Opposite: at work and (left) at rest with the author taking a good look during the last Yacht Racing Forum in Lorient… the SEAir foiling Mini Proto has been sailing for over 18 months now testing a variety of foil configurations using as a platform David Raison’s original Mini Transat-winning Magnum scow 747. SEAir itself is a foiling research project producing foil solutions for military and leisure powerboats. The trials of the project’s Mini often involve different foils each side – AC75 and AC50 style – using an electronic foil-control system requiring up to eight live sensors. For anyone caught out when the first Dali Imoca foils appeared in 2015, this (above) is Vendée Globe winner Armel Le Cléac’h testing an early configuration back in 2012
complexity and budget are on a grand scale to ensure trouble-free use. Because in many other cases the foils
can produce a vertical force that is of the same order as the boat’s weight the hull design must be adjusted accordingly. For the America’s Cup AC75 the hull needs to create as much righting moment as poss - ible when sailing slowly. This gives the most power to accelerate and reach take- off speed quickly. This would suggest a scow-style hull, and a quick look at the moulds for the first Ineos boat leaving the Multiplast yard appears to confirm this. This type of hull has been used to great
effect in the Mini Transat fleet, where boats now have an almost rectangular plan form. This style of hull is not seen in the Imoca fleet because the rules specifi- cally ban it by imposing a maximum hull width 1m aft of the stem. The problem is more subtle for the Imoca
boats – you cannot sail for hours with the boat fully flying. But you need to get up to speed quickly. Once the foil is doing its work the hull effectively weighs 25-30 per cent of its total physical weight. So Imoca hulls are becoming more slender once again as they are no longer designed to max out on the hydrostatic righting moment. The DSS system, which to a large extent
set this ball rolling for the larger boats, does what it says on the tin. It uses hydro - dynamic lift from the foil to increase right- ing moment as the boatspeed increases. The foil is extended as far to leeward from the boat as possible, and a meaningful righting moment gain can be achieved from a foil that produces a vertical force that is 10-15 per cent of the boat weight. Using this approach the designer creates a slimmer hull that goes well in the light and properly lights up reaching in stronger breeze.
The latest generation of Imoca designs
like Charal have increased their foil size and can now fully fly, so we know that the foil is producing 10 tonnes of vertical force. However, the one-design spars for the Imoca class are designed for a maximum righting moment of around 30-tonne-metres... With such a long and powerful foil the
boat can generate a huge righting moment, in fact way more than the mast could toler- ate. So the focus on foil design in this fleet is now on how to use the vertical force to reduce overall drag, not just augment righting moment. The Imoca sailors will be watching rig
loads and retracting the foil when the alarms start flashing red. Just another task to add to the stress and anxiety. Why is it such a mission to upscale the
apparently effortless performance of the dinghy-sized foiling boats? It’s all about controlling the foil lift to maintain a constant height above the water. The Moths have a wand in the bow
linked to the main foil. The International 14s have a controllable rudder foil via a tiller twist grip. The Nacra cats have a manual foil angle control at the top of the daggerboard case, and the crew can shift their weight to change the trim of the boat and thereby change the angle of attack of the foils. On a 60-footer the loads are way beyond a wand and a Bowden cable, and changing hull trim means shifting the stack from the transom to the mast. Not a strategy that yields quick results. So how do we get foiling safely and
(relatively) cheaply into the mainstream on larger boats? The current solution is to use a small amount of buoyancy from the hull to smooth out the fluctuations of foil lift as the boat moves through the waves. It’s a
bit like using training wheels on a bike: when you’re going straight they just kiss the road, when you wobble they stop you falling over. Another approach is to try some hydro-
elasticity magic, much beloved by struc- tural engineers. To do this the foil’s carbon-fibre structure is arranged so that the foil deforms to reduce lift as speed increases, thereby offering a modicum of automated control. However, because the foils need to be so
strong they inevitably become too stiff for this effect to be very large if the whole foil is considered. Instead, to increase the potential effect foils may employ a bendy trailing-edge section that straightens and reduces lift as the boat goes faster. These were used extensively, and to good effect, in the last America’s Cup but they were a maintenance nightmare. This approach is just about within the scope of a full-fruit shore crew… it is not something for the enthusiastic amateur. In my mind, for full flight you need active
control using stored power. Autopilots are an accepted part of shorthanded offshore races, they use stored power and software linked to sensors. Is it feasible to transfer this to the control of ride height? Yes of course, but there is a natural antipathy towards diluting the effect of sailor skill. But it would be perverse to make sailing
large fully foiling boats the sole preserve of professionals with physical attributes more often seen in the cast of Cirque du Soleil. Normal people want a slice of this action too. This is reflected in the new D35 Class being developed in Europe. The boat is a fully foiling catamaran for owner-drivers and will feature an automated ride height control system. Is this the future for more accessible,
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