Pause for thought
Many expected the 2020-2021 Vendée Globe to be a full-on foil fest. Enough well-prepared foilers started the 28,000nm course to suggest one or two at least would disappear over the horizon. It didn’t turn out that way. While rough weather boat preservation is a widely cited reason, once out on the ocean the science of Imoca’s two-legged stool configuration performed even worse than anticipated. Dr Robert Lainé, former chief technical officer for the Ariane space launch programme, explains one idea to improve the current situation
Introduction There has been a lot of debate about foils on monohulls and their use in offshore sailing, but rarely have we seen a fair comparison of the real performance on a course that offers a wide range of sailing conditions. The Vendée Globe is one of these opportunities not to be missed as the boats have very similar hull, rig and keel dimensions and their crew are competitive.
56 SEAHORSE
The top 10 The orthodromic Vendée Globe course length is 24,365nm. The table opposite gives the data for the first 10 boats to finish in the most recent race this year. It is to be noted that the apparent speeds
of the first five boats over that theoretical course of 24,365.7nm are very close, with a spread of just 0.6 per cent. Now let’s look at the Speed Over
Ground covered (SOG), which is a mea- sure of the true speed potential of the yachts. l The first three foilers have a SOG ranging from 14.84kt for Bureau Vallée to 15.13kt for Apivia (2% difference). l The first three non-foilers have a SOG ranging from 14.17kt for Omia Water Family to 14.22kt for Yes we Cam! (0.3% range) which is a very small difference – and here the fastest boat is first in class. l The difference of speed averaged between the top three foilers (14.94kt) and the top three non-foilers (14.19kt) is 0.75kt – a much larger difference of 5%. A 5% difference in SOG is a very signif-
icant advantage for the foilers over the non-foilers, but that comes at the cost of a longer route around the world in the wind conditions experienced that ranged from almost no wind up to 40kt+. At the end, over the finish line, the true
difference of speed is only 0.6%, meaning that only 12% of the 5% SOG advantage of the foilers is converted into real gains
over the course. Why? The reasons lay in the behaviour of the foils in open sea and the lack of adaptability of current Imoca foils to widely varying sailing conditions.
Foils in a dynamic environment To understand why Imoca foils are not performing much better in open sea races we need to go back to the lift and drag factors of these foils. Unlike with the America’s Cup AC75
foils, the Imoca rule forbids any change of the foil profile by means of flaps. The foils come out of the hull through a fixed aper- ture (gate) and the only degrees of freedom left to the skipper are the length of foil sticking out, the ‘rake’ which rotates the foil around its axis and the ‘cant’ which lifts or lowers the tip of the foil. These three degrees of freedom are all
adjusted manually via basic mechanisms, in other words they cannot be adjusted continuously by the skipper (nor automat- ically). He has to find an average setting valid for the next few hours. During that period between consecutive
adjustments the leeward foil will see a variable water flow incidence due to the waves, and the pitch, roll and heave of the boat, hence the lift generated, will vary and that will generate additional drag. In this context one will also remember
that these boats have a very wide and flat stern, meaning that the pitch variation will be essentially rotations around the transom
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