Does it ring a bell?
by John Curnow, Sail-World.com AUS Editor, September 25, 15:00 PDT
The Soviet Lun Class Ekranoplan could reach nearly 300 knots with a range of 1,000 nautical miles, provided the water was flat. © USSR
True, the delivery system is different, along with the overall mission goals, but both the AC75 and the Soviet Lun-class Ekranoplan leverage ground effect. Each is designed as a competitor. Both effectively accomplish their tasks. They are also lighter than their traditional counterparts, and they require significantly less horsepower than their kin.
The similarities are indeed striking. There’s a futuristic, distinctly military, and somewhat spacecraft-like appearance, especially when viewed through the lens of their respective eras. Fortunately, the outcomes couldn’t be more different. One is just a slight upgrade from the iconic Spruce Goose regarding achieved ‘flying’ hours, whereas the other continually piques interest in the America’s Cup and the engineering innovations that enable its success.
Perhaps the most thrilling part of AC75 racing is the bear away. Increased acceleration and an abundance of power. It’s crucial to handle everything with care. However, as attractive as all of this sounds, this discussion primarily focuses on foils—the lifting components of the setup. Not only are they conspicuously visible, whether raised or lowered, but they don’t just fulfill the ‘Up, Up, and Away’ function. They are also integral for providing righting moment.
During AC36, we delved into the sails, and I am grateful to everyone who assisted with the technical aspects of the content we produce. Previously, I hinted at possibly exploring foils for AC37. Naval Architect Andy Claughton, who also serves as the International Technical Committee Chairman of the ORC, has generously offered to share his vast knowledge. Thanks, Andy!
Clearly, the arms are one-design, and the entire arm-and-wing assemblies must maintain a specific weight to execute their secondary primary function. Long-chord, high-area foils can lift you into the air but create significant drag. Short-chord foils work well in motion but are less effective at initially contributing to the ‘flying’ aspect and can struggle with compliance during maneuvers.
Larger sections provide more area to support the necessary mass, yet a critical factor to consider is the advanced engineering embedded in the mechanisms that actuate these foils. Consider the elegance of a swan—visually stunning on the surface, but underneath that beauty, there is intricate control machinery at work. Picture an aircraft stripped of ailerons, flaps, slats, spoilers, elevators, and rudders—such a plane wouldn’t fly.
The wings feature a long flap along the trailing edge, managed by the flight controller using battery power.
Upon closer inspection, the nacelle between the wings is where significant weight is centralized, reaching nearly the furthest point when that specific foil faces the wind. If it were permissible and safe, I wouldn’t be surprised if depleted uranium was the material of choice, as its specific gravity allows for a smaller profile, thus reducing drag. That’s precisely why it resembles the bulbous bow of a ship.
Long foils become increasingly efficient the longer they extend, much like a glider. Moreover, if we draw parallels with Boeing’s famous B-52, the lower parts of an AC75 foil evoke images of that aircraft (minus jet pods and tailplane, but featuring added winglets) hanging beneath the actuating arm. One could also mention Boeing’s 787 Dreamliner, considering it utilizes an all-carbon structure…
“You aim to take off, so you need a large wing; however, once airborne, you’re better off with a smaller wing. Hence, you’re left balancing between two extremes, and as foil sizes shrink, the risk of losing lift and crashing increases significantly,” remarked Claughton.
“So if you decelerate and cannot generate enough force to lift the boat above the water at maximum lift coefficient, you’ll start to float. Once off the foils, a larger foil can help you get back on faster. But once back up, you’ll be thinking, ‘Goodness, these foils are too big,’ making it a delicate balancing act in choosing the optimal area.”
“Another primary consideration is the design, which resembles the aerodynamic challenges faced with Formula One wings, where the aim is for the wing to deflect under pressure and get closer to the ground. In the case of hydrofoils, you have the fixed wing element plus the flap at the trailing edge. Engineering the flap to maintain desired positioning under load is crucial.”
“It is essential for the flap to stay in place. In previous Cups, complex engineering attempted advanced foil operations, yet when sailing, the flap failed to hold its intended position. The distance between hinge points was too great, leading to undesired bending. Engineering plays a vital role in ensuring your foil stays as designed while supporting loads of seven to eight tons. While the basic hydrodynamic design might be straightforward, achieving it through engineering is incredibly challenging.”
“You also have to contend with cavitation, which is somewhat unavoidable. Cavitation occurs when fluid starts to boil due to extremely low pressure on the upper side, resulting in bubbles. The sensation can be jarring, almost giving you the impression your teeth might fall out. It’s far from smooth sailing. Therefore, designers constantly work to circumvent cavitation limitations. When speeds approach 52/53 knots, your foil is on the brink of cavitating, making the bear away at the top mark particularly perilous, as higher speeds increase the likelihood of cavitation,” added Claughton.
Utilizing both foils during specific maneuvers distributes the load, allowing the boat to maintain lift through turns. It’s akin to training wheels, which is beneficial since going low-rider would spell disaster. While incorporating ground effect principles from the Ekranoplan is advantageous, much like a deck-sweeper sail that aims to seal the gap between sail and deck, you want to keep the hull close to the water but not touching, as that induces sluggishness and may lead to a dangerous spiral. Clearly, pitch control is the primary focus when fully foiling—no doubt about it. That’s why Flight Control is truly an art.
“These foiling dynamics prioritize engineering over naval architecture. It’s not just about engineering foil structures but also creating highly efficient hydraulic valves; otherwise, you risk overworking your cyclists. Thus, much of the focus shifts to hydraulics, even more so than hydrodynamics. The engineering challenges extend to software considerations as well. Honestly, I would claim that the hydrodynamics of the foils constitute merely 10 percent of the overall challenge.”
“The role of the elevator/rudder has become even more critical due to the forces exerted by the primary foils. It must function as both a rudder and an elevator, which adds layers of complexity to its engineering, hydraulic control, and other elements—it’s truly immense.”
“If you find yourself racing one of these boats, you’re perpetually navigating treacherous territory because nothing feels entirely reliable. It feels akin to piloting a prototype without ever progressing to a production model. The equipment is alarmingly complex, with all systems needing to integrate seamlessly, and the cost is exorbitant!”
Should you stock extra parts, even if they cost a million pounds, for emergencies, knowing you’ll face a three-month wait if something malfunctions? A tough decision to make. Adding design and production quality control into the mix generates sleepless nights, with many staring at their ceilings at 3 a.m.
“In the past, you might have had 35 sailing team members and 10 in design. Now, the roles have completely reversed.”
This style of racing is exceptionally intense. Picture driving a Formula 1 car and slamming on the brakes just past the 50m mark before a turn. Most would faint from the G-forces. It’s similar here—the maneuvers required exceed the capabilities of the average sailor, which is precisely what makes it captivating. The control exhibited in a Moth is evident; however, the nuances of the AC75 are less apparent but no less real. I believe this is a game filled with true risk.
And there you have it. There’s much more available on the group’s websites for you. Simply utilize the search bar or the ‘edition’ dropdown at the top right of the masthead to find additional information. Enjoy your yachting, stay safe, and thank you for engaging with Sail-World.com!
John Curnow
Sail-World.com AUS Editor
