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      Abbreviated rules   07/28/2017

      Underdawg did an excellent job of explaining the rules.  Here's the simplified version: Don't insinuate Pedo.  Warning and or timeout for a first offense.  PermaFlick for any subsequent offenses Don't out members.  See above for penalties.  Caveat:  if you have ever used your own real name or personal information here on the forums since, like, ever - it doesn't count and you are fair game. If you see spam posts, report it to the mods.  We do not hang out in every thread 24/7 If you see any of the above, report it to the mods by hitting the Report button in the offending post.   We do not take action for foul language, off-subject content, or abusive behavior unless it escalates to persistent stalking.  There may be times that we might warn someone or flick someone for something particularly egregious.  There is no standard, we will know it when we see it.  If you continually report things that do not fall into rules #1 or 2 above, you may very well get a timeout yourself for annoying the Mods with repeated whining.  Use your best judgement. Warnings, timeouts, suspensions and flicks are arbitrary and capricious.  Deal with it.  Welcome to anarchy.   If you are a newbie, there are unwritten rules to adhere to.  They will be explained to you soon enough.  

Basiliscus

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  1. Thanks. Yes, it was. I was a flight control engineer in a previous life.
  2. I don't understand many of his contentions. He said tacking in the multihulls was slow. The AC45s hit peak turn rates around 40 deg/sec. To put that in perspective, the control surface actuators of a fighter like an F-15 move at 30 deg/sec. They were turning the whole boat faster than just the control surfaces on a fighter. Could an IACC boat turn at 40 deg/sec? I rather doubt it. Yes, the boats did tend to bounce from one side of the course to another. But it was a narrow course. It was scarcely possible to get into a significantly different wind than your opponent, so there was little risk of letting them go their own way. You'd be meeting them again in another 2 - 4 minutes, regardless. On a more wide open course, where a boat can get a lot of separation, then there's a real risk that one's opponents can get into a very different wind and gain big. That's why you have to go with them and cover. So ironically, the small course may have lessened the incentive for close covering and tacking duels. Even so, as the crews got better at sailing their boats - and remember, this was the first regatta with full flying tacking catamarans - the penalty for tacks shrunk and the crews were more inclined to put in an extra tack in the middle of the course to cover their opponents. I believe that had the next Match been sailed in AC50s, especially on a larger course, you'd see more classical match racing tactics employed. He talks about lead in the foils. Why would you put lead in the foils instead of making them from solid steel? I've not run any numbers to see how much volume of steel is required to meet the stated weight of the foils, but stiffness counts for a lot in a foil and to use something soft like lead doesn't make any sense to me. And then he goes on about the cost of the wingsails and having to crane them in and out every day. I'm not convinced the wingsails are that much more expensive than soft sails, if any. A wingsail is more of a capital investment, like a hull, because you can build it once and use it (potentially) for several campaigns. Soft sails are expendable items that are good only for a few races. The sail budget for an AC campaign is in the millions of $. Sure, one sail is cheaper than a wingsail, but you're not going to do an AC campaign with one suit of sails! As for the crane, USA 17, the AC72s and AC45s sat on moorings on a regular basis. Although no team did it, the dollies for the AC45s were designed so the boat could weathervane on land. This is a common technique for parking landyachts. The axles of dollies under the rear wheels are angled to a common point, and the whole boat pivots about that point as if it were on a mooring. So they could have been left outside with the wings up. But the teams found it more convenient to bring them inside each night so they could work on the boats. I was never involved with an IACC campaign, but I think it was pretty common to pull out the stick on those boats, too. No travel lift is going to be able to lift out an AC75 with the foils extended, so cranes will be needed for the AC75s just like it was for the multihulls. So I don't see where the cost savings of the soft rig will come from.
  3. A hydrofoil has two basic approaches to maintaining height. The first is called, "platforming," in which the boat flies at a constant altitude and the waves pass underneath without the boat reacting to the waves. The second is called, "contouring," in which the boat flies a a constant height above the water surface, going up and down with the waves. Platforming is used for short wavelengths - chop. Contouring is used for long wavelengths - swells. In practice, the boat will use a combination of both modes of operation. The frequency range for each mode depends on how responsive the boat is, how much clearance it has, how rough you want the ride to be, and how much power is available to the control system. In practice, the boat will employ a combination of both modes. An automatic control system would use a combination of sensors and nested feedback loops. The innermost loop might be based on vertical acceleration feedback. This would give the boat a lot of apparent inertia to keep it flying at a steady height. The acceleration loop might be driven by a pitch attitude loop that also includes some pitch rate feedback. The pitch attitude is the sum of the flightpath angle and the angle of attack. The rate of change of flying height is equal to the flightpath angle times the speed, and at constant speed the angle of attack is approximately constant. So pitch attitude tracks flight path angle, and pitch attitude feedback is proportional to the rate of change of flying height. This means pitch attitude feedback provides damping to an outer height control loop, and it makes sense to employ a pitch attitude loop inside a height loop and around the acceleration loop. So a candidate control law architecture would have a flying height command compared with a height measurement to get the error in the flying height. The height error would be multiplied by a gain to create a pitch attitude command. The pitch attitude command would be compared with a pitch attitude measurement to get the pitch attitude error. This would be multiplied by a gain to generate a vertical acceleration command. The vertical acceleration command would be compared to the accelerometer reading to get the acceleration error. The acceleration error would be multiplied by a gain to drive the flaps on the main foils. The pitch attitude error might also be multiplied by a gain to drive the flaps on the stern foils for more rapid tracking of the pitch attitude. The flying height feedback would be low-pass filtered to keep the boat from reacting to the chop. The accelerometer feedback could be high-pass filtered to remove the gravitational component and sensor biases. That would be the classical way of designing the control laws. The modern way of designing them would be to linearize a simulation model, throw the feedback signals and controls into a linear quadratic regulator algorithm, and use the resulting gain matrices to drive all the controls based on all the sensors. It may well provide tighter control, but you'd be harder pressed to figure out what it was doing and how to tune it.
  4. shroud fairings??

    One of the rule requirements in recent Matches has been that the rigging can have a fineness ratio no greater than 3:1. It's hard to get a fairing that meets that requirement to be stable in rotation about the stay. With a 3:1 fineness ratio, you're not going to have attached flow at any appreciable angle of attack, especially at the low Reynolds number of a shroud. That drives you to want to make a rotating fairing. What you need for stability in a rotating fairing is for the center of gravity of the fairing to be ahead of the rotation axis, and for the aerodynamic center to be aft of the axis. And you don't want the thickness to be any greater than necessary to enclose the stay, because thickness is the biggest determinant of the drag. An alternative approach is to accept that the flow is going to separate shortly aft of the maximum thickness point and minimize the width of the separated wake. If you view the shroud along the apparent wind vector, what you want to do is to minimize the thickness of the shroud in that view. Of course, the boat has to sail on both tacks, so for a fixed fairing you need to also view the shroud along the apparent wind vector of the opposite tack. The intersection of the width in those two views leads to a diamond shaped area in cross section. You want the shroud to fill that diamond as much as possible so as to pack the most material into the thinnest shape with respect to the apparent wind direction. Rounding off the corners doesn't sacrifice much cross sectional area and makes for a practical shroud section. That's why a 2:1 ellipse makes for a reasonable shroud design. You can do better, but not dramatically better.
  5. Sheesh. Nobody's really asked the truly important question, yet. What's it rate?
  6. If the section shape is left for the teams to define, there's no point in trying to limit the angle of attack because the section shape can be designed to produce the desired down force even with a positive incidence on the foil. I've no idea what the optimum angle would be for the arms - that's another VPP question. Say both foils were angled down at the same angle as the lee foil in the normal sailing mode. Positive lift on the leeward foil would have a vertical component to support the boat and a horizontal component that provides side force to counter the rig. Negative lift on the windward foil would have a vertical component that provides righting moment and a horizontal component that also helps to counter the rig. Because of the down load on the windward foil, the leeward foil has to support both the weight of the boat and the down load from the windward foil, so it needs more of a vertical component than it did when the windward foil was retracted. It may be that the best position for the two foils would be a little lower than shown for the normal sailing mode, but higher than shown for the stable mode. If the foils were positioned flat as shown in the stable mode, then the side force needs to come from leeway acting on the struts (assuming there are no flaps on the curved struts). The spanwise load distribution on the horizontal foils will have a discontinuity at the junction. They will need to have independently articulated flaps on either side of the strut to minimize the drag. I would expect there to be port and starboard flaps on each foil regardless of how they operate, because of the interference effects of the strut. There may also be flaps on the strut, and it would make sense for the strut to have a sharper elbow than is shown on the concept sketches so there could be a longer straight segment for the flap. It could also make sense to put some angle into the cant axis instead of having it straight fore-aft. This might be used to toe the foils in or out when fully down. It's not obvious to me whether you'd want all the side force to come evenly from both foils or to be shifted to either the leeward or windward foil. Since the leeward foil literally does the heavy lifting, it might be desirable to have the windward foil generate the side force so the leeward foil can be optimized for vertical lift with minimum drag.
  7. A VPP solves for the equilibrium forces and moments, so you need aero and hydro force models and a means of iterating to find the solution. You can do it all in a spreadsheet, using Excel's Solver add-in. The force models are typically in the form of look-up tables. The real grunt work is in generating the look-up tables. You can use any aero/hydro method you want. For conceptual design, you might use a vortex lattice method like AVL, or even a simple lifting line method, plus 2D section data from XFOIL or MSES. If you don't have MSES, you could use Javafoil for multielement wingsail sections. For more fidelity, you can use a panel code. A panel code is the simplest method that can show the areas susceptible to cavitation on a 3D foil. For higher fidelity work you'd go to Reynolds averaged Navier Stokes (RANS) CFD codes. The kicker is the number of independent variables. For a wingsail, you're going to need angle of attack (apparent wind angle - mast rotation), flap deflection, twist at a minimum. For a foil, you're going to need cant, rake, leeway, flying height at a minimum. Every independent variable you add multiplies the number of points in the table by the number of values for that independent variable. It's easy for the tables to grow to many thousands of points. You'll assign fixed values for some of the independent variables. For example, you may fix the flying height and heel angle. On a wingsail, whether you'd fix the flap deflection or twist would depend on how you intend to trim the sail. Foil cant would be fixed. You'd also specify the true wind angle if you were trying to generate a polar diagram. Some variables you'd use as controls to let the VPP vary to find the equilibrium, such as mast rotation or wingsail twist (depending on your approach for trimming the wing), foil rake, rudder angle, leeway, pitch attitude. Then chose some variable to maximize - typically based on boat speed. You'd leave true wind angle free if you were trying to maximize Vmg. Finally, you'd write the equations for summing all of the forces and moments in each of the six degrees of freedom. When you add up all the forces for one degree of freedom, the result will not necessarily add to zero. This is called the residual. To find the equilibrium a common method is to create a cost function as the weighted sum of the squares of the residuals. Then you let your solver loose to minimize the cost function. Ideally, all of the residuals will be driven to zero, but numerical errors will probably leave you with a small value. If you want to build a dynamic simulation, you need twice as many force and moment tables, because you now have to include the forces and moments due to angular rates. These were assumed to be zero for the static equilibrium used in a performance analysis. You also have more independent variables. For example, with the boat flying straight and level, the angle of attack of the foils is only due to the rake of the foil and the pitch attitude of the boat. But in a dynamic situation, the boat can be moving vertically, which changes the angle of attack. Rolling of the boat changes the angle of attack differently for the port and starboard foils. So the force models for a dynamic simulation have to be built with the appropriate assumptions in mind. Instead of solving for equilibrium, in a dynamic simulation what were the residual forces and moments become the accelerations used in the equations of motion. The equations of motion are integrated to time-step the simulation. If you poke around the net, you can probably find a Moth VPP to use as an example. Here's a bare-bones landyacht VPP that I wrote eons ago. It's very simple, but it has force models, solves for the equilibrium, and steps through the true wind angles to build a polar.
  8. No, with the foil out of the water the only RM comes from the weight of the foil. With the foil in the water, the RM comes from negative lift on the foil. The latter can be far greater than the former. After all, the foil is capable of creating enough lift to equal the weight of the entire boat. What you could end up with is the lee foil operating at maximum positive lift and sails will be cranked on as hard as possible all the time. The heeling moment will then be balanced by negative lift on the windward foil. So it could be the windward foil that gets actively controlled, rather than the leeward foil. The one advantage that does accrue to holding the foil above the water is the saving of hydrodynamic drag. So the question becomes, "Does the extra righting moment improve the performance of the boat more than the associated drag degrades it?" That's a question for the VPP. I think there's a good chance the answer is, "Yes." Only if the answer is, "No," would it make sense to hold the foil above the water. Holding the foil in the raised position has some aerodynamic drag associated with it, too. The apparent wind angles will be on the order of 20 deg, and there's no way the airflow will be attached on the lee side of the T foil at that angle of attack. And any lift produced by the raised foil has to be countered by side force on the other foil, resulting in more drag. Perhaps Mr. Bernasconi would like to share the results of VPP studies so we can appreciate the full brilliance of the concept?
  9. It was only legal then because SNG had waived the parts of the racing rules that prohibited stored energy. Had they sailed under the normal racing rules, the engines would not have been legal. The trimaran was originally designed, built, and tested using manual power alone. It was only after Alinghi opted for powered controls that BMW Oracle ripped out the cockpit of the trimaran and installed a BMW turbo diesel to power the hydraulics. Keep your eye on the racing rules for this edition to see what will be allowed.
  10. The problem with conventional mainsails is the high leech tension that is required to control twist. This leads to the need for high mechanical advantage, which requires time and energy to crank on for every maneuver. One of the big advantages of wingsails is the torsion load is taken internally, which leads to light loads on the sheet and the ability to trim the wing rapidly with minimal effort. I think a wingmast wishbone rig, ala Team Philips, is a good choice. The leech tension can be maintained by a vang to the wishbone, so the sheet loads would be comparable to a wingsail. It's reefable and different sized mainsails could be used for different conditions, with a big roached flathead for light winds and a more tapered sail for heavy winds. (From Pete Goss gallery)
  11. Depending on the definition of "monohull", it might take something like that. This Moth was judged to be a multihull because it pierced the water in 3 places: Being a former M16 owner, I think a 75 ft scow would be awesome. Think 2X A-scow. With foils! Call it Lawson's Revenge.
  12. I don't remember the photo to which you refer. However, I have observed the dust trails from landyachts, and they are a real revelation. You can see the trail crabbing sideways as it drifts with the true wind. At the same time, the yacht is laying down new dust along its course. The combination of the yacht's speed and the drifting means the dust trail is aligned with the apparent wind vector. From onboard, the dust trail simply trails back along the apparent wind.
  13. No, I believe they followed the rule. I'd rather see a different rule. I don't think salt water and electrons play well together. Trickle-down technology from the AC has a tremendous influence on all of sailing. I'd like to see the development of hydromechanical technology that would be applicable to other forms of sailing.