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Basiliscus last won the day on September 1 2019

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  1. Basiliscus

    Larry's AC50 Circus

    The SailGP Design Team.
  2. Basiliscus

    Larry's AC50 Circus

    The latest wings are a completely new design by SailGP. Different planform shape, new structure, new control system (both in concept and implementation), and made in four segments so they can be varied in size depending on the forecast winds.
  3. Basiliscus

    trickle down

    There's another form of lead that is provided by moving the wand forward. Velocity times the flight-path angle equals the rate of change of height. Pitch attitude is angle of attack plus flight path angle. When making corrections in height, the angle of attack isn't changing very much, so a change in pitch attitude is equivalent to a change in flight path angle. What all this means is feeding back pitch attitude has the same effect as feeding back the rate of change of height, which adds damping to the height loop. Without rate feedback, feeding back just height creates an oscillator from the control system. Rate feedback allows the control system to anticipate. For example, without rate feedback, the system will keep driving toward the set flying height until it crosses it, and then react by correcting back. And back and forth it goes. With rate feedback, if it is approaching the set flying height at a fast clip, it will back off or even apply retarding control before it gets there, because the rate is being subtracted from the height error and that combination goes to zero before reaching the set height. The height of the wand equals the flying height plus the distance of the wand from the c.g. times the pitch attitude. When you move the wand forward, the feedback from the wand becomes a blend of height and pitch. This is exactly what you need to add lead to the control loop. It is like having the wand in the middle of the boat and adding feedback from an attitude gyro, or adding feedback of vertical velocity from an inertial measurement unit. The farther forward the wand, the more sensitive it is to changes in pitch attitude. The faster the boat goes, the more vertical velocity feedback there is (because vertical velocity is pitch times speed), so it nicely gives you more help when you need it more.
  4. Basiliscus

    The new sailing twin skin setup

    That is indeed true when the lift is held constant. At higher speed, more air is flowing past the rig, and to create the same lift one needs to deflect it less. It is always more efficient to move a lot of air a little than to move a little air a lot. The lift is approximately constant because it is dictated by the righting moment available from the hull. Induced drag is also inversely proportional to span squared. It is another reason why tall rigs are more efficient in light winds, besides being able to generate the righting moment to get the windward hull out of the water or being able to extend up to higher true winds.
  5. Basiliscus

    Boats and foils comparison

    What they pulled off was the system that injected a polymer solution that flowed back along the amas to further reduce the skin friction. It was successfully developed and would have doubled the margin of victory. They had all the environmental permits to use it. Why they removed it is an interesting illustration of strategic decision making. When GGYC challenged, they said their boat would be 90 ft on the waterline, 90 ft long, and 2 ft deep. They didn't have to say that it was 2 ft deep, but having said it, they had to live with it. They wanted to have a 3 mm margin. When they measured in, they had zero margin, and the risk was that if they were protested and remeasured later, they could be disqualified for being over. Removing the system saved them 250 kg, and they wanted that weight more than they wanted the Vmg it provided. The riblets stayed. When the boat was sitting in the parking lot in San Francisco, you could run your fingernails across the bottoms of the amas and feel the grooves. They were too fine to see, but could be felt.
  6. Basiliscus

    Boats and foils comparison

    FYI, the last time riblets were used to win the America's Cup wasn't 1987, it was 2010. 3M supplied riblet film that was applied to the amas of 17. These riblets had a much finer pitch than those used on Stars and Stripes because of the higher speed of the trimaran compared to the 12 meter. The reason you don't see riblets being used routinely is because of RRS Part 4: 53. SKIN FRICTION A boat shall not eject or release a substance, such as a polymer, or have specially textured surfaces that could improve the character of the flow of water inside the boundary layer. This rule was waived by the Defender for the 33rd Match for the America's Cup, which allowed BMW Oracle to apply them to the trimaran. There were some misconceptions in the video. At one point they say that laminar flow has higher drag than a turbulent boundary layer, but this is incorrect - a turbulent boundary layer has a lot more drag at the same Reynolds number. And riblets have nothing to do with preventing flow separation. Riblets reduce the turbulent skin friction of attached flow. AIAA-1982-169_Turbulent_boundary_layer_drag_reduction_using_riblets.pdf
  7. Basiliscus

    Boats and foils comparison

    That's a good illustration of the higher velocity in the junctions due to interference effects. The more acute the junction, the stronger the interference.
  8. Basiliscus

    Boats and foils comparison

    Not in the manner you are thinking. Cavitation is due to the water at a given location boiling. It's not like ventilation, which is air that makes its way into separated flow on the foil.. Cavitation is all about the local pressure compared to the vapor pressure of water, so it's not going to spread unless the adjacent areas also have pressure below vapor pressure. A bulb at the root can help prevent cavitation by countering some of the interference between the strut and the foil. The most important thing is you don't want everything to get fat at the same place. You can think of the velocities around the strut being superimposed on the velocities around the foil at the junction. This causes the pressure to be lower at the junction than away from the junction. One way to help avoid this problem is to add a body that has a dumbbell shaped pressure distribution. You put high velocity on the body where the foil/strut has low velocity, and you put low velocity on the body where the foil/strut has high velocity. That way the body partially cancels out the foil/strut velocities and the net interference is not so great. A good example is the fairing that OTUSA used on their AC72 rudders:
  9. Basiliscus

    Boats and foils comparison

    Foil thickness is a major driver of the cavitation speed. Going faster requires thinner foils. I haven't tried to calculate how much volume is required to achieve the necessary weight of foil to meet the c.g. requirement of the rule. But it's quite possible the mass could drive the thickness to get the necessary volume, and this could limit the cavitation speed. A bulb would allow the foils to be thinner and less subject to cavitation.
  10. Basiliscus

    Boats and foils comparison

    My guess is they are hinges on the bottom of the Britannia foils. Moving the hinge axis down means the flap moves aft as it is deflected, like a Fowler flap. This adds area and may open up a slot for high lift at low speed to aid in taking off under marginal conditions. To be effective as fences, they'd need to be at the leading edge and on the upper surface. The Luna Rossa fairings are wider than needed for a hinge, plus there's only one on each side, so that's why I suspect they have actuators in them. Luna Rossa does look to have fences inboard of the bend at the tip.
  11. Basiliscus

    Boats and foils comparison

    My guess is they house actuators of some kind.
  12. Basiliscus

    american tragic, is really the future

    Early in the design of OTUSA's first AC72, they were considering what percentage of the weight should be carried by the foils. Michel Kermarac, the foil hydrodynamicist, put up a chart that had true wind speed along the bottom and foil lifting percentage on the Y axis. It had contours of boatlengths gained or lost in a race. There were two sweet spots. One was foil assisted in moderate winds. The other sweet spot was in the upper right corner and had advantages of as much as 40 boatlengths around the course. Michel was aiming for the first sweet spot. When asked about the second, he said, "That is when the boat is fully flying and the hulls are out of the water." Everyone thought, "Oh, well, we can't go there," because they didn't think there was a way to stabilize the boat in heave. When OTUSA heard of ETNZ experimenting with flying on SL33 cats, they initiated a crash program to experiment with flying an AC45. And crash they did - a lot. But they discovered for themselves what ETNZ had already found out, which was that coupling between leeway and heave provided natural heave stability similar to surface piercing foils. Luna Rossa was also foiling SL33s about that time. OTUSA learned to foil the AC45 and the design of their AC72 was changed to be a full-flying foiler before it was launched. In the interim, ETNZ had launched their AC72 and were actively foiling. OTUSA broke a foil on their first AC72 outing and had to cobble together a set of foils cut from a trimaran centerboard. This put OTUSA well behind ETNZ in gaining experience foiling. OTUSA's first AC72 had the daggerboard located ahead of the forward beam, which was the best position from the standpoint of performance. But it was more difficult to stabilize in pitch and heave. For the design of their second AC72, it was decided to "leapfrog," moving the forward beam forward and shifting the boards to behind the beam. This put the boards closer to the center of gravity and improved the pitch stability. With the second AC72, and a more linear and responsive board rake control system, OTUSA was finally able to master the foiling gybe. Of course, by then ETNZ had mastered the foil-assisted roll tack, which OTUSA learned to do during the regatta. So the short answer to your question is OTUSA recognized early on the performance benefits of foiling, but thought the prohibition on movable control surfaces and sensors would prevent the boat from being stabilized when flying.
  13. Basiliscus

    SailGP 2019

    Cavitation depends a great deal on the details of the specific section shape, its loading, and the 3D interference effects. For example, cavitation occurs earlier on the inside of junctions and elbows of L foils because the flow around the vertical and horizontal panels combines to increase the velocity (and decrease the pressure) there. NACA data are useful for calibrating prediction methods, but they're not that directly relevant to modern designs. Today designers have much better tools (like Xfoil) that allow them to specify pressure distributions that lie just below the cavitation threshold and calculate the shapes that will produce those tailored pressure distributions. Whether or not there's a big change in lift with cavitation depends on how extensive the cavitating region is. Depending on the section shape, cavitation at high speed may begin near the maximum thickness point and gradually spread as the speed or loading increase. Or, if the designer was successful at preventing cavitation earlier, it may set in more suddenly. Basically, when the flow cavitates, the minimum pressure becomes constant - equal to vapor pressure. So it sort of chops off whatever the pressure distribution might have been in the absence of cavitation, and that is the lift loss. It's not hard to design sections to not cavitate and still provide enough thickness for good structural stiffness up to around 40 kt. As you start to design for higher speeds, there's not a lot more that can be done with shaping, and the foils have to get thinner. Above 50 kt, it's really, really hard to make a foil that is thin enough not to cavitate, but still thick enough to carry the loads, especially through the elbow, where the bending moment is high. It takes a lot of close cooperation between the hydrodynamic engineers and the structural engineers because they both have to push things to the absolute limit. There is a substantial drag penalty with cavitation. It's a bit like the wave drag increase around hull speed. It doesn't mean you can't go faster than that speed, but chances are the boat is already pretty close to the thrust it can produce for the righting moment available, so the extra drag becomes pretty limiting with regard to the speed. More righting moment means you can sheet on harder and push through the drag increase, which is what Phillipe Presti was getting at with the rudder foil differential. The current wingsails are all-round designs that have to work in light air and heavy, and were sized by the AC50 Design Rule. It'd be interesting to see what the boats might do if they had a wingsail that was optimized for higher winds. Just like a reefed sail is faster than a bigger sail that has to be flogged to limit the heeling moment.
  14. Basiliscus

    Chase boat - trickle down?

    I think a hydrofoil chase boat would be great for an AC team, especially for the performance chase boat & its crew of engineers watching the telemetry. Something like Harry Larsen's Talaria would be a good choice. There's one big problem, though. There aren't any commercial builders of hydrofoil boats that are willing to sponsor an AC team. The teams are capable of engineering and building their own hydrofoils and control system. But an in-house built chase boat doesn't fit the business model.
  15. Basiliscus

    AC75 vs F50 and Maxis

    They are quite heavy. The Design Rule has a maximum of 1215 kg for each foil, and the center of mass of the foil + arm has to be more than 3.375 m from the cant axis. The crew weight (including guest racer) is 1120 - 1150 kg, so each foil is more than the entire crew.