Posted February 24 by Kimball Livingston
So the boys at Oracle Racing built themselves the fastest AC72 in the galaxy, and they learned how to sail it, just in time. Otherwise we wouldn’t be speculating about another Cup-n-SF vs. San Diego, Long Beach, Newport, Hawaii.
Two weeks before the 2013 match, the Oracle crew was struggling to achieve consistent, replicable settings for daggerboard rake.
Or not happening.
Tiny adjustments were critical. When you’re pushing foils through the water at near-freeway speeds—water being rather more resistant than air—Preferred Angle of Attack is significantly different from Preferred Angle of Attack Plus or Minus a Freckle.
The range of adjustment for the Oracle angle of attack, wing designer Tom Speer tells us, was “about six degrees.”
Which I would take to be an accurate ballpark that he knows any of his competitors could quickly assess, and not so accurate as to be selling out state secrets. But within those sixish degrees, guesswork was unacceptable.
“The problem with a manually-powered hydraulic system,” Speer says, “is that you never know what level of pressure you’re supplying to the actuators. Any normal hydraulic system will have an engine-driven pump and an accumulator that maintains a steady supply of pressure to the valves running the actuators.” In that framework, you can build an architecture of predictable inputs and predictable outcomes. But. “We weren’t allowed engines or accumulators because stored energy was not allowed. As the guys were cranking, they were building pressure, but as soon as you moved something, pressure would drop. The pressure in the system varied according to what was going on—how hard the guys were pumping and whether or not the boards and wing were being trimmed.
“You might open a valve a given amount,” Speer says, “and sometimes in response the actuator would move quickly; sometimes it would move slowly.”
I will be so bold as to say for Tom, inconsistency is a killing force.
I will also report that, not being an engineer, I don’t fully get this quote: “When we mounted the valves on the board trunk, that implicitly provided a mechanical feedback so that now the valve would cut itself off. With low pressure, it would move slowly but stop at its given place. With higher pressure, you got a faster response and the same, predictable stop.” Speer has since expanded upon this, and you will find his words at the bottom of this piece.
So, Jimmy, here’s your new world: You want an attitude change of half a degree? On each of your wheels (mounted at an angle because it was just so hard to cram stuff in), you have two buttons. Press one, and you get a daggerboard rake adjustment of half a degree. Press it three times and you get 1.5 degrees.
Speer again: “Making that change helped us catch up on our maneuvers.”
I dare you to accuse the man of insufficient understatement, remembering that Oracle Racing’s USA 17 #2, reimagined in-build after the #1 Oracle boat pitchpoled in spectacular fashion on October 16, 2012, sailed into the 2013 match still behind the curve.
Before we leave the subject of hydraulics, however, let’s detour through one sideways detail. “Our wing self-tacked,” Speer says. “Air loads would make the wing fold through to the new tack. New Zealand’s wing had to be cranked through hydraulically. In race eight we surprised them—they expected to be able to cross on port because they had ‘always’ been able to cross—but they couldn’t cross and they had to tack quick. They weren’t ready. They didn’t have pressure ready. But their guys did have the a-m-a-z-i-n-g presence of mind to keep cranking, and keep cranking.”
And keep cranking, kinda like those legendary Edwards test pilots reeling off airspeed numbers after the wings fell off . . .
If you’re reading this, you’re 99.9 percent likely to already know that Oracle skipper Jimmy Spithill spent part of almost every press conference talking large about overnight modifications to the boat. So was that reality-based, or was it theater?
Speer says, “Crew work was the biggest difference.
“We didn’t change our daggerfoils at all, but we were experiencing cavitation between the rudders and the rudder wings. It was enough to strip the paint in every race, and we had to repaint them every time we went sailing. Then the team came up with a sort of Coke bottle-shaped bearing that went into the junction. That shape worked because it has a high velocity on the fairing where the foils had low velocity. Adding two factors together, it sort of smoothed out the velocity at the junction and gave us maybe 30 kilograms of drag reduction.”
A – A – A – And that’s all, folks. That was the comeback.
The future. Well, Yosemite is looking as if it could blow, big. There’s no good news in the weather for the USA, whether you’re freezing east and north, and sometimes down south, or parched out west. And cable news is not going away. We forgive you, Ted Turner. And an AC35 is as close to a cosmic inevitability as anything in sailing.
We’re told to expect a new protocol, cum design rule, before the end of March, but probably not with a venue announced, which makes it a leetle bit harder, eh, to set designers in motion? Average wind. Sea state. Mere details.
Yes, there is movement toward renewing the AC45 circuit. Yes, it would make sense to go foiling on said circuit, but that would add cost at what crossover in value and . . .
Speer says, “We foiled 45s for test purposes, but it took 20 knots to get them foiling. To make it work in light air, you would have to make them wider, with bigger wings.”
And a good engineer will leave it at that.
Returning to the matter of controlling daggerboard rake
I have this from Tom, in response to my question about OTUSA’s adjustments to their system. Take it away, Tom Speer—
“The essence of a feedback control system is the system works on the difference between what you want and what actually exists. This difference in control engineering parlance is called the error signal. It can be anything, as long as there is this difference operation is going on. The difference can be electrical, as when an electronic sensor provides a measurement and the subtraction is done in the control computer. But you can also set it up so the difference is done mechanically. This is what OTUSA did.
“The desired board position was provided by a small electromechanical actuator. This could be something like the servo you’d use to move a control surface in a model airplane. Judging from the photos, I’d say the actuator used in the board control system was essentially an electrically driven screw. The important thing is it had a rod that went in and out in response to the buttons on the steering wheels or buttons that the crew could control. Each push of a button moved the rod a given amount. A certain number of inches of rod movement corresponded to a certain number of degrees of board rake. Control engineers would call this the command signal.
“The next element in the control system was the valve. This was a three-way valve – it had positions for positive board movement, no board movement, or negative board movement. The important thing is whether the valve is open or closed depends on the position of the internal valve element relative to the valve body. If you have the valve body fixed and you move the valve lever, you turn the flow on or off (or reverse the direction of flow). But it would work just the same if you nailed the valve lever down and moved the valve body back and forth. So it’s really the difference between the position of the lever and the position of the valve that counts. If you wave the valve in the air, both the body and the lever move the same and there’s no difference between them, so there’s no flow through the valve.
“The last element of the control system was the board rake actuator. Flow from the valve moved a piston in the actuator. The valve could send the fluid to the front side of the piston or the back side of the piston in order to move it in either direction. When the valve was closed, no fluid could enter or leave either side of the piston and it was hydraulically locked in place. When the valve was open, there was a flow rate proportional to the supply pressure.. When there’s a constant flow rate, the board actuator moves at a constant rate, just like blowing up a balloon.
“Of course, a common way to use the valve is to bolt the body to fixed structure and just move the lever. In this case, the position of the electromechanical actuator would correspond to the rate of change of board rake because movement of the rod would change the lever and that would control the flow rate through the valve. The difference represented by the valve position would be the difference between the lever position and zero, because the valve body was fixed. To move the board, you have to open the valve for a time, letting fluid flow to the actuator, and then close the valve. So the electromechanical actuator controlling the lever has to move one way to open the valve, and then move the opposite way to close the valve. Two movements – on, then off. If the supply pressure was constant, you could use a fixed time interval between the two actions to move the board a fixed increment. But this doesn’t work when the supply pressure is variable, because the flow rate will vary and you will get a different response for the same time interval. You might measure the supply pressure and adjust the timing accordingly, but that was against the rules.
“The way OTUSA solved the problem was to take advantage of the fact that the flow rate really depended on the difference between where the lever was located in space and where the valve body was located in space. When something depends on a difference, you can exploit it for feedback control. You just need to make one side of the difference correspond to what you want, and the other side of the difference correspond to what you have. OTUSA made the location of the valve body correspond to board rake by mounting on the board trunk. They made the lever position correspond to the rake they wanted by anchoring the small electromechanical actuator to the hull structure. Now the end of the rod from the electromechanical actuator corresponded to the desired rake of the board. Rod forward to rake forward, rod back to rake back. When they connected the rod to the valve lever, the feedback loop was closed.
“When the rod moved, the valve opened, just like it did when the valve was bolted to the hull structure. But now the valve body moved with the board instead of staying fixed to the hull. When the body and the lever were back in alignment, the flow shut off. Only one movement of the electromechanical actuator was needed, and the position of the small actuator was proportional to the desired position of the board. The movement of the board itself turned the flow off when the desired rake was obtained, as indicated by the body matching the lever location. I called this an implicit feedback, because there was no explicit sensor that you could point to that measured board position. It was the valve itself that did the measuring.
“Locating the valve on a moving thing is a very common control technique. It’s used in all kinds of hydraulic actuators. It’s quite common for the valve to be mounted directly on the hydraulic cylinder itself, with the rod end anchored to fixed structure and the cylinder end attached to the moving thing. Now a linkage from the fixed structure to the valve provides the desired position and the valve moving with the cylinder measures the actual position, in the same I’ve described above.”
Thanks, Tom, and now we really will leave it at that—Kimball
This article was syndicated from BLUE PLANET TIMES