Pressure vs Flow
A slight detour into high school physics is needed at this point.
Pressure and velocity of a fluid are inversely proportional. Consider your garden hose. If you turn on the tap but keep your finger over the hose end, the flow speed is zero (ie; none is coming out) but the pressure (set by the house’s system pressure) is at a maximum.
When you take your finger off, the flow increases and the pressure drops. With hydraulics, if you want high pressure you have to accept a low flow rate. Conversely, if you want high volume you have to accept a lower pressure.
A yacht’s hydraulic systems need to match the requirement of ‘load vs speed’ to ‘pressure vs volume of flow’.
For a ‘normal’ canting keel maxi yacht, the hydraulic winch systems require high flow rates but work at relatively low loads (around 180200bar), while the keel canting rams operate in the 300bar region, needing much higher pressures to move the highly-loaded keel.
To provide some perspective of the loads on the foiling cats, the board systems on the AC72S were running at around 700bar.
Every demand on the system downstream of the accumulator will result in a reduction of system pressure. A certain minimum pressure is required for some tasks – especially the high-load board lift and lowering – and so the input pressure from the crew needs to be replenished.
The boards may be subject to high side-force with very high frictional resistance of the supporting bearings. Certainly on the AC72S the boards were raked forward to generate excess lift to help raise them against the bearing friction and, conversely, were set to negative incidence (through aft rake) to draw them down.
This adds to the complexity of the manoeuvre since, as soon as the board is at the required extension it needs to be swiftly set to the appropriate cant and rake for the point of sail.
HUMAN EFFORT
America’s Cup races are only 45 minutes long. But for most of it the grinders will be working at over 80 percent of maximum heart rate and at the ragged limit of the aerobic capacities to keep the hydraulic pressure at a level that allows the boards to be operated.
Every blip of the button to trim the board is bite out of the system pressure. A bottom mark rounding, last minute call, two
boards to raise and lower, rake and cant, winches to spin, sails to furl and sheet – then a sprint to the other side – will be a punishing manoeuvre. Doing it three or four times in 45 minutes without any respite up the beat is a killer.
It’s possible that the ETNZ cyclors are able to provide higher power for longer for a given level of exertion – thereby maintaining the hydraulic system pressures for longer, allowing more hydraulic demands to be met in the same timeframe.
This would translate in to an ability to move boards faster against a given load as well as carry out other operations before the line pressure dropped below that minimum required threshold.
My own personal observations of ETNZ in Auckland was that their rate of turn when tacking was much faster and with less speed lost between tacks than anything I’d seen from the other teams in their online footage. This may well be partly down to the fact the boards can be dialled in faster because of the greater hydraulic power the cyclors can provide.
THERE’S AN AMERICA’S CUP TRUISM…
Race 1, leg 1 will tell us all we need to know. Generally the first match sets the tone. While Oracle has muddied the waters by joining in the the Challenger series to better gauge the competition and prepare accordingly, the team that is furthest ahead at the start and continues to improve (and that is key) will win.