Left: the seventh Imoca built for Alex Thomson by loyal sponsor Hugo Boss is lifted gently into the water after having its keel fitted. The previous Hugo Boss 6 drew attention with its cut-away foredeck and attention to aero detail, but the speed of HB6 mostly derived from a long and quite straight DSS-type foil designed for lift over sideforce, combined with a hull that bucked the trend by going narrower to exploit the straighter foil’s dramatic contribution to righting moment. Some of those elements are pushed harder still with HB7 but the new boat carries big C-foils in place of flatter boards – which would limit the potential to lift the boat clear of the water, as is now the norm. Balancing a pencil on your finger (above)… 7.5-tonnes of Imoca supported only by a canted inclined keel and one enormous – and enormously loaded – foil as Jérémie Beyou flies Charal without the benefit of rudder T-foils for pitch control
have a tiger by the tail. The foil can lift us out of the water and we run the risk of blowing the rig off the boat. The skippers will be anxiously watching the board load cells to make sure this doesn’t happen Fortunately the system is somewhat self-
limiting – as the boat rises the immersed area of the foil reduces and with it the lift, and also as the foil nears the surface venti- lation will reduce the lift force further. Plus the foil can be retracted into the hull to bring the lift force closer to the hull thereby reducing RM. A final level of automatic control may
be introduced using the hydro-elastic deformation of the foil. The foils deflect quite a lot when they load up, the tip will bend upwards by 500mm or so as the foil loads up… this is visible to the naked eye. But as the foil bends there are more
subtle twist deformations that can affect the foil angle of attack. These twist defor- mations can be controlled to some degree by the distribution and orientation of the carbon fibres in the board. Get this right, then as the board loads up the shape changes to reduce the angle of attack and therefore the force produced. This is edgy and complex stuff to design and build. It’s quite an ‘exam question’, how to
balance the undoubted speed increase of powerful foils in moderate conditions with hazards posed by chasing along at 30kt+
in the Southern Ocean in the pitch dark. The boats launched to date show differ-
ent approaches. First out of the blocks was the VPLP-designed Charal. Clearly the boat can foil, as the photo above shows, and this boat won the Imoca class in the Fastnet Race. But keeping the boat fully flying without a T-foil rudder is like bal- ancing a pencil vertically on the end of your finger, possible but not for more than a few seconds without your full attention. Three other boats have now come out of
the shed: Hugo Boss (VPLP), Arkea Paprec (Juan K) and Apivia (Verdier/Dalin). In terms of hull shape they are all quite similar, pushing as far toward a full flat scow-bow as the Imoca rules permit. There are, however, some differences that hint at controlling trim to ensure that the foil is at the right angle of attack as the speed builds. It’s clear no one is planning to do much upwind sailing in light air. All the topsides and decks show careful
attention to aerodynamics, along with pared-down foredecks to save weight. The most ambitious is Hugo Boss with a very sheltered cockpit and coachroof that offers a close seal with the boom. In Alex’s words ‘the Vendée Globe is like going to war’, so it’s important your dugout is as comfort- able as possible. Exposure to the elements is debilitating and should be minimised. It’s not been a popular approach in the
past. When I suggested back in the day that the Volvo Race boats would be safer, drier and more comfortable if we added half a metre to the freeboard of the new designs the sailors looked appalled. I think the prevailing view was that this would make the sport appear more effete, although that wasn’t the word they chose. The new Imoca foils are characterised by
a very wide wing span. Span is extremely important to keep drag down when the foil is heavily loaded; however, once up to speed the lift needs to be reduced, so I don’t think this is entirely about drag. You can see the Arkea-Paprec foil tip is raked aft, which both moves the lift centre aft and also creates a bending moment in the shaft that will feed the hydro-elastic behaviour as it trims out lift with speed. The Imoca boards can retract with only
one degree of freedom, in other words the part of the board that runs between the two bearings (one where it leaves the hull and another in the deck or inside the hull) must be straight or have a constant radius. The hull exit positions are all similar but the geometry of the retracting part of the boards is quite different. Apivia has an upward-facing arc (centre
of rotation below the water), so that the inside bearing is below the deck; so as the board is extended the tip of the foil becomes more horizontal.
SEAHORSE 47
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