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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.  

Danny Boy

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About Danny Boy

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  1. DC Designs

    I've got so many ideas for an unstayed una IC that I'd consider selling a kidney on the black market to finance it. Shit I hate the way that boats bleed you dry but I can see it will always be that way.
  2. DC Designs

    BalticBandit, I'm not completely down with some of the stuff that you're saying here. I agree that generally you'd possibly sheet freer on a gyber, as if you take the AOA as the angle of the sail to the track rather than the centre of the boat then this would be consistent. The second thing to realize is that an increase in boat speed on a high aspect foil, even a slight increase in boat speed, results in dramatically more lift. 49ers experience this a lot. Off the line it is important to be able to get your nose out so that you can ease your jib sheet about 2" at the lower spreaders. This increases the power to the sails enough and velocity goes up enough that the increased lift on the blade compensates for the lower angle of sail I think you have this a little wrong - naturally the amount of lift that can be achieved before stall increases with speed - who hasn't ended up going sideways after a crap tack? Or getting hiked/trapezing before getting the boat moving from standstill? Basically Trying to generate full RM without enough speed and stalling the foils. In the context of actual sailing, for the 49er example, you'll be twin stringing in upwards of say 7 kts breeze. At this full righting moment condition the actual lift itself remains constant. If you put the bow down a few degrees and put on a couple of knots, the RM is the same, therefore for athwartships equilibrium your sailing sideforce will be the same. The lift coefficient actually reduces (with the square of the speed at constant lift). Because your lift coefficient reduces, the lift is generated at a lower AOA (leeway) and therefore your induced drag coefficient reduces (although this is tempered by the fairly linear increase with speed of the frictional drag). So max VMG occurs when the sum of the induced component (reduces with speed at constant lift) and the frictional component (increases with speed) is at a minimum. Naturally there are plenty times on the course where you're forced to sail a few degrees high of optimum VMG, and similarly on a quick boat like the 9er if you can nudge forward enough you'll usually foot off over the boats to leeward to neutralise them. This is obviously tactical suicide on most keelboats where you'll barely go faster as you bear away. In the pinching mode, whilst you are sailing a bit high with a higher induced drag component, and the lesser speed means you'll be making more leeway, you're very unlikely to get near the ultimate stall angle (about 20 deg) and thus the ultimate lift producing characteristics of the foil. I'd respectfully say that if you're pinching so hard and going so slow that you're approaching 20 degrees leeway that you should have tacked off or footed out a bit earlier.. So in practice a gybing board boat goes forward a bit faster and sails a bit higher. But how should we think about this Another way to think of a gybing board is that it is effectively an assymetrical foil. Low speed foils that do not have to operate symmetrically have long used assymetrical foils to optimize lift. STOL aircraft, Gliders that don't have to fly inverted all use Assym foils. For a boat, if we only have to go fast on one board, Assym foils would be the way to go (Bilge-boarders do this all the time). But of course we can't. Except when we can. A gybed daggerboard essentially creates an assymetrical foil that automatically deforms on each tack appropriately. This is the same general theory and mechanism of the "flapped" boards - and is the same mechanism that Dennis Connors' (its hard to read DC without thinking Dennis Connors) AC Cat used. The benefit of this is so great that This Paper estimates that a properly shaped assymetrical rigid wing has a 50% lift/drag improvement over a symmetric foil. I agree that a decent assymetric foil section is far superior to a symetrical ones, provided it is being used at the intended Cl, Rn ect, but without reading this paper I don't follow how an unflapped central dagger that needs to perform on both tacks can be assymetric, unless some pretty interesting flexibility has been built in. IACC yachts have flapped keels whereby when the flap is deflected, the low pressure (windward) side is completely fair. IACCs typically run enough flap to actually make a couple of degrees of negative leeway - this is because the assymetry of the heeled shape of a yacht, even a narrow cup yacht means that it will still be creating sideforce (and therefore induced drag) when it is at zero yaw angle. So the Reynolds numbers on water and air are dramatically different. Lets say that the lift/drag improvement is only 5% (because the gybed blade isn't a fully shaped assym foil, but instead the aft 80%-90% of the full foil), it still gives me the option of having the same lift and decreasing my drag by 5%, or increasing my lift by 5% and leaving my drag the same. Or I can compromise and decrease drag by 2.5% and increase lift by 2.5%. No problem with this - have the option of higher speed at same track or higher track at same speed. Ok, decreased drag means more speed. In fact drag is 2nd order of magnitude effect of speed so a boat with 97.5% of the drag of Boat A will go roughly 11% faster. Means on a .5 nautical mile course with a stock boat doing 10knots, The decreased drag boat will gain 3-4 seconds which at 10 knots is 30-40 feet or 3-4 Boat Lengths. Not always - depending heavily on the hull shape and length/displacement ratio, hull drag is only roughly proportional to speed - this is due mainly to "humps" and "hollows" in the residuary resistance (made up of mostly the wave making drag) due to alternating constructive and destructive interference between the bow and stern waves as the speed increases. Also there is the famous "displacement hump" at about Fn = 0.3 (about 4.5kts on a 16ft 49er) where the boat starts sailing uphill on its own bow wave, and a heavy boat's resistance will start to increase at a much higher rate. Obviously 49ers ect have been designed to minimise the effect of the displacement hump, furthermore a 9er rarely sails as slow as 4.5kts. When a boat is planing (true planing is Fn greater than 1.0, about 14kts on a 16ft 49er) then the drag starts to rise at a lower rate more linearly proportional to speed for most boats. Something like a speedboat hullform is horrendously inefficient below planing speed - many of us will have had to fairly bury the throttle on a RIB or dory to get it onto the plane, then to back loads while staying on the plane. Here the resistance actually reduces with speed for a while after hullspeed has been exceeded. Luckily this effect is rare in sailboats as most need to be efficient over a broad range of speeds, mostly spanning the difficult semi-planing range. Similarly 2.5% more lift reduces the distance you need to sail to weather because you will be sailing a better VMG. Net net, with only a 5% improvement in lift/drag you will see a benefit of probably 8-10 seconds, which at 10 knots translates into roughly 80-100 feet at the weather mark. I think you've made a few big assumptions here, and you may be using the term VMG wrongly - if "better" VMG was gained by sailing higher, then sure less distance travelled. However if optimum VMG was achieved by higher speed at the same heading or even at a lower heading then you'd be sailing the same distance or even more. Now if you can lock your board going downhill, you don't pay any price for this other than some added complexity in the boat. For weight regulated classes, there isn't even any weight penalty for the additional hardware. Agreed. Note that looking a gybing board as part of an assymetrical/cambered foil also explains why its so important for the foil to be high aspect ratio and built by a top builder. A thick chord blade or a blade with its max chord too far aft will not do a good job of conforming to the aft 75% of a cambered foil, whereas a thin high aspect ratio blade will. I don't really understand this statement. Now I don't know about you, but anything that gives me 160' for every weather nautical mile sailed, is damn well going to make a difference. Still think you've assumed a lot with this value, but sure any racing sailor will take any advantage however small - we've all lost plenty of races by sub - boatlength margins, right? Note that if this was only about the induced drag of the hull, you could combat this by starting to leeward of a gyber and go into massive pinch mode. Massive pinch mode would put both boats into a mode where hull drag is in play, but the air drag of the sails and then AoA drag on the blades trumps any hull drag benefits. Yet in "pinch mode" is where the gybing board REALLY kicks butt. Its not optimal VMG, but relative boat to boat the Gybing board boat will trounce the normal bladed boat. Sure pinching is limited by the induced drag "AOA drag" of the hull/foils. I guess that if the hull of the gyber is being dragged sideways at a slightly less ruinous angle then it will suffer less. On the flipside, if boats are footing, going faster, less leeway for constant lift, then the dagger may actually be set at too high a gybe angle and the hull may be making a slightly negative leeway angle, thus losing out a little against its optimum. Bearing in mind leeway angles or board gybe angles are typically circa 3 deg then being able to accurately set the angle, and having everything aligned is pretty crucial. Now if gybers are sailing against gybers, there is going to be an optimal upwind angle that trades off reduced drag vs. increased lift, and you will have lanes that are as narrow as you see in that fleet based on speed and rig. This holds true for any type of upwind sailing. The big difference occurs when gybing boards are sailing against non-gybing boards. ONLY THEN will you see a difference in performance. And what you will see is a boat that can "climb out" from other boats, hold narrower lanes, or in some cases have noticeably better forward boat speed. so I disagree that you don't get better forward speed with a gybing board. Whilst I don't want to appear chickenshit, this'll prob be my last post on this thread as I think fellas disagreeing on matters of sailing physics is probably not all that pertinent to "Kick Ass Canoes," and there are plenty other threads on this forum for that kinda chat! I'm unkeen on just talking the talk when my circumstances mean that I'm not going to be in the market for a DC for some time. Dan
  3. DC Designs

    To be honest I reckon that the finn/europe masts are so constrained by their rules that the external dimensions are pretty much fixed and there isn't much difference between the top guy's kit - I believe the europes post '96 tried to word their mast rule to create a 1 design (female) mould situation in order to control development costs, but I think a certain nation applied the sledgehammer approach and made its own tooling at vast expense, making a bit of a mockery of the rule. From these same moulds different sailors can specify different layups to suit their required bend characteristics, which isn't too hard to do for those in the know. I don't think that the IC would need to go that specialised - you'd get 99% of the performance for a fraction of the cost and hassle if the basic characteristics were remembered. The OK class is a good case in point - after carbon sticks were legalised, contemporary europe/finn thinking was applied to the bend curves - normalized for mast length and RM. Most of these masts are circular made on male mandrels, just with modern bend characteristics. Sure, the aero performance compared to a moulded wing section will be a little poorer, but this is almost a secondary concern, even less so if a sleeve luff were to be used. Dan
  4. DC Designs

    I think we're all singing from roughly the same hymn sheet. Dynamic reponse is just response to anything dynamic, whether gust or seaload. Like you say most wave encounters are pretty "transient" or quick - often too quick to trim to. Gusts are usually slower, although a decent rig will soften the effect of the gust, "automating" the rig. The stiffer mast whipping about imparts more force back onto the boat if it is stiffer due to higher sectional weight (i.e same material, same diameter, thicker wall). If the mast is stiffer due to either a different section shape (larger diameter, thinner wall) or higher modulus material, then provided the sectional weight is the same it will feel the same to the sailor, only it will respond quicker (higher natural frequency - proportional to the root of the stiffness to weight ratio) I think Hysteresis is maybe not the right term - I think strictly speaking hysteresis is a difference in the magnitude of one variable depending on whether its dependant variable is increasing or decreasing. I think its seen quite a lot in woven sailcloths - strain being less when load is being applied as compared to when the load is reduced. Quite a lot of examples in electromagnetism but forgot all that as soon as I could. As for the endplate stuff - I did a wind tunnel project at University about the boom/deck gap. Like most of university I can't remember much of it (blame Nige's cousin) but between my dodgy experiments and proper papers I read, it seemed that the gap was pretty important and its effect less absolute than the 25mm gap Bill Hansen talks about. Some good stuff out there somewhere on theoretical optimal spanwise lift distribution for maximising driving force/righting moment ratios for varying endplate gap. Like I say though, lots of water under the bridge since then, time and beer addled my memory. Dan
  5. DC Designs

    I used to be a pretty serious Laser sailor, that most technical of classes. I mostly windsurf now but dabble a bit in other stuff. I believe one of my best mates is your cousin Tim..
  6. DC Designs

    Andy P – I can understand that an actual windsurf rig, even stayed, would be far too bendy and the sail shape all wrong for the canoe, which has orders of magnitude more RM. Absolutely hear you re: the mast weights – my belief is that an unstayed canoe rig would have to be proportionally stiffer than the finn rig, therefore weigh, say, 10+kg. Obviously this is not a trivial amount of weight in the context of a 50kg canoe, nor is the cost of 10kg of carbon/epoxy… Just to illustrate, I reckon that a finn and moth with appropriate sized sailors would generate similar sized RM, so a unstayed moth mast designed to the same concept would be approx 7 or 8kg!! On the flipside the benefits, if done well, would include no windage/cost/complication from stays, and poss slight structural simplification of the boat, as well as the obvious ability to square the boom downwind with no interference on sail shape from stays. I do believe that technology has moved on enough since the 70s that an unstayed rig would be far less compromised by a lack of staying than it would have been back in the day, particularly on such a skinny boat. The luff curve would be so much less that cunningham to prebend the mast would be uneccesary. K76 – the IC has a max mast height that practically limits the aspect ration one can achieve. Also the canoe, esp DC has a very narrow shroud base, especially compared to the huge base of the A class and large base of the moth. If the shroud base issue is extrapolated to the natural conclusion of zero, then we have, in effect an unstayed rig but with huge compression, hence why an unstayed rig might work well on this kind of boat. Phil S – I can well believe you were dynamite downwind – I’d wager that the sloop rig becomes less efficient anything lower than a really broad reach. I’m sure diamonds would def improve the side stiffness lower down and thus pointing. The rig would still be wobbly around deck level but the lower leech would be more controlled. Another point is that the una upwind will suffer from having a high foot and losing out on endplate effect – maybe a longer foot, nice and low on and forward of the carriage then cranked up to give the helm space would work, although this would require some funky boom action, prob impractical. Amati - A shorter mast would not need to be so stiff to maintain a similar ratio of midspan deflection to cantilever length at sailing righting moment so sawing the relevant amount off the top of a finn mast would be a good start. One of the better finn lofts would be able to have a stab at this I reckon – North dominate at the moment I think, but I think Quantum, or WB in finland might help. A very rough order of events would be: shorten finn mast, add material to stiffen approx 30% while maintaining the distribution of f/a and athwartships bend. Do some static bend testing, measuring tip and deflection at bands at quarters. This can be nondimensionalised by the guys at the loft for the RM and mast length. Ask them to take their best Finn mould shape (predicted flying shape under sailing loads) and adapt it to an IC planform (much shorter foot, roach, full battens) and the bend data that you’ve supplied for the luff curve. This would prob give a respectable first stab. Ideally you’d get someone to run an aeroelastic simulation using the flow/membrain software or whatever the Quantum equivalent is, which would speed up the prototyping no end. This ain’t cheap though so would prob rely on having friends on the inside. It would be unlikely to get any development cash as the captive market isn’t that big. Dan Right, Gust Response. I’ve not seen Emmit Lazich’s prose, but my take is that “gust” response is a bit of a misnomer, and that “dynamic response” would be a more appropriate term. Gusts, apart from at the onset, really aren’t particularly dynamic events. In the non existent steady state, then dynamic response would be irrelevant, but in the real world, the boat is subject to motions from 1) the seaway and 2) the movement or inputs of the crew. The effect of driving the boat through waves, or the crew moving about is to subject the boat to accelerations in the 6 degrees of freedom – heave; pitch; yaw; sway; roll and surge. In the context of sailing upwind, coupled heave and pitch are by far the dominant motions. As anyone who has jumped up and down on scales will know, accelerations magnify a static force, therefore any of the equilibrium forces acting on the boat can be affected by motions in a seaway, or crew kinetics. Imagine sailing along with a completely stiff rig. Hit a bit of chop which instantaneously imparts acceleration onto the boat, and suddenly the heeling force multiplies by the magnitude of the acceleration. This will require corrective action from the crew, whether steering, sheeting or kinetic, to maintain the equilibrium heel angle, course ect. Liken this to driving a F1 car over rough ground. With a bendy rig and or foils (which, to one extent or another, all foils and sails are), the sudden increase in load will result in the mast bending more, the mainsail leech twisting open ect ect which will serve to lessen the effect of the overpowering force, and reduce the amount of input necessary from the crew to keep the boat sailing on an even keel. Liken this to driving a rally car over soft ground. (N.B not all aeroelastic effects work in the right way to shed or gain power dynamically – i.e under greater loads, badly supported masts will bend between the hounds, increasing jib luff sag, which is opposite to what is required, hence sloop masts are kept pretty rigid below the hounds, and all the dynamic response comes from the topmast) If a mast is changed to one that is lighter for the same stiffness, the dynamic force caused by the mast whipping about aloft will be less, and also reduce the amount of corrective input required from the sailor – (think rally car on rough ground with lighter wheels/suspension components, or my previous post for the windsurf mast analogy.) This is what Jim C describes as manageability. The mast will recover its undisturbed shape sooner too (higher natural frequency). Finn evolution over the last 30 years has basically been use of material improvements to increase and increase the stiffness of the mast within an acceptable level of manageability (corrective sailor input), allowing sails with less luff curve and consequently more control on shape. Another variable to consider here is that a fitter sailor of the same weight will be able to maintain corrective input for longer than a less fit sailor, thus allowing a stiffer rig with less luff curve. Dynamic response can also be provided by other factors, such as sailcloth stretch (if it is in the right place!) Apart from planform aero factors, the square top mainsails currently in vogue will help out here – the area outside the straight line between the clew and head will stretch dynamically and depower the rig. This is why the threadlines on the 3DL sails on 18 footers and AC boats are generally pretty sparse in this area, allowing stiffer masts and more genoa (or spinnaker in the 18ft case) luff control, decoupling dynamic response from the mast alone. I have writers cramp now and will have to work a bit later. Ah well. Dan
  7. DC Designs

    Sorry Chris - good point which I forgot in my rant, cunningham acting like a backstay on the bendier stayed masts. Taken to the extreme of a windsurfer rig the sail is set entirely on masses of downhaul. Less of an effect on my flight of fancy really stiff unstayed telegraph pole! Not sure whether the sleeve luff camber sails are any more efficient than a decent wing section mast. Obviously then you have to reconcile disparate aero and bend requirements. Either way, stiff or bendy mast with stiff battens and lots of cunningham means rigid camber is set into the sail which doesn't "feather" when sheeted out, which is how the formula boys hold down those ridiculous sails without getting backhanded. I reckon the sleeve luff / camber setup could be made to work on the really stiff unstayed rig, giving a better aero entry from a cylindrical mast (given correct bend characteristics..) Dan
  8. DC Designs

    Speng is right about the sloop rig and the higher Cl due to it being a "slotted wing" but this only really applies upwind where the jib can be sheeted correctly with the slot working. Much lower than this and the jib cannot be trimmed efficiently, especially on a narrow boat like the canoe thereby negating this effect. Down towards the dead run the jib is both blanketed, and a horrendously inefficient shape, hence the relative superiority of the una when sailing deep. Upwind, all things being equal the sloop should be better until all of the righting moment is used, as the max Cl available from this type of rig is higher for the aforementioned reasons. In theory the una should be the equal of the sloop in breeze once the performance is driven by the driving force/righting moment ratio, rather than just the absolute driving force when the wind is light. The other sloop efficiency gain is in reducing the size of the separation bubble behind the mast. Another one is that when overpowered and spilling wind by easing main, the luff of the main backwinds near the CLR, meaning that the f/a balance isn't affected much as the jib and the leech of the main are still pulling. Doing the same in a una means that the C of E moves aft, loading the rudder up too much (unless a centreboard is used and raked back). I think that the contemporary Finn rig is a pretty highly developed peice of kit and some adaptation of this would be the "optimal" una rig for the IC, tweaked for the greater righting moment and shorter foot / larger roach. Any rig like this is a comprimise between amount of broadseam and luff curve to achieve the "optimum" flying shape - i.e vertical distribution of camber and twist for a given righting moment and leech (and luff / foot) tension. Back in the old days the aluminium finn masts could be set up well for moderate winds, with the ideal section stiff low down athwartships, bendier up top to twist the sail open as the boat hit waves or the helmsman bounced. The f/a bend would be greater, and more constant curve to allow depth adjustment with leech tension. The problem with these sails was that in big breeze, the sail had to be flattened with more leech tension, reducing twist and making the sail over critical to steering or kinetics - i.e too demanding of the sailor. Furthermore in light breezes there was so much luff curve that leech tension had to be used to flatten the sail enough to be effective upwind, although at the expense of killing off twist and flattening the top of the sail so much that it was ineffective. The luff curve heavy style also necessitated the use of soft cloth with lots of bias stretch, which also meant that the sail would become fuller and more draft aft than intended as the breeze came up, also bad for performance. The laser rig is a crude simplification of this style. Ideally this situation could have been mitigated by a mast that was much stiffer sideways, meaning that a lesser range of f/a bend could be also be used, therefore less luff curve and a better "off design" sail shape. The reason why this was impractical lay in the dynamics - the impulse that the mast imparts to the boat as it hit chop/gusts would be too great above a certain mast sectional weight. A good analogy to this would be the difference between a 30% and 100% carbon windsurf masts (both with identical bend characteristics). In a steady state environment the circa 1kg weight difference would be undetectable, however in a realistic environment with the mast whipping around as the board hits chop, the 100% mast would feel smooth, but the identical rig with a 30% mast would be pulling the rider all over the place. Any windsurfer who has tried a similar experiment will concur. A further analogy would be using light alloy wheels on a racing car - the weight saving is negligible in the context of the system, but the dynamic loads imparted to the chassis as the car hits bumps ect are proportionally smaller, significantly improving ride and roadholding. Anyways back to the Finn rigs - the advent of carbon, and more recently high modulus carbon has meant that the designer can have far more stiffness for a given section weight (or dynamic load on boat) meaning that a rig with much less luff curve can be used. This means that the shape is driven more by the broadseam, which gives more control. Also, stiffer fabrics can be used with less bias stretch, which hold their designed shape over a larger wind range. These sails probably look very similar to 70's sails in the "ideal" 12kt scenario, but aren't far too full in the light or the breeze, so can be closer to an ideal fullness but with more appropriate amounts of twist. All told with this style of rig you can have a near ideal shape nearly all of the time, without the added control of shrouds/spreaders. The main problem is that it necessitates a very close match between luff curve and mast bend characteristics. From what I can tell from Phil S's cracking 1st attempt at a una, I can only imagine that the rig is far too bendy, especially sideways low down. This can probably be tuned to be good over a narrow wind range, and on flat water, but the bendiness means that out of range the sail shape will not be ideal, and in waves his mid and lower leech will not stand up, killing pointing. I've been looking at una rigs for a europe style boat (about 70% Finn Righting Moment), and to get similar bend numbers (adjusted for mast length and RM) to contemporary Finn and Europe rigs on a 56mm ID cylindrical mast, I have 5mm wall on the sides and 2.5mm front and back at deck level. Obviously this all tapers down with distance up the mast, but this is very ball park stiffness wise with Phil's boat which has nearly double Finn RM. My Theory is that, especially with the high compression from the narrow shroud base, a nicely done una could be a good bet in all apart from upwind in light airs. Can't remember what the finn mast height is (IC legality) but I reckon a fat bloke's finn mast with extra material added, and the right sail (inc shorter foot, full battens and roach) could be a good first attempt. Apologies for the long ramble Dan
  9. DC Designs

    OFP, Sorry I probably wasn't clear enough on the sheeting angles. What I was referring to was the optimum sheeting angles when the sails are pinned in sailing upwind, i.e when the helm is steering to the telltales with the AOA of the sails constant with respect to the centreline. The optimum sheeting angle would be the optimium jib lead angle for a sloop where upwind generally the main will be centred to keep the slot open. For Una rigged boats this angle is the boom to centreline angle (Una rigs i.e finn, laser) A mirror dinghy will have a fairly wide jib sheeting base, whilst an IACC yacht will be down to about 6 degrees. Similarly with Una rig boats, a finn usually sails with about 9 deg until it gets windy. I you tried that on an oppie (Less Aerodynamically and hydrodynamically efficient) you'd go slow and sideways. Similarly an nice efficient A class cat would have a fairly narrow sheeting angle. Obviously this is different to sailing off the wind where the course stays largely the same apart from when hunting waves, and the sails are trimmed to the apparent. JimC Can't remember off the top of my head where I got my info from the last post but it will probably be hidden in some textbooks like Aero/Hydrodynamics of sailing by Marchaj or Sailing Yacht Design Theory/Practice. I did my Nav Arch degree at a Uni where the consultancy wing do a fair bit of AC testing, so I'vbe got a reasonable handle on why stuff works in that respect. With regards relative leeway angles, I think that most dinghy classes with non gybing daggerboards will actually be suprisingly similar in this respect, just with more efficient classes making the same amount of leeway whilst pointing higher and/or going quicker. Sizing of foils is quite important - too big and you will be incurring too large a penalty from the extra wetted surface, too small and the induced drag penalty from running too high a lift coefficient will start to bite. For boat shaped foils operating at boat reynold's numbers, there exist optimal angles of attack at which the optimum balance between the induced drag ( increasing with smaller foils) and the friction from the wetted surface ( increasing with bigger foils) is found. This is a simplified explanation and will depend on things like foil section shape, aspect ratio and a bunch of other factors. This is blurred a bit for quick dinghies that plane upwind, for which freeing off 10 deg and going a fair bit quicker doesn't hurt the VMG too much. The sideforce generated by the foils remains the same assuming that the crew continue to generate the same righting moment, but since the speed has shot up, the lift coefficient will be much lower, meaning that the boat will now be making less leeway, and have more than the optimal amount of foil in the water. Dan
  10. DC Designs

    Gents. The reason that gybing daggerboards work (if done properly) is this: The sideforce that opposes the forces from the rig and propels the boat forwards is provided by the daggerboard, but also the rudder (usually by virtue of a couple of degrees of weather helm, as the rudder operates in the downwash of the dagger), and the hull, which usually has a lateral area of comparable size to the dagger. Whilst the dagger and rudder are fairly effective lifting devices (high - ish aspect ratio, good section shape) The hull is a highly inefficient lifting surface (very low aspect ratio, dodgy shape) and as such cannot produce anything near as much side force for its area as the dagger or rudder. More importantly, the induced drag penalty of such an inefficient body when dragged at a yaw(leeway) angle is huge when compared to deriving the equivalent amount of lift from the foils. A boat with non gybing daggerboard sails upwind through the water, with the hull and dagger making a course a few degrees lower than the heading (the leeway angle) The yaw angle or leeway is necessary to have an angle of attack over the lifting bodies and produce sideforce. If set up properly, the rudder should have some weather helm, such that it is carrying some of the sideforce, unloading the dagger and hull somewhat. The foils generate lift fairly efficiently, but the hull will have a proportionally massive induced drag for its modest contribution to the total side force. The benefit of a gybing dagger is that, if correctly designed, the dagger gybes a few degrees each side of centreline meaning that the dagger can experience an angle of attack while presenting the hull at zero angle of attack - i.e no leeway. This means that all of the lift creating the sideforce is generated by the (efficient) foils, and none by the (inefficient) hull, which now has no induced drag. This net reduction in drag means that the boat can either go faster at the same heading, or higher for the same speed. The latter is usually far more valuable in a fleet racing context.. Whilst gybing the board means that the foils are slightly more highly loaded, and thus run slightly higher lift coefficients than if using the same foils fixed, it is inconsequential in the context of the reduction of induced drag from the hull For this system to work well, the amount of daggerboard gybe has to closely reflect the leeway angle, which varies from class to class. Furthermore there has to be a mechanism to "centre" the dagger downwind, else it will slop around pointing in a different direction to the heading of the hull, thus creating unecesary induced drag between the two bodies again, not to mention control probs. Obviously there are some tricky things to get used to when first using a gybing board - little or no leeway meaning that conventionally foiled boats "fall away" to leeward Also, where people have had good judgement of their boat's abilities on laylines, the addition of a gybing board will prove tricky at first! As for the track of boat / sheeting angles - I think that optimum sheeting angles should be made with respect to the track of the boat, rather than the centreline of the boat. This would infer that a boat with gybing dagger should sheet a little freer than the non gybing boat, by an equivalent amount to the leeway angle of the non gybing boat. However, optimum sheeting angles are generally a function of the efficiency of a boat, with mirror dinghies sheeting far wider than IACC yachts. As the boat with the gybing dagger has improved its efficiency, it could go to a closer optimum sheeting angle, thus negating somewhat the previous paragraph. Basically, I reckon amidst the general noise and variables oif dinghy racing, the last 2 points cancel out. Dan