ORC
Challenging the algorithms
The ORC VPP works pretty well for predicting speeds of displacement and even planing boats when they are monohulls with a set of appendages oriented more or less vertically. The program’s under- standing of the lift and drag produced by the keel and rudder(s) combined with the hull form’s lift and drag forces are the result of decades of study, starting with tank testing and reverting now to using digital tools – particularly extensive CFD studies. However, this year’s adoption of the Offsets 2.0 VPP code now facilitates the defining of any appendage configuration, with ORC mathematicians confident that the contribution to performance of non-vertical appendages such as DSS and the curvilinear horizontal foils found on Imoca 60s and Figaro 3s can also be predicted… these predic- tions capturing the base effects of a vertical force that lifts the boat, and the augmented righting moment. With the Mills-designed Flying Nikka, however, there is now a
completely new issue: how to model performance when the hull lifts completely clear of the water. Davide Battistin is the ORC VPP programmer who has been working on this problem and explains the situation. ‘When modelling performance of normal boats the challenge is identifying the elements of the force balances to find the equilibrium condition. With all the measurements provided of the boat, rig, sails, crew weight and so on, this is fed into the VPP to determine how at each wind speed and angle the forces are balanced, and thus the predicted performance of the boat in these varied conditions. ‘Fully foiling boats have very different physics,’ Battistin explains.
32 SEAHORSE
‘While foiling the hull volume is no longer contributing to the force balance, so we obviously had to rework the VPP to account for this condition while the boat is foiling and at full power.’ A critical component in this modelling is to identify at what boat
speed the amount of lift from the foils will equal the displacement of the boat, the so-called take-off speed. This of course is a concept familiar to America’s Cup sailors, and spectators, as everyone saw those boats lift free of the water, maintain foiling speeds and then crash back down to displacement mode when the boat speed dropped for any reason. In contrast with the AC75s, coastal racing demands that Flying
Nikka also incorporates a deep fixed bulb keel, but otherwise Roberto Lacorte’s flying machine has been designed to behave in exactly the same way as its purist AC counterpart. So the challenge has been in developing an accurate VPP that can account for these two conditions: semi-foiling where some hull is in the water (Battistin suggests that this is at about 50 per cent of the hull volume immersed) and fully foiling where the hull is completely airborne. Yet it’s not a simple switch between these two modes: there
must also be recognition of the transition space in the polars from non-foiling to foiling, and once foiling the full span of wind speeds where foiling can be sustained. And there are even more details to account for, such as when
the wind forces are great enough to upset the equilibrium and depowering is needed. On a displacement-mode monohull it is simple enough to understand this with changes in heel angle, but
FABIO TACCOLA
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