The AC75 cunningham has evolved from a simple luff tensioner into one of the most critical aerodynamic controls in modern sailing. The legend traces back to Briggs Cunningham, the man who steered Columbia to a famous 4-0 victory in the 1958 America’s Cup against the British yacht Sceptre. Unhappy with the mainsail shape on his 6-Metre yacht in the 1930s, Cunningham noticed the sail had stretched at the leading edge. With no time for a re-cut ahead of a weekend’s racing, he inserted a cringle eye on the luff and arranged a pulley system to tighten it. From that moment, the cunningham became one of the most important controls on every racing boat from a dinghy to the America’s Cup.
Nicknamed the cunningham in his honour, it remains an ever-present control capable of making the difference between winning and losing. On a conventional racing yacht, the cunningham appears relatively straightforward. A sailor pulls on it to increase luff tension and move the draft forward, or eases it to make the sail fuller.
On an AC75, that same principle has been taken to an extraordinary level. The cunningham has become a powerful hydraulic sail-shaping tool, capable of transforming both mainsail and jib as the boat accelerates from low-speed take-off mode to more than 50 knots.
Today on the AC75s, the cunningham is a highly loaded, highly engineered control capable of substantially changing the aerodynamic characteristics of the sail. That matters because an AC75 asks its sails to operate across an almost absurd performance range. The boat may initially be struggling to accelerate onto its foils and requiring maximum aerodynamic power. Seconds later, once flying, boat speed increases dramatically, apparent wind builds, and the same sail that was producing desperately needed power can suddenly be producing unwanted drag.
Imagine an aircraft wing. During take-off, a plane extends flaps to produce a deep, highly cambered wing section capable of generating large amounts of lift at relatively low speed. Once cruising, those flaps retract and the wing becomes cleaner and lower drag because the aircraft no longer requires the same high-lift configuration. An AC75 faces a surprisingly similar aerodynamic problem. During take-off it needs power and lots of it. The sails must generate sufficient aerodynamic force to accelerate several tonnes of yacht through the high-drag displacement and transition phases until the foil produces enough lift to raise the hull clear of the water. Once airborne, hydrodynamic drag collapses and the AC75 accelerates rapidly. The apparent wind rises dramatically and moves forward. The deep, powerful sail shape required moments earlier becomes too powerful and too draggy, so the yacht needs another gear. Cunningham load is one of the principal ways of getting there.
The double-skinned mainsail makes this particularly sophisticated. Unlike a conventional mainsail, the AC75’s main consists of two skins affixed on tracks to the mast, creating a soft aerodynamic wing. The AC75 Class Rule severely restricts where the sail can be controlled. With upper control lines above the first third of the sail removed, the cunningham, mainsheet and traveller became the primary mainsail controls.
That places enormous importance on controlling the bottom of the sail. Pulling cunningham load into the structured luff can substantially flatten the mainsail, but it does more than that. The cunningham can influence draft depth, draft position and mast bend. Recent Recon reports on the AC75 systems describe twin hydraulic cunningham rams imposing substantial loads down the mainsail luff, simultaneously flattening the sail, controlling draft position and helping induce mast pre-bend. This makes the cunningham part of the entire rig-control system. Change luff load and you are not necessarily changing only the sail but the entire relationship between the sail and the mast. Because the forestay is attached to that rig, changes elsewhere can influence jib behaviour as well. Everything interacts.
The jib cunningham works on the same fundamental aerodynamic principle as the mainsail, although the geometry and consequences are different. The current AC75 Class Rule specifically permits a cunningham system near the jib tack. Increasing jib cunningham tension changes the load running down the luff, altering the sail’s shape and draft characteristics. The objective is not simply making the sail tighter. The jib needs to operate efficiently across very different apparent-wind speeds.
At the lower end of the range, particularly when trying to accelerate or recover from a touchdown, the trimmers may want a deeper, more powerful section. Once the boat is fully foiling and apparent wind increases, that depth becomes increasingly expensive in terms of drag.
More cunningham allows the jib to become flatter and cleaner, but there is an additional complication. The jib is the first major sail surface encountered by the airflow, and its wake then interacts with the mainsail. The jib and mainsail cannot simply be optimised independently as they form an aerodynamic package. Change the jib’s depth and you potentially change the airflow arriving at the mainsail. Change the mainsail and you alter the pressure environment in which the jib operates. The optimum is the combination, and that is what the teams are spending hours on the water training to understand.
This interaction between jib and mainsail is fundamental. The airflow passing between them, traditionally called the slot, is influenced by the geometry, camber and angle of attack of both sails. If the jib is too deep, the airflow can become compromised and drag increases. Too flat and the boat may sacrifice useful aerodynamic force. The cunningham therefore provides another tool for tuning that relationship, and the adjustment on an AC75 can be extraordinarily precise. The trimmer is not simply looking at tell-tales and pulling a sheet until the sail looks right. The designers already have target aerodynamic shapes and the whole control system must allow the sailors to reproduce them.
The extraordinary acceleration of an AC75 explains why cunningham control has become so important. A conventional yacht experiences a relatively modest change in apparent wind as it accelerates. A foiling AC75 can transition through a huge apparent-wind range very quickly, meaning the optimum aerodynamic section is continually changing.
Consider the process of a tack. The boat enters at high speed, loads change, the yacht turns, the sails cross, foil loading transfers, boat speed falls and the apparent wind shifts. Out of the tack, the AC75 accelerates rapidly, so the optimum sail shape entering the tack may not be the optimum shape halfway through it, and certainly not the optimum shape several seconds after the exit. The cunningham therefore becomes part of the sequence. Load can be adjusted through the manoeuvre, potentially flattening the sails when drag reduction is critical and then changing shape again as the yacht accelerates away. Modern mechatronic control makes these adjustments fast, repeatable and extremely accurate.
The cunningham is no longer simply a device for tightening the luff. On the mainsail it helps control depth, draft position and mast behaviour. On the jib it provides another powerful means of changing aerodynamic shape across the AC75’s enormous speed range. Together, the two controls allow the trimmers to continually alter the aerodynamic characteristics of the rig. On an AC75, the sails are not static engines but variable aerodynamic surfaces, and the cunningham is one of the principal controls that allows the sailors to change their shape. What looks like a simple downhaul has become, in effect, one of the AC75’s most important aerodynamic gear levers.
