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Monday, October 8, 2012

Rule Driven

I prepared this diagram for the last installment of the series on foils to be published in Australian Sailing + Yachting Magazine. It illustrates a great example of how rule spaces drive solutions that may differ to address the same problem.

This a seminal time in the development of dynamically stable foiling systems.
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.

Given that constant radius ‘C’ foils on multihulls cannot be stable in ride height because their dihedral angle increases with altitude, the race is on to invent a multihull foil configuration that is stable, fits inside conventional beam dimensions, and works without adding complexity.

One solution will be unveiled on our new A Class currently in the final stages ofconstruction. Another much discussed solution has been tested recently and spectacularly on the first ETNZ AC72.


Returning to the initial point about the effects of rule peculiarities, the AC72 rule seeks to restrict the major speed producing factors to keep racing close.
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.
By restricting beam and displacement, righting moment is fixed, equalising the loads that determine the sizing of stays, fittings and sail handling gear.
When formulating the rule, one concern was the possible use of the windward foil to generate downward lift to augment righting moment. 
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. 
The argument was that, with variations in pitch and heel, scrutineers could never be certain that a foil did not, at some time, generate some downward lift. 
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). 
I therefore voted (in my capacity as a Challenger representative at the time), for mandating that the windward board be raised during straight line sailing. 
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.

Closed Door

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.
Since vertical lift is a component of foil force and is therefore tied to sideforce by dihedral angle, if sideforce is below a critical value, foil force cannot exceed boat mass without adding sideforce artificially.
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

So any foiling solution for an AC72 must rely on surfaces at or near the leeward hull. 
The windward rudder can contribute but stability should remain positive when it clears the water at moderate heel angles.


The ETNZ solution appears at first glance to use an ‘L’ foil with the vertical part pulling sideways and the horizontal part lifting upward. This is indeed the case in upwind mode where boatspeed is still in the foil assisted range and speed control is relatively easy, so stability in ride height is not an issue.

However the tight inflected bend in the top part of the foils makes them cant inward when partially retracted. 
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.
Pitch attitude is again provided by rudder T foil control surfaces.

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.


  1. A very clear explanation. Thank you.

    You seem however to neglect a critical aspect of these foils: drag. The vertical portion and the horizontal portion interact when the foil is partially retracted, due to what is effectively interference drag, amplified by proximity to the free surface. The magnitude of this drag can be seen from photos of ETNZ in flight, published on their blog, where a wake large enough to surf upon is noted coming off the foil tip to windward, and a huge valley in the surface of the water is noted above the foil itself, which is a logical consequence of the water from the vertical and tip being deflected away from each other. On a boat this large, the drag may be enough to preclude foiling to windward as efficiently as a displacement boat.

    Essentially this rule forces completely bastardized approaches to foiling which are, in effect, silly. One hopes the compromise in performance will not be enough to allow a non-foiling boat to win the competition and thereby establish yet another reason for people to say that lift > displacement foiling does not work well on course racing multihulls, or that it cannot work well on all points of sail.

  2. You are absolutely right. In fact I make the point in the 'A Cat State of the Art' series ( and in recent Australian Sailing articles on foiling that foil drag is most probably greater than hull drag on long slender hulls at most relevant speeds for course racing multihulls with beam and other restrictions. Foil drag being understood as the additional drag incurred as a consequence of producing vertical lift.

    The crux is whether such extra drag comprised of lift induced drag, parasitic drag and secondary effects such as surface losses is less than the saving in hull drag...
    I suspect that is precisely why ETNZ uses the inflected top to make the foil upright at full extension and transition to foil assisted mode when sailing upwind in moderate conditions.
    The option is available to transition to foiling and 'foot' for tactical reasons but best VMG is probably with the hull in the water.

    Similarly the A Cat has a transition speed below which foiling is more draggy than using the slender leeward hull first in displacement then in foil assisted mode.
    My view is that every rule forces its own peculiar tradeoffs and this is not necessarily a bad thing, it is an inherent feature of the structure and framework necessary to focus development.
    Yes, the AC72 solutions will be 'bastardised' in their own way and the A Cat solutions will be pushed in a different direction, just like the Moth was forced into T foils instead of sticking with the original tripod arrangement.
    The aim and the challenge is to get around the course as quickly as possible in as wide a range of conditions as possible within an agreed set of restrictions. Restrictions that were decided without envisaging all possibilities that may arise over time. Clever rule interpretation does the rest.

  3. I take your point on rules being arbitrary constraints. I also looked at your previous posts and enjoyed them; thank you for taking the time to explain things as they relate to the A-cat.

    For the AC72, I heard early on that 18 knots was the magic number; above that and a foil wins the drag contest. Since boats were projected to move that quickly (or faster) on all points of sail, foils seemed essential. But I am not so certain; this V-foil configuration might raise that crossover speed number substantially.

    I think some clever rule interpretation and a clever approach to managing loads could easily permit a wand-driven articulating foil within the AC72 rule. It surprises me that no one appears to have gone this route, but I imagine they have written off the loads as too large to manage, instead of just finding efficient ways to manage them.

  4. Angle of Attack.
    The angle between the foil and the oncoming flow:

  5. Team NZ is very clever.Good on them. PC