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      A Few Simple Rules   05/22/2017

      Sailing Anarchy is a very lightly moderated site. This is by design, to afford a more free atmosphere for discussion. There are plenty of sailing forums you can go to where swearing isn't allowed, confrontation is squelched and, and you can have a moderator finger-wag at you for your attitude. SA tries to avoid that and allow for more adult behavior without moderators editing your posts and whacking knuckles with rulers. We don't have a long list of published "thou shalt nots" either, and this is by design. Too many absolute rules paints us into too many corners. So check the Terms of Service - there IS language there about certain types of behavior that is not permitted. We interpret that lightly and permit a lot of latitude, but we DO reserve the right to take action when something is too extreme to tolerate (too racist, graphic, violent, misogynistic, etc.). Yes, that is subjective, but it allows us discretion. Avoiding a laundry list of rules allows for freedom; don't abuse it. However there ARE a few basic rules that will earn you a suspension, and apparently a brief refresher is in order. 1) Allegations of pedophilia - there is no tolerance for this. So if you make allegations, jokes, innuendo or suggestions about child molestation, child pornography, abuse or inappropriate behavior with minors etc. about someone on this board you will get a time out. This is pretty much automatic; this behavior can have real world effect and is not acceptable. Obviously the subject is not banned when discussion of it is apropos, e.g. talking about an item in the news for instance. But allegations or references directed at or about another poster is verboten. 2) Outing people - providing real world identifiable information about users on the forums who prefer to remain anonymous. Yes, some of us post with our real names - not a problem to use them. However many do NOT, and if you find out someone's name keep it to yourself, first or last. This also goes for other identifying information too - employer information etc. You don't need too many pieces of data to figure out who someone really is these days. Depending on severity you might get anything from a scolding to a suspension - so don't do it. I know it can be confusing sometimes for newcomers, as SA has been around almost twenty years and there are some people that throw their real names around and their current Display Name may not match the name they have out in the public. But if in doubt, you don't want to accidentally out some one so use caution, even if it's a personal friend of yours in real life. 3) Posting While Suspended - If you've earned a timeout (these are fairly rare and hard to get), please observe the suspension. If you create a new account (a "Sock Puppet") and return to the forums to post with it before your suspension is up you WILL get more time added to your original suspension and lose your Socks. This behavior may result a permanent ban, since it shows you have zero respect for the few rules we have and the moderating team that is tasked with supporting them. Check the Terms of Service you agreed to; they apply to the individual agreeing, not the account you created, so don't try to Sea Lawyer us if you get caught. Just don't do it. Those are the three that will almost certainly get you into some trouble. IF YOU SEE SOMEONE DO ONE OF THESE THINGS, please do the following: Refrain from quoting the offending text, it makes the thread cleanup a pain in the rear Press the Report button; it is by far the best way to notify Admins as we will get e-mails. Calling out for Admins in the middle of threads, sending us PM's, etc. - there is no guarantee we will get those in a timely fashion. There are multiple Moderators in multiple time zones around the world, and anyone one of us can handle the Report and all of us will be notified about it. But if you PM one Mod directly and he's off line, the problem will get dealt with much more slowly. Other behaviors that you might want to think twice before doing include: Intentionally disrupting threads and discussions repeatedly. Off topic/content free trolling in threads to disrupt dialog Stalking users around the forums with the intent to disrupt content and discussion Repeated posting of overly graphic or scatological porn content. There are plenty web sites for you to get your freak on, don't do it here. And a brief note to Newbies... No, we will not ban people or censor them for dropping F-bombs on you, using foul language, etc. so please don't report it when one of our members gives you a greeting you may find shocking. We do our best not to censor content here and playing swearword police is not in our job descriptions. Sailing Anarchy is more like a bar than a classroom, so handle it like you would meeting someone a little coarse - don't look for the teacher. Thanks.

Basiliscus

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  1. 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.
  2. Foil types are defined by the size and extent of the cavity they form. Subcavitating foils have no vapor cavity. Transcavitating foils have a vapor cavity over some part of the surface, but the cavity length is less than the chord length. Base ventilated foils are wetted on their upper and lower surfaces, but have an air cavity that trails from a squared-off trailing edge. Supercavitating sections have a cavity that extends from the leading edge to past the trailing edge, leaving the entire upper surface, and base, dry. You could operate a thin, lightly loaded foil at speeds above 50 kt and still be subcavitating, so 50 kt is not a magic number. It's just very difficult to remain subcavitating at that speed. This chart is very busy, but it gives some idea of what the foil characteristics would be like for a very high-speed subcavitating section. The Eppler E817 is a 12% thick section designed to be very resistant to cavitation. However, it is likely to form bubbles when running above 40 kt. If you scale down the section to make it thinner, you reduce the local velocities when lightly loaded, and raise the cavitation speed. Scaled to 5% thick, it could hit speeds of 60 kt without cavitation, but only over a narrow lift range. When loaded more, it will cavitate on the upper leading edge, and when loaded less it will cavitate on the lower leading edge.
  3. The terminology I used vs what Steve uses: main element = element #1 tab = element #2 flap = element #3 I use "wing" to mean the assembly of all the elements.
  4. Just for the record, the rake wasn't changed for the races. Raking aft was tried on the lay day, but it didn't relieve the lee helm. The head wasn't pushing to leeward because of the flap twist, so moving it back did not improve the helm. The original rake was restored for the races. Instead, the lee helm was alleviated by adjusting the strings controlling the tab so as to open the slot. This had the effect of increasing the load on the flap while decreasing the load on the main element, and the resulting aerodynamic moment countered the lee helm. The effect on profile drag was also predicted to be neutral from the tab change, with an increase in drag on the flap and a decrease in drag on the main element. The tab changes were made only low down at first, then additional segments were added in subsequent races, but the change never extended to the entire span. In addition to the tab changes, the flap twist profile was changed to raise the mid leech and twist off more at the head. This increased the load on the part of the wing that was behind the ball and pushing to leeward. It also allowed the wing to produce more lift for the same heeling moment. The December 2013 Seahorse magazine has an article that presents data showing these effects. Moving the ball was actually considered. But it was judged too risky to accomplish in a day and a half.
  5. Here's a presentation that describes the wing control system on the AC45 one-design boats. It's a direct descendent of Dave Hubbard's C-class wing control systems. The AC45 control system can control wing rotation, camber (flap deflection) and flap twist. It is self-tacking, meaning it flips through onto the opposite tack by itself when the angle of attack switches to the other side, without the crew having to take action to reconfigure the wing for the new tack. The camber and twist settings will be the same for the new tack without the crew having to do anything. Due to friction in the system, the wing sometimes needs some help from the crew to invert, especially in light winds. However, it cannot force camber into the head, and it cannot invert the flap deflection at the head. It can only let the head of the flap fall off compared to the foot. Hubbard's concept is to make the upper control arms slaves to the bottom control arm. The cool wrinkle is the control cables cross over as they go up the wing, so the port control arms are connected to the starboard control arm at the bottom and, and the starboard control arms are connected to the port control arm at the bottom. The air loads make the wing want to fold up like a taco, because it is restrained at the ball and clew (by the sheet/traveler line). Say it's on starboard tack. As it folds, the starboard control arm at the bottom feeds slack into the port control arms up the wing, and the port control arm on the bottom pulls on the starboard control arms up the wing. This action holds the leech of the flap to weather against the air loads. At the foot, the master control arm has a third line that goes to the centerline of the main element and limits how much the wing can fold - that's the camber control. If all the control lines dead ended at the ends of the master control arms at the foot, then the flap deflection would always match the foot all the way up. But instead, the control lines lead through the ends of the master control arms and dead end on a quadrant that can be eased off. The lines leading to each pair of control arms are attached at different distances from the quadrant's pivot, so they are eased off by different amounts with the top control arms being eased more than the lower control arms. This controls the twist. Like the line limiting the flap angle, the line limiting the twist quadrant is led to the centerline of the flap, which makes the twist symmetrical on both tacks. Since the twist quadrant moves with the flap, the twist stays the same as the camber is changed.
  6. Now that I think about it, twin stabilizers like those on Harborwing's wingsail, could be permanently attached to a wingmast and the sail raised between them without interference. When the sail was dropped, the mast would become an aerodynamically stabilized and damped wingsail just like the ones on Greenbird and Saildrone. Saildrone has demonstrated this kind of rig is survivable at sea, albeit at a much smaller scale. Saildrone has both the characteristics of mass balance and 360 deg rotation.
  7. Flutter of a wingmast is a serious issue. I've pitchpoled a landyacht backwards when the wingmast fluttered while going through a tack. Flutter is due to coupling between the rotation of the mast about its axis and the sideways movement of the mast due to slop or stretch in the rigging. To reduce the susceptibility to flutter, you should arrange for the center of gravity of the mast to be ahead of the rotation axis of the mast. This will cause lateral acceleration of the mast to result in a stabilizing rotation of the mast. One way to achieve this is to have a mass balance arm sticking forward from the leading edge. The ballast should be placed high, since the linear accelerations are greater at the head than at the foot. A logical location would be just above the hounds, since you need reinforcement in that area anyway to take the point loads of the stays, and on a lot of masts the head is behind the rotation axis. You can see this kind of mass balance used on Richard Jenkins' Greenbird, which holds the world's landsailing speed record. Although Greenbird uses an aerodynamically stabilized wingsail rather than a wingmast/soft sail rig, the same principles apply. Another example is the water ballast tank attached the the base of the America's Cup Catamaran wingsails when they are being stepped and removed. The water ballast shifts the center of gravity of the wingsail forward so it doesn't flutter and sail in the breeze before the foot is restrained by sitting on the ball. In that case, the wingsail is hanging from the crane, so the foot of the wing experiences more lateral motion than the head, and the best place for the ballast is at the bottom. When operating under bare poles, it would also be a good idea to attach a tail surface similar to the one on Greenbird to stabilize the mast and keep it aligned with the apparent wind. It only requires a modest torque applied to the mast to trim it for nonzero force, and the sheet can be released to feather it like a conventional sail. Randy Smythe's Sizzors uses a wingmast that can rotate through 360 deg without interference from the stays. That's a good feature to include, too, although it would mean the mass balance (and tail) would have to fit underneath the stays, and it complicates having direct control of the rotation of the mast. The arm holding the ballast for the mass balance must be strong. Acceleration of the ballast due to dynamics of the mast can be high - 40 gs would not be unreasonable as a design load. Between a proper mass balance to tame the dynamics and 360 deg rotation to control the static loads, I think many of the issues with large wingmasts can be significantly reduced. A large wingmast offshore is still not a beast to be taken lightly, however.
  8. Wingmast/sail combinations have been around for a long time, and were popular in the C-class (and other open class cats) before wingsails supplanted them. (drawing by François Chevalier) It looks like the Scissors rig differs in using a light-weight structure for the wingmast, and in running the stays to the masthead so it can rotate 360 deg. It doesn't look like the mast would rotate 360 deg with the sail raised, so it wouldn't be able to gybe-around like a cantilevered wishbone rig. I agree with all of Bench Warmer's advantages for the rig. Aerodynamically, I don't see any difference between Scissors's rig and traditional wingmast/sail rigs. That's not to take anything away from the performance of the rig - I'm a fan of wingmast/sail rigs. In some applications, they can be every bit as competitive as full wingsail rigs.
  9. And it has a turboencabulator.
  10. One is English, the other is metric.
  11. You're right that the pitch attitude is referenced to the horizontal. But the trajectory of a foiler, like an airplane, has a third dimension. The force on the foil depends on the angle of attack, which is referenced to the velocity vector, not the horizontal. If the velocity vector is horizontal, then the pitch attitude and angle of attack are the same (when the boat is sailed with level heel). But if the velocity vector is pointed up or down, then for the same angle of attack the pitch attitude will also be more up or down. Another example is a boat that maintains the same pitch attitude, but loses speed. The angle of attack has to increase to maintain the same lift (lift = weight at near steady-state conditions). The boat starts to sink down, resulting in the velocity vector pointing downward and the angle of attack increases because of the change in the velocity vector instead of a change in the pitch attitude. You can get a similar effect with the controls. Say you used the stern foil to maintain pitch attitude and you made a step increase in the main foil flap deflection. The increase in lift from the flap will accelerate the boat upwards, and very quickly the new trajectory will reduce the angle of attack and bring the lift back into equilibrium as the boat continues to rise at a steady rate. In all these cases, vertical direction of the velocity vector (flight path angle) summed with the angle of attack (angle between the velocity vector and the reference line) equals the pitch attitude (angle between the reference line and the horizontal). What the sailor wants is for the trajectory to be level. The lift on the foil depends on both the flap and the angle of attack, and the boat has to find the combination of flap angle and pitch attitude that brings the lift into equilibrium with a level trajectory. The pitch attitude and the flying height don't respond at the same time to the flap control. A designer has the option of exploiting the phase difference between pitch and heave at the expense of more complexity in the control system, due to having to link a forward mounted wand to the flap on the foil. Or the designer can opt for a mechanically simpler arrangement that doesn't try to feed back pitch attitude to the flap. In that case, maybe the designer adds area to the stern foil to increase the pitch stability and damping. The wetted area will be higher, increasing the drag, but for a one-design the simplicity may be worth the tradeoff. There's no single way, or even one best way, to design the control system because the requirements for each boat are different. That's why design is an art.
  12. The pitch attitude of the boat is the sum of the angle of attack of the foil and the flight path angle of the foil through the water. The slower dynamics of the boat play out at almost a constant angle of attack, and changes in the pitch attitude of the boat reflect changes to the flight path angle. If you multiply the flight path angle by the velocity, you get the rate of change of the flying height. So the pitch attitude times the speed also corresponds to the rate of change of the height. Putting the wand ahead of the foil means the feedback signal is a combination of both height and the rate of change in height. This adds lead to the control system and improves the damping. If you consider that, at constant speed a change in angle of attack is needed to change the flight path angle, then pitch attitude feedback adds even more lead because it anticipates the change in the flight path angle as well as the flight path angle's effect on height. If you make a feedback system that has only position feedback (like pure height measurement), what you create is an oscillator. The feedback will make the system correct back toward the set point, and the feedback goes to zero at the set point, but the trajectory is already passing through the set point and headed toward the opposite extreme. As error accumulates in the opposite direction, the system will reverse and correct back, but it will overshoot again in the original direction. When you add rate feedback, the reaction of the control system is stronger when it is headed away from the set point, and the feedback goes to zero before it gets back to the set point. The control system will already be applying control to slow things down as it gets to the set point, so any overshoot the other way is less than for the pure position feedback design. This makes the response settle down and converge to the set value. If you have enough rate feedback, there won't be any overshoot at all and the system will be heavily damped. That's why putting the wand forward helps to make the boat easier to sail. It's not so much that it anticipates the waves, but more that it anticipates what the boat will do, even in flat water. Sure, you can put the wand at the foil and get damping from other sources. But moving the wand forward is a powerful way to add damping, and pitch attitude feedback is especially effective when the boat is going fast.
  13. http://www.tspeer.com/Aclass/A-ClassCatamaranFoils.pdf ...Drag increases as boat flies higher Lift and drag shift to leeward board ... This analysis only considered the induced drag due to lift, not the total drag. The parasite drag will decrease as the wetted surface decreases. So the total drag should initially drop as the boat flies higher, but then increase as the wetted span is shortened to the point that the increase in induced drag outweighs the decrease in parasite drag. But the effect of leeway on lift and righting moment should be correct, within the limits of lifting line theory. Unfortunately lifting line theory doesn't capture the effects of the free surface, so there are still a lot the physics that are left out.
  14. http://www.tspeer.com/Aclass/A-ClassCatamaranFoils.pdf
  15. “History never repeats itself but it rhymes,” said Mark Twain After Reliance, didn't Herreshoff create the Universal Rule as a less expensive alternative to the Seawanhaka Rule? Reliance = USA 17 trimaran J Class = AC72 12mR = AC Catamaran There are even echoes of the Dunraven Affair in recent Matches. The same controversies are playing themselves out, just with a faster tempo. It's fascinating to read the Lawson History of the America's Cup and compare the first 50 years of the AC with recent decades. The discussions sound very familiar.