Above: Eric Tabarly starts the 1982 Route du Rhum on his foiler Paul Ricard; the interesting solution to supporting the big early square-top main suggests parallels with the ‘headboard’ control systems now being developed for the AC75 monohulls. For the ultimate salty sea dog Tabarly’s thirst for innovation was unquenchable. Left: where will it end… it’s only just begun. A much larger foiler based along similar lines to this electric runabout is now being trialled on the River Seine for its potential as a future commuter craft
At low speeds the foils cause drag and slow the boat down, but as speed picks up the water flowing over the foils starts to lift the boat, drag is progressively elimi- nated and so the speed increases further making the foils even more efficient. The biggest question when designing a
foil for a sailing boat is to decide on a target performance range and therefore shape. Looking around at some of the newest foiling yachts it’s clear that there is most definitely no consensus on shape. The latest offshore foiling movement
was given a boost by British yacht designer Hugh Welbourn as he doggedly refined his original concept of a Dynamic Stability System (DSS). At its heart DSS is simply a lateral lifting foil that projects out from the side of the boat perpendicular to the hull and parallel to the water. When the boat heels the foil dips into the water and, a foil being a foil, it starts to generate lift which in turn increases righting moment and reduces the boat’s displacement as it tries to lift it out of the water. Sounds simple… The top Imoca designers were intrigued
and of course wanted to incorporate this technology into their latest designs but they ran into trouble with the rules that govern the class. The rule stated that there may be no more than five appendages and traditionally this meant the keel, double rudders and a daggerboard either side. It’s pretty hard to get rid of any of those
but yacht designers can be clever and that’s when, following the 2012 Vendée Globe, they set to work on the idea of combining
64 SEAHORSE
DSS with the daggerboards to get the benefits of both in the same appendage. What you end up with is a shaft that pro- jects laterally out of the side of the boat, an elbow where the foil makes a sharp turn upward to a triangular tip. The shaft acts as the DSS and the part that is turned upward replaces the daggerboard and limits the boat from slipping sideways. These new combination foils were given the nickname ‘Dali foils’ in honour of the iconic Spanish artist’s distinctive moustache. But the latest generation of Imocas
being prepared for the 2020 Vendée Globe have pushed the former Dali foil to new heights, and with widely differing opinions on what will work best. In fact, most of the new foils dispense with the ‘moustache’ flavour altogether, going instead for much longer and more powerful lifting surfaces that in many cases suggest a more refined return to the original DSS concept, but still allowing for the need to resist leeway. The newest foils are overall much
bigger, particularly in span, especially the foil as opposed to the shaft which in many cases now effectively form one continuous surface. Some offer a gentle curve while others are more abrupt where the shaft becomes the foil. In the case of the latest VPLP design, Hugo Boss 7, the foil is now of a constant radius – extremely long and with no surface discontinuities at all. It’s clear, however, that the Imoca
designers now expect their skippers to spend most of the time above rather than in the water and with this in mind for the
first time in sailing history the tables have been turned. Historically it has been the hull design that has dictated foil design. Now it’s the foil that is the driving force when it comes to designing the hull; this reversal was first hinted at in the design of the previous Hugo Boss, again by VPLP, with a more slender hull and longer, flatter foils than her rivals – ironically pointing to a return to the core principles of DSS. As Guillaume Verdier, the brilliant
French designer who has played a big part in many of the most successful designs, from Imoca to Ultim to America’s Cup multihulls (and now AC75s), says – and I paraphrase – ‘All we care about with regards to the shape of the Imoca hull is how quickly we can get the boat foiling. That’s the number one priority.’ As a further indicator of where foil
evolution may take us next, the foils in the new AC75 class have allowed any form of conventional ‘keel’ to be dispensed with altogether. Now we have retractable foils with ballasted foil arms, which together deliver both righting moment and lift as well as a component to resist leeway. Early observations of the first sea trials
of the AC75s suggest that this latest foiling evolution will take a little longer to settle down than the inherently more stable multi hull foiling platforms used for the last three America’s Cups. However, with this latest foiling genie out of the bottle, even- tually the technicians will find solutions to not only make for reliable AC75 America’s Cup yachts but, once again, perhaps for elements of their latest brilliance to flow down to more humble Mom and Pop craft. We shall see.
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EASTLAND/AJAX/DPPI
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