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The forward cockpit and coachroof give an impression of the rig being further aft than usual on Sodebo Ultim 3 but that element of the boat at least is relatively conventional. What it means, though, is that the aft half of the deck is completely clear, allowing the rig to be lowered and producing the precious endplate at the foot of the mainsail. Expect a small spray screen for the helmsman (left) to put in an occasional appearance – 60 knots of apparent wind is daily fodder for Coville


charge of VPP development at American Magic. However, even if this core proces- sor and the way it converges to the equilib- rium of a boat are important, reliable results will always depend upon the consis- tency and quality of the individual num - erical models feeding the VPP, and these are specific to each boat. Any VPP is only ever as reliable as the accuracy of the models that feed it. The higher quality of these inputs and


handed already implies some compromise on performance to guarantee a safe dynamic balance of the boat. The passive stability inherent in our foil


design can be divided into two criteria: l The global stability l The local stability The global stability determines the flying


range provided by the foil design, being the speed range across which the boat remains in good balance without requiring further adjustment. The wider this range the more stable the foil and therefore the boat – and the easier it will be for the solo skipper to maintain the high average speeds required for success when racing over a long course. The local stability comes from the heave


stiffness provided by the foil and the rear foils. This is similar to a spring; the stiffer it is the more energy is required to com- press it. The greater the heave stiffness the greater the energy required to modify the flight height and pitch of the boat and so the greater the inherent stability of the foil. For Sodebo Ultim 3 we emphasised the


need for a large flying range to get suffi- cient and safe inherent stability, giving Thomas a high average boat speed with minimum action at the expense of maxi- mum top speed.


VPP – the object of all fantasies The recent dramatic change in the behav- iour and performance of the Ultim is testa- ment to the ever greater requirement for excellence in R&D in a demanding, finan- cially (especially) high-risk field. Building a


€10-12million Ultim that doesn’t work is an unthinkable outcome… it is certainly a career ender for some of those involved. The constant increase of boat speeds


and flight range introduce new and often complex physical phenomena requiring substantial investments in research. This is why Bañuls Design invest a significant portion of their resources into gaining a better understanding of boat behaviour, developing our design modelling over many years in close collaboration with two-time America’s Cup winner Joseph Ozanne, who has been responsible for the VPP and numerical models for numerous different types of racing boat. (Joseph is Team Leader Simulator Development at American Magic for the 36th AC). The conception and analysis of the


performance of offshore racing boats are still primarily processed through the use of velocity prediction software. The VPP remains essential to assist in all the impor- tant choices through the design process: the better the VPP for the boat being designed the better the outcome. A good VPP accu- rately solves the balance of the different forces and moments applied to a boat, in the six degrees of freedom. It combines the various theoretical models (sails, hulls, appendages) to simulate the overall speed and attitude of the boat. Today’s tools are excellent but there is still a long way to go, particularly in modelling sea states. Bañuls Design are currently using the


core VPP developed by Giorgio Provin- ciali, the Italian engineer who is in overall


the more accurate way that we can now interpolate them to get what we call ‘a response surface’ are a crucial develop- ment. These numerical models come from several sources of numerical simulations, each with different levels of fidelity to real sailing conditions and with different time- frames and costs: l Analytics models from basic calculation l Panel codes l Computational fluid dynamics (CFD) l Experimentation (towing tank tests, wind tunnels) The supply of the aero and hydro


numerical models needed to run a com- plete VPP requires an extensive campaign of numerical simulations. Even an Amer- ica’s Cup budget and timeframe, focusing on a single type of boat, running an exten- sive CFD simulation, barely reaches an entire high fidelity response surface. For Sodebo Ultim 3 the schedule and


budget led us to develop a method of com- bining high and low fidelity models and the unique experience accumulated by Thomas and his team on the previous Sodebo 2. This gave us a consistent response surface for our VPP, which gener- ated very reliable results. These results were not only a very helpful assistance in the key design choices, but they are just as essential today for our design office to work with Thomas and his team in the fur- ther successful development of the boat.


Dynamic VPP Until recently most VPPs were based on the hypothesis on the static behaviour of the boat (constant speed on a straightline tra- jectory), which ignores all sorts of dynamic phenomena such as acceleration, waves, loads, wind gusts, dynamic stability and sailor or autopilot interactions, making this approach less than optimal. Bañuls Design are gradually integrating


space and time into our VPP for specific simulation cases. The VPP, with these addi- tional dynamic bricks, not only gives static equilibrium but also dynamic behaviour 


SEAHORSE 47


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