UNDER THE PUMP
The energy equation
The AC50 foiling catamaran is predominantly one-design – the wing sail, platform and hulls are all identical to prescribed geometries, leaving the deck layouts, systems and foil designs as the principal areas for exploitation by the individual design teams.
The adoption of one-design elements is because the hulls are airborne most of the time. In fact, with the advent of flying tacks and gybes, the hulls are almost certainly capable of remaining dry from start gun to finish – so if they aren’t in the water, why sweat the details?
The adoption of the standard wing geometry represents a freezing of technology to limit design costs, allowing the microscope to be turned on the foils and their control systems that have a fundamentally more significant impact on performance.
So it’s about maximising the knots-per-dollar ratio. But even then limits have been placed on the foils and the controls systems. Only two sets of boards are allowed, and all teams have opted for what are broadly described as a ‘light airs set’ and a ‘heavy airs set’.
The forces generated by the foils – be that lift (righting moment) or side-force (reducing leeway) – vary with the square of boat speed: if you double the speed you get four times the force. Higher
windspeeds produce higher boat speeds and the boards can generate higher forces, so the designers can utilise a smaller, skinnier, less ‘draggy’ foil.
The ‘light airs’ and ‘heavy airs’ are defined by the Protocol under Rule 32.1 which sets the wind-speed limits for racing between 6kn and 25kn measured at 5.5m above the water. Teams may position their design anywhere along that spectrum.
Modifications to the foils after measurement are limited to 30 percent of the board by weight under Protocol Rule 35.10 (d) (i) before they are counted as a ‘new’ board. So some on-going refinement is possible, but not wholesale change of design or construction.
Since the dagger foils and the rudders are the only parts in contact with the water for the majority of the race, the precise control of those at high speeds and through high-g manoeuvres is a critical aspect of the design.
Most of the moves require a complex choreography of board rake and cant on the leeward (loaded) board going into a tack or gybe, lowering the ‘new’ board and retracting the ‘old’ one, all the while ensuring a seamless transfer of forces from one side to the other to maintain level flight, stable pitch and constant ride height.
If the use of PLCS (Programmable Logic Controllers) wasn’t prohibited it would be a simple matter of hitting the ‘tack now, Skipper’ button and calling ‘right turn, Clyde’ as the helm goes over. The boards would follow a pre-ordained sequence of cant, rake lift and lower, rake, cant, taking input from multiple gyros that would sense the rate of turn, True Wind Angle, pitch and heave to adjust the foils in milliseconds, leaving the crew sailing out bound from the tack, smiling nonchalantly at the photographer in the media RIB.
Reality is a little different as all inputs for board controls must be manual. This puts great emphasis on the hydraulics and their control systems. The hydraulics for the sailing systems on-board are powered by the crew grinding – or pedalling, in the case of ETNZ’S self-named cyclors. Rule 16 of the AC50 Class
“Grinders work at over 80 percent of maximum heart rate”
Pedal power?
While this article isn’t about the merits/demerits of grinding vs pedalling, the topic is relevant in terms of ‘input power’ to the hydraulic system. The last Cup saw much analysis of grinding output, with figures varying from 1.2 to 1.5kw as peak output, and 0.9 to 1.1kw sustained for 20 or 30 seconds. But this is for the top performers – the ‘normal’ grinder was at around 1.0kw peak and 0.6kw sustained. A dip into the world of road cycling reveals a mountain of data about power outputs, watts per kg of bodyweight, and a plethora of other metrics. No doubt ETNZ has relied on more than a Google search for its decision to adopt pedalling rather than grinding. From available Tour de France data, peak outputs of 0.9kw are recorded, but those guys are 70kg whippets. Biomechanically, compared to arms, legs are a bigger muscle group, with longer limbs, operating in a natural rotational motion at hip and knee, with momentum to help carry over top-dead-centre. Shoulders and elbows do not lend themselves to the plane of rotation required. Furthermore, the upper body muscles used for grinding are involved in breathing, so cyclors can breathe more effectively as well as use their hands for other tasks (navigation, board control and so on). It would explain why hand-cycling performance vehicles cannot outstrip leg-powered bicycles. There are many variables to consider, but another significant one is cadence (rate of turning the handles/ pedals) and torque being matched to the characteristics of the hydraulic pumps through gearing. The cadence of a leg-driven system, when geared to suit the rider’s torque output, could conceivably offer higher outputs.
Rules places limits on stored power to the extent that effectively, none is permitted.
Class Rule 16.2 (b) states that no more than 0.25 litres of hydraulic oil shall drain from the system when locked off and de-pressurised from full pressure. This is to prevent the expansion of the hydraulic hoses or the installation of longer than necessary hose runs – in an attempt to store power under pressure.
ACCUMULATORS
Accumulators are permitted under rule 16.2 (d) and (e) – one for the board rake and cant, and two for the lift-and-lower system respectively – one set per hull. However, despite their 4.8 litres maximum capacity, they are also subject to 16.2(b) and so cannot be pressurised to store power. As such they are more of a reservoir than an accumulator. Sitting between the input of the grinders and the output of the rams, their real role is to smooth out the peaks and troughs of the pressure in the hydraulic lines as a result of the grinders/cyclors efforts, delivering a consistent line pressure that can be harnessed for moving the rams attached to boards.
The Class Rule specifies publicly-available hydraulic equipment from a list of suppliers – so there’s no developing custom pumps – and the equipment determines the maximum pressure the system can safely handle.
In that sense there is a level playing field. But how you generate, distribute and replenish that pressure is the trick. B