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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. Oracle Team USA

    A single soft sail costs less than a wingsail. But race boats don't use a single soft sail. Soft sails are expendable items, good for only a few races (not to mention all the sails made for testing), while wingsails are more like capital investments like hulls. An AC soft sail budget is at least as much a wingsail. A wingsail is good for the entire campaign, and probably several campaigns if the Design Rule is stable. I'd like to see your cost data for typical wingsail repairs compared to the replacement costs of sails - I doubt you'd see a patched sail used in an AC race. As for the crane, teams need a crane to step and unstep a mast, so it might as well be used to launch the boat, too, and save the cost of a travel lift. When wingsails were selected for the AC72s, there was a lot of concern about the cost of fragile wingsails compared to soft sails. But in practice, the wingsails have proven to be remarkably robust and I'm not convinced campaigning a wingsail is any more expensive than campaigning a soft sail rig.
  2. Goodbye Russell ... ?

    This issue was contested by Thomas Lawson a century ago. Check out The Lawson History of The America's Cup. It makes for interesting reading. Written after the first 50 years of the AC, the same controversies were going on then as now.
  3. Are Z foils that much slower?

    Computer control is not needed for a canard. Just look at all the successful Burt Rutan homebuilt designs - Variviggin, VariEze, LongEze, Quickie, Voyager - none of which used computer control. Computer control is needed for supersonic designs that have the c.g. so far back that they are unstable at subsonic speeds, but this isn't required to make a canard stable.
  4. Are Z foils that much slower?

    Canard planes do not stall because they cannot be allowed to stall the wing. If they did, they'd flip over backwards. The canard is designed to stall before the wing, which results in the nose dropping before the canard develops enough control power to stall the wing. You can avoid stall with an aft tail by limiting the control power - the Ercoupe is a good example. Every study looking at the drag of trimmed aft tail vs canard vs tandem wing configurations has shown the aft tail has the least drag and the tandem wing has the most drag. You can make an aft tail have positive lift for trim by putting the center of gravity far enough aft and enlarging the tail to maintain stability. This is commonly done on freeflight model airplanes. Some friends and I tried a canard configuration for controlling a wingsail, and we quickly found we couldn't make it work. The drag on the canard contributed to the unstable moment from the canard, and at a certain angle of attack it was impossible to bring the wingsail back. We switched to an aft tail, and that was entirely successful. With regard to sailing hydrofoils, the best location for producing the side force and the best location for producing the vertical lift may not be the same. That's one reason to consider alternative locations for the rudder.
  5. Are Z foils that much slower?

    Decreasing the pitch attitude will decrease the angle of attack on both foils. Increasing the leeway angle will increase the angle of attack on the leeward foil and decrease the angle of attack on the windward foil. You need to solve for both angles to get the foil to balance the weight and side force at a particular speed. You also need to either solve for the center of gravity (crew position) to balance the heeling moment, or you need to vary the side force to balance the righting moment available from the crew. So at a minimum, it ends up being a problem with three unknowns to solve for simultaneously.
  6. The foil of the future

    For the third America's Cup Match in a row, the winning boat - hulls, beams, pod, wingsail, jib, rigging - was designed by OTUSA.
  7. Are Z foils that much slower?

    I think a single, controlled Z foil could have some advantages over L foils. With negative dihedral, at high speed the shaft could be entirely out of the water, with all of the side force coming from the inclined wing and the winglet. This would be a big reduction in wetted area, and it gets the thickest part of the foil out of the water. As the flying height increases and more wing comes out of the water, the dihedral angle effectively becomes more negative, as evidenced by a line drawn from the water surface penetration to the foil tip. This would minimize the chances of the boat slipping to leeward if it flew too high, as happens with L foils. The drawbacks are a reduction in righting moment as the load moves toward the tip; and a reduction in wetted span, which increases the induced drag. There's also a structural challenge as the load moves toward the tip and the bending moment increases. Whether or not the loss of righting moment affects the performance more than the reduction in drag from reduced wetted area would depend on the overall configuration. If the beam were large compared to the span of the foil, the proportion of righting moment lost would be small. If the foil extended almost to the centerline, the loss of righting moment would be considerable.
  8. Are Z foils that much slower?

    Using a flap has an advantage over changing the incidence in that it moves the sweet spot of the foil. When you increase the incidence, the angle of attack or lift coefficient for minimum profile drag does not change, and it's likely the foil will be operating away from minimum profile drag (or best profile lift/drag ratio). But when you increase the lift using a flap, the minimum profile drag lift coefficient increases, as does lift coefficient for best profile lift/drag ratio. This makes it more likely the foil will be operating in the zone where the profile drag is minimized. Of course the best performance would come from a combination of flap and incidence control so the foil always operated along the envelope of least drag for the required lift coefficient.
  9. Are Z foils that much slower?

    WRT AC35, you need to go to the Design Rule: 6.10 In measurement condition and appendage measurement condition, no part of the AC Class Yacht shall extend more than 2.400 m below MWP. 11.2 Daggerboards shall penetrate the lower surface of the hull entirely between 7.300 m and 8.050 m forward of the stern plane, and entirely within 0.150 m transversely either side of the hull centerplane. 11.5 The maximum dimension of any daggerboard shall be 4.200 m in any direction, measured along a straight line. 11.6 When fully retracted, daggerboards shall extend no more than 0.400 m below MWP. 11.7 Daggerboards, in any and all positions, shall not exceed maximum beam below MWP. Daggerboards (including fittings and control systems) may exceed maximum beam above the diagonal line formed by a point 1.000 m above MWP at maximum beam, and a point 3.000 m above MWP at 0.500 m outboard of maximum beam. No part of a daggerboard or daggerboard system shall extend beyond 0.500 m outboard of maximum beam, regardless of height above MWP. 11.10 A daggerboard shall only: (a) be retracted or extended; and (b) rotate around no more than two axes (or combination of the two axes) whose limits shall be determined as follows: (i) The maximum rotation about the cant axis shall be 15 degrees with the rake axis rotation set to appendage measurement condition; and (ii) The maximum rotation about the rake axis shall be 12 degrees and the rake axis shall be within 15 degrees of horizontal with the daggerboard set to appendage measurement condition. These rules pretty much dictated L boards. The reverse curvature of the shaft brought the elbow out to maximum beam for maximum righting moment, but was limited by 11.7. The dihedral angle was limited in the negative direction by 11.6, and to some extent 11.6 also put a maximum on the radius of the elbow, depending on how much of the elbow could be tolerated in the lower bearing. I've not analyzed such a shape, but I'm pretty sure the minimum drag would have been obtained by a daggerboard wing that was at 90 deg to the total force vector for its entire length. But this wasn't possible because of the depth limit of 6.10. Instead, the wing either had to be curved so the tip was at maximum depth, or it had to have a kink that allowed the wing to run along the maximum depth line. The higher the elbow, the more negative the dihedral could be, but that made the hull fly lower. A Z foil could not have the amount of negative dihedral seen on the A-Class. The A-Class has a span limit for the Z foil, but no depth limit. The winglet of the Z foil would have been useful to produce side force at high speed, but the span of the winglet would have come out of the shaft above the elbow, raising the entire wing up above the maximum depth line. A Z foil would also have violated the 400 mm maximum depth when retracted, even with the board canted in by the maximum 15 deg. The winglet of the Z foil would have only been possible if the wing had enough dihedral to keep the tip of the winglet above the 400 mm line. Draw the box yourself and see if you can fit a Z foil in it.
  10. Length of time between challenges

    As a practical matter, the Match needs to be held in odd numbered years. The reason is on the even numbered years, there's either the Summer Olympics (N. Hemisphere Match), Winter Olympics (S. Hemisphere Match) or the Soccer World Cup, and they soak up all the global sports TV coverage. If you want to develop a completely new kind of boat, two years is not enough time. You need to hire the team, experiment with subscale surrogates, design the boat, test and develop it, then do race prep at the venue. It's certainly possible to design and build a new class in the Deed minimum of 10 months, but it's not the way to go if you want multiple challengers. If you are going with an existing class of boat, then two years is enough to design an improved version and campaign it. Six years is too long to go between matches, so that's why it's tended to be 4 year intervals.
  11. Are Z foils that much slower?

    I agree with all the above. Flaps are a much better way to control foils than raking the whole thing. I would prefer fixed rake and flaps for control on the daggerboards. That would significantly reduce the power demands on the crew. The control system would also be much lighter. The stabilizer could have variable incidence through rudder rake or a hinged mounting to a non-raking rudder. Or an elevator could be added to fixed rudder & stabilizer for pitch control. The reason the AC cats weren't allowed to use the windward board is it's impossible to ensure the windward board isn't used to generate righting moment through down force. If a means of adding righting moment exists, the teams will exploit it. One of the reasons for the pronounced bow-down attitude of the AC cats at high speed is they were using the allowed stabilizer deflection differentially to generate down force on the weather hull, which left them with essentially fixed collective deflection that was little different from the AC72s. If the windward board is used to generate righting moment, then as the wind increases the structural loads increase without bound. The team that wins is the one that operates on the very verge of breaking without going over. It also becomes extremely dangerous when the windward board ventilates or broaches, and the righting moment it was generating is suddenly lost. The Vampire solution is good for performance, but it could be a disaster in close-quarter maneuvering before the start. It would be very difficult for the pilot in the windward hull to judge the clearance between the extended leeward board and another boat. The reason for the beam limit on the retracted part of the AC50 boards was to prevent them from taking out the shroud on a competing boat in the event of a collision. A retracted Vampire board could be an issue in the same situation. When you factor in the performance and safety aspects, I think the windward-retracted L foil configuration, with either L or T rudders, is the one best suited for inshore match racing. Now, whether the rudders are forward or aft of the foil could be an interesting question and left to the teams to decide. Another part of the Design Rule to revisit is the maximum depth restriction. That's what forced the kink seen in some of the daggerboard wings. The 4.2 m circle would have been sufficient to keep the boards from becoming too long, and the combination of cant range and depth limit when retracted would have sufficed to control the wing dihedral. If it hadn't been for the depth limit, I think you might have seen leeward-only Z foils used.
  12. Too much data ? Having a guess at Polars

    Cavitation depends on the foil's loading. When the loading (lift coefficient) increases, the pressures are lower on the suction side, particularly toward the leading edge. At low lift coefficients, the sections are shaped so the minimum pressure occurs well back on the chord. As the loading increases, the pressure distribution on the suction side becomes more negative and flattens out. The section is typically designed for the pressures to be very close to the water vapor pressure along the entire forward half of the chord at the design loading and speed, leading to a flat "rooftop" shape to the pressure distribution. At higher loadings, a leading edge suction peak forms, and the foil needs to be operated at much lower speeds to avoid cavitation. Thickness decreases the pressure on both sides of the foil, and the shaft sections need to be thicker than the wing sections because of the higher bending moments reacted by the shaft. This leads to cavitation starting first on the lower shaft and elbow (due to interference between shaft and wing), then spreading up the shaft and down the elbow as speed increases. Upwind, the loading on the shaft is greater because of the higher side load from the rig. This leads to a lower cavitation onset speed compared to downwind.
  13. Best moment of the AC35 trophy presentation.......

    What I didn't understand is where were the yacht clubs? The America's Cup is not a competition between teams, it is a competition between yacht clubs. They just put the Cup out there, and then Ashby and Burling picked it up. There wasn't so much as a mention of the yacht clubs. What should have happened was the Commodore of the Golden Gate Yacht Club hand it over to the Commodore of the RNZYS and then the Commodore of the RNZYS pass it to the team. That's the way it was done in Valencia, and IIRC, in previous matches as well.
  14. Uptip Foils

    The steady-state value of leeway can be positive or negative. The stability is affected by the variation about the equilibrium value. If the leeway is negative, then the leeway acting on a board with positive dihedral will produce a positive lift due to the negative leeway. However, if the boat rises up, the same mechanisms apply - the reduction in vertical area will result in the leeway becoming less negative, the vertical lift less positive, and the boat will tend toward coming back down. I don't think this mechanism is as important for the ACCs as it was for the AC72s. The control systems have improved to the point that the teams are trying to reduce stability as much as possible under the Design Rule for reduced drag, because the unstable modes of motion have times to double that are within the ability of the crews to handle. So you're seeing the jig (unloaded) rondure shapes banging the limits of the Design Rule.
  15. Wings

    I have a hard time believing the points in the upper left corner, with Cl>2 and Cd<0.04. The flow is separated on half the flap, and the drag is less than for the small points where the flow is fully attached - really? I think this video is a good illustration of the role of the slotted flap. It's job is to decelerate the flow from the main element's trailing edge dumping velocity to the apparent wind velocity, without separating. At 0:12, the flap tab (#4 element in C-class parlance) is deflected +10 deg, and the pressure increase that would result from attached flow separates the flow. At 0:15, the flap is deflected more, but the flap tab has a-10 deg deflection, which flattens the pressure distribution and doesn't stress the tired boundary layer so much. This keeps the flow attached. At 0:22, the flap has straightened out so it is similar to the existing flaps, and this is about as good as the section can do. From there, the lift doesn't necessarily get any higher, and the drag goes up as separation sets in. This is consistent with what I've found using the MSES code to design wingsail sections. I couldn't make a flap tab (#4 piece) increase the performance, either. A comparatively thin flap with flat contours for most of the flap worked as well as anything.