. This is enough curvature to work in the full length top batten slot in a Cup headsail.

Getting the stiffness you want is critical agrees Mike Schreiber.. "and also the amount of flexibility and durability in it because it is a pretty unique environment these things are operating in. They are really slamming against the rig in the pre-start and they get wound around the shrouds and unwound. So we found durability was the biggest hurdle. You start out with something really light and you just beef it up until it stops breaking!"

In terms of length the longest battens are at the top - typically in the 3m+ range, while down low they are shorter. "In that case the primary function of those is to clean up the local curvature at the leech where sails without battens, particularly carbon sails tend to get a little bit of leech dump just because it starts to get beat up. So down low it is just leech support, cosmetics and as you go up the sail it helps to support the roach and influences the shape," says Schreiber.

In practice for the Cup, the battens are left permanently inflated, but teams check the pressure before heading out on to the race course. In theory one changes the pressure within the batten according to conditions, but this only tends to happen before leaving the dock and in fact changing pressure doesn't affect the sail shape greatly. "You get to the point where if you blow it up harder it doesn’t get must stiffer," says Vallings. "You have the generic stiffness when it has zero pressure in it. So if you bring it up so that it won’t kink, maybe 20-30psi, it has a reasonable stiffness and if you up it to 60 it doesn’t double the stiffness."

Mike Schrieber says that Alinghi they tend to pressurise their batten differently according to what part of the sail they are in.

After, one imagines, considerable teething problems and some impressive bangs from aloft, we understand that the reliability of the inflatable battens has come on in leaps and bounds.

"I am amazed at how well the whole package is working," says Mike Schrieber. "We have learned a lot in one Cup cycle. The sky is the limit in terms of what we could do from here."

The future

Inflatable battens have many applications in other classes, even though overlapping headsails are now something of a rarity. To our knowledge it hasn't been tried on the roach on TP 52 mainsails which are limited by having to have a single fixed backstay - but what if inflatable battens were fitted at the top of the sail to allow a bigger roach to flip under the backstay?

In recent months C-Tech have been moving into the superyacht market fitting inflatable battens to roller furling headsails, complete with 4mm air lines laminated into the 3DL running up the luff. Unfurl....inflate... "About 30 seconds before the top mark you disconnect the hose, the air comes out of the hoses, furl it. It is good." says Vallings. The first try out was on board Hasso Plattner's 147ft Visione and has since been tried on Charles Dunstone's new Hamilton 2 and Harry Macklowe's Unfurled.

Could air replace that level of stiffness? "It would have to be around 100mm in diameter at up to 3,000 psi to create the stiffness of a 25mm carbon batten! At this stage it is unfeasible."

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