Basiliscus

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About Basiliscus

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    Port Gamble, WA, USA

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  1. 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.
  2. Basiliscus

    The new sailing twin skin setup

    I found Edmond Bruce's Design for Fast Sailing to be quite a revelation when I read it in the '70's.
  3. Basiliscus

    The new sailing twin skin setup

    I discovered the same thing when I was playing with a simple landyacht VPP. I had arranged it so the angle of attack for maximum aerodynamic L/D was below maximum lift. But I discovered the best performance was with an angle of attack higher than best aero L/D. It took me a while to realize that because the side force on the chassis was dependent on the side force applied from the rig, that the L/D of the rig and the L/D of the chassis were not independent. By loading up the chassis more, it improved the L/D of the chassis and gained performance even at the expense of the aerodynamic L/D. So while you're right that you can't purely optimize the L/D of the rig independently from the boat, it's still the place to start. Drag reduction always yields a performance gain. Increasing the lift may improve performance, but not necessarily. It depends on how much the drag increases, both aerodynamically and hydrodynamically, as the lift is increased. OTUSA sailed their AC72 with a daggerboard lifting pole that acted like a slat, which allowed the windward daggerboard shaft to have attached flow. The aerodynamic thrust from the daggerboard offset most of the drag from the entire windward hull. However, the increased side force increased the drag on the leeward daggerboard, and the slat-shaped lifting pole on the leeward hull had more windage than a round pole. The drag from daggerboard and leeward pole negated the gains from the windward pole and daggerboard, and the aerodynamic lifting pole was scrapped. When adding lift, you really can't say much about the effect on performance unless you have a VPP that considers all the interactions. Dave Hubbard did an interesting analysis when Oracle Racing was sizing the soft sail rig for the trimaran, USA 17. There were no limits on the rig design, so it all came down to the fundamentals. Hubbard showed that there was an optimum rig size for each wind strength. If the rig were too high or the sail area too large, the lift had to be reduced to stay within the righting moment available, and the parasite drag was higher than need be. If the rig were too short, then the induced drag was increased and performance suffered. So there was an optimum area and height for each wind strength. It turned out that the best aspect ratio was remarkably similar across the wind range. It looked like the entire rig was just scaled up and down for different wind strengths. Just what the optimum aspect ratio was depended on the righting moment, windage, and the resistance of the hull. If the double-luff sail is to have an aerodynamic advantage over a rigid wingsail, I believe it's going to be the ability to vary the size according to the wind strength. It's not going to have an edge on maximum lift or control of camber and twist. Of course, the rationale for the double-luff sail may not be aerodynamic at all. It could be all about logistics. Or fashion.
  4. Basiliscus

    The new sailing twin skin setup

    Actually, there is a fundamental relationship between lift/drag ratio and sailing performance. The basic sailing performance equation is: Vb = boat speed Vt = true wind speed gamma = course to true wind (0 = straight upwind) beta = apparent wind angle, measured between apparent wind and course through the water Vb = Vt*sin(gamma - beta)/sin(beta) Vmg = Vt*sin(gamma - beta)*cos(gamma)/sin(beta) This comes directly from applying the law of sines to the wind triangle, and is exact for all sailing craft when the terms are defined as above. It turns out that the apparent wind angle, beta, is the sum of the aerodynamic and hydrodynamic drag angles: lift = force component at right angles to the apparent wind or course through the water drag = force component parallel to the apparent wind or course through the water ea = arctan(aero_drag/aero_lift) eh = arctan(hydro_drag/hydro_lift) beta = ea + eh At any point of sail (gamma), you want to minimize the apparent wind angle (beta), and this leads to maximizing the aero and hydro lift/drag ratios. Maximizing the force comes into it when it improves the hydro lift/drag ratio. For example, a boat that sails dead downwind has a hydrodynamic lift/drag ratio of zero. So heading up to get more side force improves the hydro lift/drag ratio and speeds the boat up. Likewise, operating the rig at angles of attack past maximum aero lift/drag ratio can improve performance when the additional hydrodynamic lift results in a better hydro lift/drag ratio. Very high performance craft (such as landyachts, iceboats, and foilers) approach the ideal of the "constant beta boat", in which the apparent wind angle is constant for all points of sail, and the lift/drag ratios are also constant. If you assume a constant apparent wind angle, you'll find best Vmg occurs near 45 deg and 135 deg to the true wind, and maximum speed near a beam reach. So there are good reasons why we tend to sail at approximately those angles. And it's all about lift/drag ratios.
  5. Basiliscus

    Larry's AC50 Circus

    A hydrofoil can ideally operate in two different modes. One is called "platforming", in which the boat sails at a fixed altitude above the mean water level and the waves pass underneath without disturbing the boat. The other mode is called "contouring", and the boat flies at a fixed distance above the water, following the shape of the wave. In reality, a hydrofoil uses a combination of these two modes because the waves have a mix of frequencies and amplitudes. It lets the shorter, high frequency waves pass by, and it follows the longer, low frequency waves. Just which wavelengths are ignored and which ones are followed depends on the size of the boat and the speed at which it's operating. For the sheltered waters in which the last two America's Cup matches have been held, platforming predominated and the waves weren't a big issue. Hydroptere once turned into a pretzel when it failed to contour properly and stuffed a foil into a wave. So waves can definitely affect a hydrofoil.
  6. Basiliscus

    Larry's AC50 Circus

    Yes. The orbital motion of the wave affects the apparent velocity of the foil, as well as the angle of attack. And, for a surface piercing foil, the wave height affects the submerged area. These all have a significant effect on the lift and the motion of the craft. Flying into the waves is easier than flying with the waves. When flying from trough to crest into the wave, the apparent velocity is increasing and in the middle the angle of attack is increased. These effects help lift the boat up and over the crest just when it needs it most. Approaching the crest from behind, the angle of attack is reduced and then the apparent speed drops off. This tends to make the boat want to dive into the crest. For chop, the encounter frequency is so high that the boat can't respond to the waves and it just flies level, letting the waves pass underneath. But for swells, the boat needs to follow the wave contour. AIAA-62767-951_Hirsch_Seakeeping.pdf
  7. Basiliscus

    trickle down

    Looks like an AC75 surrogate to me. What's it rate?
  8. Basiliscus

    AC75 vs F50 and Maxis

    Ah, now I get it. You aren't measuring static pressure at all. What you really want is the stagnation pressure of the water. And you're right that the leading edge is the place to do that.
  9. Basiliscus

    AC75 vs F50 and Maxis

    The leading edge is the worst possible place to measure static pressure. At high speed, the pressure at the leading edge ranges from stagnation pressure to the vapor pressure of water, depending on changes in the angle of attack and flap deflection. There are ultrasonic sensors that are much better at measuring the distance to the water. The problem isn't determining where the water surface is relative to the platform, rather the issue is what to do with the measurements when you get them.
  10. Basiliscus

    AC75 vs F50 and Maxis

    The flight control sensors don't have to be navigation grade. I would think readily available solid state flight controllers would be adequate. In addition to rate gyros and accelerometers, I would add attitude gyros and GPS velocity as well as the ultrasonic height sensor. I think you're on the right track with regard to combing the sensors. I would use a complementary filter to combine them, with the accelerometers and rate gyros providing the high-frequency feedback and the height sensor providing the low-frequency feedback. The rate of change of height is approximately equal to the speed times the pitch attitude, so using attitude feedback provides damping to the height loop and attitude would be good for the medium frequency band in the complementary filter. The filter would provide not just an estimate of flying height, but also the first and second time derivatives of height without the phase lag that comes from numerical differentiation. The filter would roll off high frequency nose and wash out bias errors in the inertial sensors. I would favor a control law that had nested loops for height acceleration, height rate and flying height.
  11. Basiliscus

    AC75 vs F50 and Maxis

    That's not so easy. The static pressure varies as a function of speed and local angle of attack as well as depth. And don't forget the depth includes wave height. The difference between the local static pressure and freestream static pressure is known as position error, and calibrating it for a foil would be extremely difficult if it was possible at all. Its hard enough to calibrate the position error for a static source on an aircraft fuselage, where the position error is small and doesn't vary as much.
  12. Here are some letters to the New York Times talking about the thrashing the America gave to British yachts. LettersToNYT.pdf
  13. Basiliscus

    AC36 CLASS RULE

    We finally have some numbers on the performance of the FCS. It's to move from 70 deg to 120 deg in 3 seconds - a rate of 17 deg/sec. Call it a peak rate of 20 deg/sec when times for acceleration & deceleration are taken into account.
  14. High-performance boats do not preclude dialups - there was a massive dialup between the trimaran and catamaran at the start of race 1 of the 33rd AC. A dialup is a tactic you use in a windward start. I don't see how it makes a lot of sense for a reaching start, because the boat closer to the line has a huge advantage. You'd get the hook, take them up, and then peel off for the line before coming to a stop in a dialup. Crossing the windward boundary of the starting area gets you both offsetting penalties, so it's not like taking your opponent to the course side of the start line and then ducking back to start before them. The 36th AC will have windward starts, so expect dialups.