The design and build phase is over. America’s Cup pundits are saying that the 35th battle for the big silver prize may have already been won or lost in the design of the underwater appendages. The daggerboards and rudders have subtle design differences that you will not see unless you look very closely.

There is a wing you can see — the hard mainsail that powers the boat through the water. There is also a wing you cannot see — under water, the daggerboard foils and the rudders that lift the three ton boat out of the water and make it fly.

Daggerboards also provide stability and resist the sideways force of the wind on the sails, but they also cause drag and that is where the tradeoffs lie.

Nick Holroyd is the foiling daggerboard design guru for SoftBank Team Japan. At the San Francisco Cup, he was the mastermind behind Emirates Team New Zealand’s 72ft catamaran’s foiling breakthrough when the Kiwis got the jump on opponents by being the first to make them foil. He left the Kiwi outfit with skipper Dean Barker in June. Along with other designers he brought on board the SoftBank program, Holroyd brought a wealth of intellectual property with him, too.

In a recent interview Holroyd said that “one real performance area where America’s Cup teams can differentiate themselves and put their research to0 work is in the daggerboards and the wetted surface they project and the drag they entail.”

Each challenger gets to design six boards for their AC45 test boats and only four final boards for their AC Class 50s. Teams may repair damaged boards, but are only allowed one design change per board and then only up to 30% of the weight. This can be significant as Holroyd points out but it takes time, up to three-to-four weeks to repair a badly damaged board.

Although building a new board is a three-month process, the time to build, test and learn a new board is more like seven months. This includes learning how to sail with the design and learning what the design limits are.

Each new board Holroyd said, “is a discovery. That’s the fun part. You have a period of design and work and then you put it into the boat where there is always some aspect of performance, design or behavior that is a surprise.”

“The “S” shaped foil is evolving for most teams, he adds, “because that shape moves the foil’s lift point more outboard from the boat creating more stability and more righting moment. But it also creates more drag. This is where we are making tradeoffs.”

“A less stable foil has less control and less drag and more speed… but if you can’t sail the boat in this less stable mode, that is not fast either.”

At the slower wind speeds likely in May and June, the daggerboard foils need to be more efficient so the boats will be able to foil earlier. The ultimate goal is for the foils to fly the hulls at slower wind speeds. The span of the underwater wings for light air needs to be longer, a deeper draft. This is a real challenge for structural engineers as the board is more difficult to control as it flexes and as the crew changes the rake and cant positions.

Holroyd points out that in 6kts of wind the boats have both hulls in the water, slow sailing. In 6.5kts they will lift one hull and the speed will double from 6-12kts. In 7kts the boats foil and reach boat speeds of three times the speed of the wind. “There is a huge transition,” he says.

The next sets of foils will be quite different and there is little time for any significant changes.

Holroyd is watching for board failures on other teams as he assesses the structural engineering that other designers have chosen in an effort to solve as many of the performance tradeoffs as possible.

“To make these boats efficient you’re forced into making some extreme structures”, Holroyd said. “There’s an incentive in light air boards to have a lot of span — which is structurally difficult — and for heavy air boards you’re trying to design them to not cavitate, and that’s also structurally difficult.”

Asked how a team may fair if they had broken a foil during practice at this point in the competition, he said that the time to build a new dagger board has long since passed.

“You’re well inside the timeframe to built a new board in time so if you have a major structural issue at this point that warrants ‘open heart surgery’, that can take you off the water for three-to-four weeks.” These patients take a while to recover.

“If you were having structural issues in your boards at this stage of your campaign you would be nervous.”

For America’s Cup news or updates, Talbot Wilson can be reached at 595-5881 or 278-0143