This a seminal time in the development of dynamically stable foiling systems for racing.
Since the limits of foil assisted sailing are now regularly being reached, their inherent limitation (inability for lift to exceed displacement) must be overcome to unlock greater performance.
Length, beam, displacement and sail area (wing height, girths and sail corner points) are controlled so that the key ratios are precisely similar between boats.
It was deemed desirable to discourage an arms race in the pursuit of additional righting moment. However no satisfactory wording could be agreed that would have the desired effect without requiring physical measurements of actual foil force.
My thoughts at the time turned to past experiences such as the ‘Hula’ of TNZ and the double rod rigging loophole in the 2003 AC cycle (in both cases proving that two elements did not touch when in use opened a proverbial can of worms).
This in turn required definitions for circumstances such as tacking and jibing, but such provisions would be reflected in the racing rules, and seemed relatively simple to enforce on the water.
An unintended consequence of this rule is that, since the windward foil is not in the water, it cannot be set to increase sideforce. This counter intuitive arrangement is used by ‘tripod’ foilers such as the Hydroptere: by setting the windward foil with ‘toe-in’ such that it pulls up and to leeward, sideforce is increased. This forces the leeward board to produce a greater hydrodynamic reaction force compared to what it would when just working against sail force.
If vertical lift cannot be made to exceed the value given by the component of sideforce determined by dihedral angle, a single angled or curved foil cannot support the mass of the boat except at very high speed and sideforce values.
|Forces on Hydroptere tripod configuration when sideforce is small – so vertical component is less than displacement|
|And at high speed/sail force/sideforce. Both possibilities are precluded in the AC72 Rule|
The windward rudder can contribute but stability should remain positive when it clears the water at moderate heel angles.
Partial retraction is desirable for downwind sailing because speeds are higher and sideforce is a smaller component of total sail force so less lateral foil area is required to limit leeway.
The key is that, as the foils ‘kick in’, the horizontal portion of the L rotates such that the tip (inboard) is higher than the root (outboard).
As ride height increases and vertical foil area diminishes due to the top portion clearing the surface, initially leeway increases.
This reduces the AoA on the kicked up L tip, reducing lift and allowing ride height to settle.
Elegant and effective if specialised.
The stable regime is narrow but can be tweaked by adjusting foil rake and dihedral.
Rake is controlled by displacing the top bearing forward / aft, and dihedral changes with retraction thanks to the inflected top portion.
More to Come
Look out in future for sophisticated interpretations of the rule that permits only a single axis of rotation as teams explore adjustability to extend the stable regime.