Tech Review: Maximum Lift
Tech Review: Maximum Lift
Speed in a sailboat is a matter of balancing lift and drag. Thanks to two significant technological advancements—one above the waterline and one below—no race boat has ever done it better than the AC72.
The problem with a pure 90-degree turn, as with the foil that Oracle Team USA’s AC72 was using when it capsized last October, is its poor heave stability. “A pure L is good in terms of drag, but you can’t control it,” says multihull designer and foil expert Martin Fischer.
One solution is to put a bend in the top of the daggerboard. By virtue of the bends, these boards are principally vertical when fully lowered, but cant inward, and provide vertical lift when slightly retracted. In effect the L shape becomes more of a V, which provides much improved heave stability.
But beyond this there has yet to be much consensus among the teams. Thanks to catamarans having twin hulls, teams have often trained with a different type of board in each hull to gain valuable data on the characteristics of each.
Of course, while the shape of the board is vital for its orientation and in turn its horizontal lift, vertical lift, and heave stability characteristics when it is raised or lowered, crews can also control these factors by canting and changing the pitch of the boards.
The specifics of each team’s daggerboard configurations, in particular how they alter the cant and pitch of the boards, are among the most closely guarded secrets of this Cup cycle.
Building daggerboards capable of handling the vertical loads, both static and dynamic, plus giant side loads created by a boat careering along at 40-plus knots is no small task, either.
“They are complex pieces of engineering because they have to withstand the entire weight of the boat,” says Ozanne, “7.7 tons of vertical load, and up to around 3.3 tons of side force, all on this little piece of carbon.”
Beside the intricacies of how to make the AC72 foil in a controllable manner, there is the issue of when to make the boat fly. It may seem mundane, but could prove pivotal, especially if the wind limits for racing are lowered. AC72s don’t foil upwind because doing so would result in too much leeway. But there will be times when they won’t foil downwind because there isn’t enough wind. The average wind speed for San Francisco Bay is quite high throughout the summer months, so this might not be an issue. Each team will have to make a call on the wind strength above which they wish to foil and design their boats, and their foils, to that. “If people choose a higher speed, then they’ll probably wind up with a higher top speed,” says Schnackenberg, “but it will also mean that they struggle a bit more when it’s light.”
At press time, there was less than seven weeks remaining to the start of the Louis Vuitton Cup—and just over three months to the Cup match itself—and while each team has made remarkable progress considering this class didn’t exist two years ago, one gets the impression from the teams that they have barely scratched the surface with this technology. There are still significant leaps to make.