As this is being written, five days before racing starts, your scribe does not yet know who “they” are – the designers who came up with the winning foils. We do have a pretty good idea of the tradeoffs and design challenges they faced. We all know the old saw, that the fastest boat  wins the America’s Cup.  Designers used to measure boatspeed differences in tenths or even hundredths of a knot. Today’s AC Class foiling cats can show differences of five knots or more depending on whether they are up on their foils or bobbing in displacement mode. The boat ahead at the windward gate will head downwind 10 knots or more faster than their opponent who is still working their way upwind. Merlin designer Bill Lee is known for saying, “Fast is fun.” Ted Turner said, “I spell fun W-I-N.”

The quest for speed is complicated by other considerations. The Protocol limits teams to four daggerboards, plus two spares in case of unintentional damage. They can make a combined total of four changes of 30 per cent by weight of any board, although changing back to a previous shape does not count as a modification. Unlimited changes of 10 per cent are permitted. A 30 per cent change would allow a different shaped wing and 10 per cent would allow a different tip or trailing edge. The AC Class Rule limits the maximum distance between any two points on a daggerboard to 4.2 metres. With these limitations, what design tradeoffs do we make?

Daggerboard design decisions are part of a set of bigger questions and priorities. What importance do we place on… top end speed; light wind lift-off speed; stability; manoeuvrability; pilot user interface; reliability; and energy efficiency?

Board shapes are tightly coupled with control system design and ergonomics. The AC72 daggerboards operated in a stable equilibrium where ride height and lift were coupled and self correcting. Flying on AC Class boards is an unstable equilibrium requiring constant adjustment from the pilot to maintain stable flight. How accurately and quickly can the pilot adjust rake? And, how efficiently does the control system use the hydraulic pressure generated by the grinders? The grinders do more than grind. In 2013 Oracle had an elaborate choreography for tacking. As the windward board dropped, it was raked to generate negative lift to pull it down. Once fully down, the board was raked to generate maximum lift and get the new leeward hull flying before a third rake adjustment set the board for stable flight or skimming, depending on the chosen mode. Control systems will have evolved to be more efficient raising and lowering the boards. Another way to lower a board efficiently is by curving the shaft in the longitudinal axis of the hull. A curved board changes the angle of incidence of the daggerboard’s wing as it is lowered, giving downforce at first, then transitioning to positive or neutral lift, reducing the need for hydraulic pressure.

The Protocol requires all adjustments to be initiated by manual input but it allows timing sequences in the control systems. A single button push could initiate a board drop with the rake being changed by a timed sequence of adjustments. This might be less accurate than having the crew make each rake adjustment or by the curve of the daggerboard shaft, but it might also be more suitable in the heat of battle.  

Lift off speed and stability are more important than top end speed in light winds. In six knots of breeze, the AC Class yachts will have both hulls in the water and will be slow. At six and a half to seven knots they will fly a hull and in about seven and a half to eight knots they will be foiling. Foiling upwind and staying up on the foils through manoeuvres will decide light air races. High lift, stability and manoevrability will be more important than top end speed. Light air daggerboards, called “whompers” by some wags, have shorter vertical shafts and longer wings that are “cranked” (have a spanwise change in dihedral angle). Shortening the shaft and cranking the wing keep the maximum linear dimension under the 4.2 metre rule-imposed tip-to-toe limit.

For medium to strong wind, teams are likely to have “all purpose” boards (AP’s) rather than boards optimized for top end speed. Some promotional messages talk about speeds of 50 knots but we are unlikely to see that. To go 50 knots would require a low thickness to chord ratio to avoid cavitation. But a minimum thickness is needed structurally, so the chord needs to be increased, meaning more wetted surface and more drag at lower speeds.

Designers talk about the “cavitation bucket” – the range of boat speed and coefficient of lift that can be achieved without cavitation. This is a function of the foil’s sectional shape and that thickness to chord ratio. As usual, there is a tradeoff. Put simply, you can have a range of about 30 knots of boatspeed over which the foil does not cavitate.

Design a foil section that doesn’t cavitate below 50 knots and you pay the price of higher liftoff speed. Better to have AP’s that enable liftoff further down the wind range. The crossover with the whompers will be key. What is the team’s racing strategy? Is it worth going for high top end speed to gain a boat length or two on the reaching start but giving up three or four lengths on the other legs when cavitation is not an issue? Or is it your strategy to lead around the first mark and then defend because there are few passing lanes?

Another crucial design factor is the elbow where the shaft and wing join. This is one of the most highly loaded structures on the yacht. It’s also where cavitation first sets in because of the superposition of the flows over the wing and the shaft. A gentler radius reduces that effect. The elbow can be reinforced with steel to increase resistance to breakage, but that adds weight and flexes more than carbon composite.

The unloaded dihedral angle gives an indication of design philosophy. Negative dihedral compensates for more flex and allows a thinner board with less drag. But it may require cranking the wing to stay inside the 4.2 metre limit.

Lots of tradeoffs that were made months ago. Who came up with the best solution? If Bermuda delivers a lot of days with the ideal 12-15 knots, the winning foils will be the best AP’s. If there is a lot of light air sailing, the whompers and the weather calls will play a big role.