The latest America’s Cup foiling cats are providing a wonderful spectacle as they scorch round the racecourse, but how did foiling start and what are the issues involved? Matthew Sheahan investigates
‘C’, ‘L’ and ‘S’
But not everyone wants to fly. In recent years there has been a great deal of development in foils with complex shapes from ‘C’ shapes, to ‘L’ and ‘S’ shapes and now ‘S’ foils with winglets. Generally speaking, these foils are less concerned with flying and more with the balance between generating sideways lift to drive you upwind and vertical lift to reduce displacement. Much of this development work took place in the former ORMA 60 multihulls.
“By canting the leeward daggerboard you can change the balance between lateral and vertical lift,” explains Vincent Lauriot-Prévost of designers VPLP, experts in high-performance multihulls and some of the world’s more extreme designs such as USA-17 and L’Hydroptère.
“When you’re sailing upwind, side force from the daggerboard is very important, as it is in any boat, but as you sail freer and faster you need less foil to create the required side force. Normally you would lift the board to reduce the drag, but if you leave it down and then cant it in towards the centreline, you create some vertical lift which then helps to reduce the displacement of the boat.
“Another way to achieve a similar effect is to have a curved daggerboard. A simple ‘C’ shape provides more vertical lift the further it is pushed down. The next stage of development was then to try ‘S’ shaped boards which, again, vary the distribution of lift and side force depending on where the boards are set.”
Although more expensive to build than straight boards, curved daggerboards have the advantage that they do not need complex canting mechanisms. Some classes and rules ban the latter so this is an alternative approach.
“Our ORMA 60 design Gitana II, which won the Route du Rhum in 2006, would see a reduction in displacement of 40 per cent at 20 knots,” says Lauriot Prévost. “At 26-28 knots the displacement would reduce to 70-75 per cent for a boat that weighs 6.5 tonnes fully loaded.”
This puts the issue of displacement reduction into perspective for cruising boats. Half a tonne of lift at ten knots from a curved daggerboard might sound a lot, but when the boat displaces 22 tonnes loaded, the displacement reduction is just over two per cent. While this may still have a beneficial effect on performance, it becomes easier to see that the future for cruising hydrofoils is not so clear cut.
Changes to trim, stability and displacement through subtle alterations in the alignment of foils is also a growing trend in the Open 60s and now the Volvo 65. Angling the keel pin up at its forward end by a few degrees provides a positive angle of attack on the keel fin when canted out to windward. When swung out to one side, the fin acts like an aircraft’s wing and helps to support the boat, although the lift also tends to reduce the effective righting moment.
Nevertheless, according to at least one top designer, the effort is worth it. He says: “There are some other performance details involved with this that I can’t speak about!”
Foils of the future
Where is the current craving for foils taking us? One of the biggest foiling experiments at present is in the America’s Cup. With their notoriously big budgets, Cup campaigns are well-known to accelerate the development of ideas that can eventually trickle down. What kind of technology breakthrough will wingmasted cats on foils provide?
The answer is not a simple one as the AC72s used in the 2013 Cup were never originally intended to foil. A loophole in the rule, exploited by the New Zealand team and deemed legal by the jury, sent teams off exploring the possibilities, but with little scope to control the foils themselves as no moving parts are allowed.
“The Kiwis look good on foils, they are stable in pitch and heave and their foils appear to be reliable. But this will all come at a cost: drag,” said Oracle’s Joseph Ozanne at the time. “Less pitch stability, as our boats had, can reduce drag, but it makes the boat very tricky to sail. We did a lot of wheelies when we were learning to sail our boat. Regulating flight height is a nightmare.
“Modern aircraft are designed to be more unstable to improve manoeuvrability and also to reduce drag, but they have systems that keep them under control. We are not allowed these systems, so it’s the crew that have the control. So the key foiling lessons from the Cup could be learning about foil shapes and how to handle instability.”
For Sailrocket II’s foil designer Chris Hornzee-Jones, another aeronautical engineer, hydrofoiling means speed. Having set a new world record at 64.54 knots and broken into new territory, Hornzee-Jones believes this is just the beginning.
“I’m convinced we can get to 70-75 knots with subtle developments to this foil,” he declares. “Beyond that it becomes progressively harder, but the fact that powered craft on foils have achieved 80-100 knots confirms that there are foil shapes that will work at this speed. I think it may be difficult to get a single foil to perform over the entire speed range. We already use two on Sailrocket. Nevertheless, I do see over 80 knots as possible.”
The lessons learnt in this new speed territory could also have implications at far slower speeds and for much less extreme boats.
“Thick foils like the one we developed are good structurally,” says Hornzee-Jones. “Creating a foil that ventilates at low speeds means that at, say, 20 knots you are already into low drag.”
Like a gearbox and engine combination that allows you to engage top gear at 20mph and accelerate though to 100mph without changing gear, versatile, vice-less foils could indeed transform the behaviour of our boats in the future. Add to that the possibility of active control using sophisticated, compact and efficient electronics and new possibilities emerge.
Opening our minds
Although the average cruising monohull may not want to get up and flying, efficient low-drag stabiliser foils, for example, controlled by a tiny chip, could make the notoriously rolly tradewind conditions on a transatlantic a more comfortable and efficient affair.
“Foiling Moths have opened people’s minds,” says Vincent Lauriot-Prévost. “The future will depend on having good control of efficient foils. We will need to change camber profiles effectively and develop automatic trim regulation. But, we must also think weight, otherwise the systems won’t work.”
Perhaps it is no surprise that after more than 70 years of sailing development at the leading edge of the sport, many are still scratching their heads at how to make such a promising concept work for the rest of us.