Foiling toiling
Andy Claughton admires the progress made with the latest Imoca designs… but he is less convinced about the chances of widening the foilers’ magic circle
Over the last year Carrington Boats have crafted (built doesn’t do them justice) two extraordinary yachts: Hugo Boss 7 and the Ineos Team UK America’s Cup 75. The former involved 30,000 design
hours followed by 50,000 man-hours to complete, and the latter surely no less than this. That’s 25 craftsmen for the best part of a year for each boat, not to mention the hours needed to create the working drawings. Alex Thomson talks about how designs are scribbled on napkins, or in the condensation on the windscreen as we wait in the ferry queue. Ideas and concepts are the easy bit, the commitment, attention to detail and organisation needed to give these ideas flesh are extraordinary. This story is also being played out in
France, Italy, America and New Zealand as the boatbuilders put the final touches to the Imoca and America’s Cup yachts. We have known for a while what the America’s Cup yachts will look like but there has been genuine uncertainty about what the new crop of Imoca 60s look like, both in terms of hull shape and the foils. There are three threads to the design of
an Imoca foil: 1. vertical lift to make the hull effectively lighter, or even fly. 2. creating side force to help the keel reduce leeway. 3. and increasing righting moment by having the vertical force act on the leeward side of the boat. The boats that raced the 2016 Vendée
Globe show how different the foils look when you change the recipe for these three ingredients. The Dali foil has a large verti- cal tip – this is looking for leeway reduc- tion; the other two parts are byproducts. The HB6 (Hugo Boss 2016) foil, seem-
ingly so mundane it doesn’t warrant a nickname, focused on vertical force and righting moment (RM). This is because the keel produces the lion’s share of the side force, despite being canted and heeled. It is such a long foil compared to the foil tip, it is very hard for the shorter tip to make a meaningful contribution, and if it does there is quite a high drag penalty. When sizing the foils it is important to understand just how powerful they are.
46 SEAHORSE
The working part of the foil that creates vertical force is about the size of a park bench (450mm x 1,800mm), about 0.8m2 in area. Depending on the speed of the boat and angle of attack the foil will produce lift and righting moment as shown in the table above. An Imoca 60 weighs about 8,500kg so
at 30kt the foil alone can lift the boat clean out of the water if the angle of attack is great enough. Bear in mind also that the keel, by virtue of its pivot being inclined upward by 6°, is also producing about the
same upward force as the foil. When the foil and keel lift the boat the bow trims up and the angle of attack of the foil increases further. Five degrees of hull trim is not unusual. Finally, the one-design Imoca rig is designed to a righting moment of just over 30-tonne-metres (see the table…). Designers love the foil; it lifts the
boat, reducing drag, it produces righting moment with no added weight, and it can help reduce leeway. This is all good when the boat speed is in the 20kt zone, but when the boat speed gets above 30kt we
MARK LLOYD/ATR
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