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Basiliscus last won the day on September 1 2019

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About Basiliscus

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    Port Gamble, WA, USA

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  1. I learned to sail by rigging my 16 ft canoe for sail. I put the lateen rig from a Sunflower on it, which is probably about the same size as the Opti sail, or larger. For supporting the mast, I drilled a hole through the fiberglass stern seat and glued a wooden block with a hole in it to the bottom of the boat as a mast step. I chose the stern seat and sailed the canoe backwards because the stern seat was closer to the end of the boat - a lateen rig has the mast well forward of the center of area of the sail - and it allowed me to put my weight closer to the center of the boat. Instead of a daggerboard, I made leeboards. These were bolted to a thwart with vertical ends, making it H shaped. This thwart was clamped with U bolts to two aluminum rails that ran from the stern seat to the center thwart. I was able to move the daggerboards fore and aft to find the location where the balance of the helm was best. I never did make a rudder for it, and just steered with a paddle. In light winds, I could make a J or sweep stroke to maneuver when the boat had little steerage way. The leeboards were made from mahogany and shaped by eye to a cambered airfoil shape with the flat side to the outside. The top of the leeboard had a raked back handle, from which a shock cord ran forward to the stern seat and a pendant with a hook on the end running aft. When the pendant was released, the shock cord held the board up in the retracted position. To put the board down, I reached forward, grabbed the pendant, and hooked it on the center thwart. It was no problem balancing the boat by shifting my weight. However, I did notice that when I had to sit on the gunwale, there was a noticeable twist to the boat! I think something similar would be appropriate for your rowboat. I recommend leeboards instead of a daggerboard. They don't require putting a trunk in the hull, and you wouldn't have to compromise the ability to row it. I think you could put a reinforced hole in your deck with a matching step for the mast. The leeboards could go just ahead of the riggers and retract up under the riggers. Add a transom hung rudder, and you'd be set. Decide on the leeboard position first, then position the mast so the sail and boards have the right amount of lead. Given the freeboard under the riggers, you might need to go with a fairly large span for the boards in order to get the area you need. That is goodness from a hydrodynamic point of view. Since the rowing ability would not be compromised, in shallow water you could retract both boards and row until you got to deep enough water to put the boards down.
  2. Basiliscus

    Boats and foils comparison

    Thickness has the effect of raising the velocities on both surfaces. Lift comes from the difference in the two pressure distributions. So if you make the section thicker, the first thing that happens is the upper surface velocities exceed the cavitation threshold because you're moving both of those curves upward. That drives you to reduce the lift to bring the upper surface back under the threshold. But you need the lift to meet the design requirements. So you need to rearrange the lower surface velocities to get the lift back. You can do that by dropping the velocities on the aft half of the lower surface, which makes the section more aft loaded. You can't reduce the lower surface velocities on the forward half because that will make the section thinner. He describes the pressure distribution in the video as being aft loaded, but I would say the aft loading on that section was quite modest, and the lift was fairly evenly distributed over the chord. Making the velocities lower/pressure higher on the aft underside is what creates the pronounced hollow you see on a lot of sections. If you push things to the limit, what you end up with is both the upper and lower surface velocities come together on the forward portion of the section because you're trying to get as much thickness as possible for the structures guys and both surface velocities are pushed up against the cavitation threshold. This makes the forward part of the section almost symmetrical. All of the lift at max speed is carried by the aft part of the section. The limiting factors on how much aft loading you can stand are both structural and hydrodynamic. The rapid increase in pressure at the beginning of the hollow can lead to separation. The separated flow will probably reattach before the trailing edge, forming a separation bubble that essentially bridges the hollow. It means you're not going to get the lift you need from the underside. Separation limits the slope at which the pressure can increase going into the hollow. If you need more lift and you can't make the drop in velocity greater, then you need to start the drop farther forward. And that eats into the thickness. As the hollow gets more pronounced, the trailing edge gets thinner. Since it's heavily loaded, it becomes harder to make it strong and stiff so it doesn't distort. A thin trailing edge can also be more susceptible to singing at high speed. The aft loading also results in a strong nose-down pitching moment on the foil that increases with the square of the speed. As a result, the foil is strongly loaded in torsion and wants to twist off when going fast. If the torsion loads are not accounted for in the design, the twist will unload the outer part of the foil, transferring the load inboard. Now the inboard stations are carrying more load than intended and will cavitate at a lower speed. So the section design needs to be closely coordinated between the hydrodynamic and structural designers in order to fit within all the constraints.
  3. Basiliscus

    The new sailing twin skin setup

    Not a database, and it doesn't deal directly with the tramp, but you can find some of the motivation for raising the clew here. (Unfortunately, that page was created in the days of 300 baud modems, so you have to click on links to the figures to view them.) You could use a tool like AVL to investigate your own designs.
  4. Basiliscus

    Larry's AC50 Circus

    The SailGP Design Team.
  5. 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.
  6. 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.
  7. 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.
  8. 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.
  9. 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
  10. 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.
  11. 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:
  12. 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.
  13. 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.
  14. Basiliscus

    Boats and foils comparison

    My guess is they house actuators of some kind.
  15. 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.