Working towards the Prada Cup
When the 75ft monohulls took to the air, travelling at speeds that would get you a ticket ashore, it was inevitable that the focus would be on the foils. The ability to lift a 7tonne vessel using a platform barely any bigger than a surf board is an incredible feat in itself.So, it was perhaps inevitable that commentators and observers would start focussing their attention on whether a particular set of foils were fast or slow, whether they were more efficient at lifting the boat out of the water or providing low drag at high speed. Their shape and their configurations has been debated at length, as has the issue of whether a particular teams foils are flat or have anhedral, where the foils are angled down to the water. There have also been discussions as to the pros and cons of foils that are straight along their length, or whether they have kinks in them, as well as the overall aspect ratios.
Yet as far as understanding this new world goes, it is still early days and there is little in the way of consensus among observers. Meanwhile, teams are staying tight lipped as they keep their learnings to themselves.
But, in trying to understand the issues that lie behind these and many other foil features, it’s easy to forget about the other side of the equation, the rig and sail plan. Creating sufficient power at the right time to lift the boat out of the water presents different requirements to that of delivering power at speed.
In crude terms, the issue is similar to driving a planning powerboat. When you’re travelling slowly at displacement speeds and trying to get onto the plane, you often need to move the throttle more than you do once the boat has broken free of its displacement mode. Then, once the boat has climbed up onto it’s bow wave and is planning you ease the throttle back as the boat accelerates more freely.
It’s a similar deal with the AC75s. Once on their foils, the drag drops significantly and the boat accelerates rapidly. This sees the apparent wind speed, (what the crew and the boat feel, as opposed to the actual wind speed if they were stationary), increase rapidly too. This means that the amount of driving force that is required is now quite different to when the AC75 was plugging along pushing water ahead of its bow trying to climb out of the water.
The recent AC World Series provided the clearest indication so far as to how nimble teams are in this transition phase. The confines of a race course meant that we were able to compare performances for the first time.
But the ACWS also provided the first indications as to how teams handle their machines.
We also saw just how advanced the Defender’s sail plan is. Here, one of the clearest pieces of evidence was from the onboard camera mounted on the stern looking forward where it was easy to see just how little Emirates Team New Zealand’s sail moved from side to side on the traveller when compared to those of the Challengers.
Unsurprisingly this was one of several indicators that drew the attention of North Sails president and two time America’s Cup helmsman Ken Read.
"I think a sign is, is how much the travellers go up and down," he said. "And the more a traveller moves, probably the less control you have of the rest of the sail because you're really having to dump the traveller on a bear away. We saw during one of the races the Americans were banging the traveller car into the bottom of the boat."
So, given their shortcomings in the light weather, it was little surprise to hear that the British team stepped a new rig shortly after the ACWS. As with all the teams, details are sparse at present, none of the Challengers wants to reveal what they are up to while their competitors have an opportunity to copy or modify their own systems.
But this has not been the only alteration, their foils are currently different too. During the first week of training in the New Year Britannia appeared to be on the second generation of foils raising speculation that the third generation set that they had used in the ACWS are currently being tweaked.
In the American Magic camp, the big visual difference is the new, larger bustle/skeg which is far deeper than before. Unlike that of other teams, theirs does not run all the way to the back of the boat and stops around half to two thirds back from the bow. The object of this is to create a better aerodynamic seal between the hull and the water’s surface, in other words, a better end plate that increases the aerodynamic efficiency of the rig.
During their training sessions when they were up and running it was easy to see how much closer the team were able to keep the boat to the water’s surface and how the bow down trim that all the teams adopt, was matched by the position of the new bustle.
According to skipper Terry Hutchinson the team had gone into the ACWS with concerns over their light weather performance. In the event they appeared to cope well in these conditions, but the addition of an extended bustle looks likely to be a modification to help develop maximum power at the lower end of the wind range.
If there was one lesson that was clear for all, it was how punishing poor light airs performance is if your opponent is still up and flying. No one can afford to be caught out.
So far, Luna Rossa Prada Pirelli have been harder to assess during this first week.
During the ACWS the Italians appeared to be the quickest of the Challengers in light winds. They also appeared to be working their sail plan differently to the others. For starters they sailed the event without the stays that support the mast in the fore and aft plane, the running backstays, in an attempt to reduce windage. But that wasn’t the only difference.
"They were very clearly setting up their mast a lot straighter than everybody else was," said Read. "That's adjustability, that's power. So, I think teams are seeing really different setups and learning what others are doing well."
On the occasions when the Italian team has been out on the water the boat appears to have new aerodynamic fairings around the crew area which if nothing else, provides further evidence as to just how important aerodynamic drag is in this America’s Cup.
But so far, Luna Rossa Prada Pirelli has been seen less in training than the other Challengers, fuelling speculation that bigger modifications to the boat may be in progress.
Whatever is going on behind closed doors to improve performance ahead of the start of the Prada Cup, boosting the power seems to be at the heart of the changes.