COMMENTARY: Paul Cayard is wrong
Published on July 10th, 2013
By Dave Hollom, yacht designer
In an update from Artemis Racing CEO Paul Cayard on July 5, he states that “there never has been a ban on elevators on rudders in the AC72 Class Rule. All AC72’s have rudder elevators because the Class Rule allows them.”
Paul Cayard is wrong.
An elevator is an essentially horizontal control surface whose lift coefficient (Cl), either positive or negative, can be adjusted to alter and maintain the angle of incidence and hence the Cl of the main plane. This can be achieved by either an all moving surface or a surface that incorporates a moveable flap. Either way, it either all moves or has an element that moves which the AC72 rule specifically disallows. Any AC72 sailing with an elevator is therefore illegal.
Nevertheless, to foil successfully you must have some method of adjusting the incidence angle (Cl) of the main plane. The problem is that lift squares with velocity and yet the boat remains (excluding any aerodynamic effects) the same weight at 20kts as it is at 40kts. As the lift, for the same angle of attack, increases fourfold between 20 and 40kts, the incidence angle of the main foil must reduce to a quarter in order that the lift force remains constant, which it must to produce a constant ride height.
If, because of the rules, you cannot adjust the Cl of the mainplane by altering the angle of an elevator you must alter the angle of the mainplane directly by altering the rake of the daggerboard to which it is attached or, alternatively, by using free surface effects to automatically reduce the lift, progressively, as the foil nears the water surface, or a combination of both.
In either case, longitudinal stability is more easily achieved with a horizontal surface, well separated from the main plane, on the rudders. This, under the rule must be fixed and is thus a horizontal stabiliser not an elevator. It does what it says on the tin. It ensures that the back follows the front and that, whatever incidence the main plane is set at, it is approximately maintained.
Altering the angle of the stabiliser before the start merely adjusts the starting point for trimming the main plane. Its relationship to the angle of the mainplane will remain the same at any given speed and weight. However, the trim angle at which the boat naturally wants to run will alter. It will be either more bow up or bow down which I guess is the reason for allowing the angle to be changed before the start.
But, as others have observed, how do you ensure that it is not then subsequently continuously altered during the race and thus becomes an illegal elevator. Also, if some means could be found of ensuring that the stabiliser is not subsequently moved, weather is very fickle. If running bow up is safer, and that is not necessarily so, how can it be safe to set the boat up at a more bow down attitude in lighter airs when, during the course of the race, things could freshen up?
The easiest, safest, and perhaps fastest, solution to the problem is to use a rudder mounted stabiliser that is moveable at all times, i.e. an elevator. But that is illegal. Having mastered the legal, more difficult and perhaps slower solution you can understand why Emirates and Prada are a little less than chuffed when Oracle, who seem to have been unable to master the more difficult but legal approach are offered the easier, but under the original rule, illegal solution in the name of safety.
The purpose of the safety rules may well be to bring safety to the whole fleet and to the event but when they unfairly favour one team to the disadvantage of others then they are wrong. If any team can produce a safe boat under the original rules, then any team that cannot and realises that their boat is not safe and are unable to fix the problem under the existing rules, should withdraw.
This brings us to another of Paul’s points. Paul seems to think that Emirates and Prada would like to see Artemis out of the event and that this is the reason for their objections. I say this in the nicest possible way. Are Artemis, at the moment, in any position to offer a threat such that it is worth trying to prevent them from sailing? I think not.