wilecoyote

@skibrain, don't go there! There's also this one. http://www.youtube.com/watch?v=IV5aL_BcNCo

@eleeski You have no idea how badly I'm missing the fundamentals of running bouys! I'm sorry too. I have a good instructor, and I do what he tells me to. Well I try to at least. I'm in no way trying to come up with a new radical way to run the course or advocating rearing up at the apex. And I'm not looking for some magic piece or gear that will turn me into Nate Smith. My plan for the winter is to set my ski to factory, (if I can ever get the numbers), remove the wing, work on my pulling posture, put the new footbeds I bought from Wiley's in my bindings, and annoy everybody here with my hydrodynamic analysis until the ice melts. Seriously though, often a design element corrects a flaw elsewhere in the design. When I see holes and a wing, I'm thinking there are some major design issues being corrected here that might be more elegantly dealt with. But to come up with a better fin, we need to figure out what the current one is doing. But those holes... those annoying holes. So back to this tedious analysis of ski behaviour. I completely disagree that drag is unimportant simply because you have unlimited horsepower. Imagine a bag dragging in the water behind your ski sea anchor style. Same hp, same hydrodynamic lift as before, just a lot more drag. I don't think anyone would argue that this would be a bad thing. A more efficient ski will convert more of the side pull energy into cross course angle. Also if drag can be reduced, then you get to bring the tip a little higher at the apex without stalling out. The benefit, a slightly tighter turn. The "design is more art than science" argument is held by many of the board shaping community as well. I wholeheartedly disagree that it needs to be this way. Trial and error is a tedious path especially if the trial part is not informed by solid hypothesis. Virtually every aspect of ski design (rocker, aspect ratio, planform (outline) edge chamfer,) have been studied in controlled environments, and quantified. I've read some of this material but there's still tonnes that I haven't read. I'm not so naive to think that you can just read all this literature and build a kick ass ski, but if you approach the problem as an engineering challenge, you can conceivably make a major improvement almost overnight. These guys did just that. http://www.sailrocket.com/. The hydrodynamics and aerodynamics required to get this thing going that fast is far and away more complicated than slalom ski design, but they applied the theory did the maths and said this thing should do more than 65 knots, and they were right. @skijay. You may be right about the turbulance being from the wing. I'm not sure.

Wow where to start? First, @Horton, most of what I've stated is solid physics, it doesn't mater whether it's a slalom ski, a surfboard, a wakeboard an ironing board. It is fact that right around 5 degrees angle of attack (AOA)is the highest lift to drag ratio (L/D) a planing surface can have. Anything higher or lower than that will add proportionally more drag. Slalom skis, are usually operating well above 5 degrees AOA, so whenever you lower the tip, the drag should go down, that's a fact. However, if you look at the edge change photo above, the ski is riding right around 5 degrees. If you lower the tip at that point, the ski will fall below the magic 5 degrees and drag rises. Another major factor, is rocker. One thing that rocker does is stabilize the trim angle (AOA) The thing with rocker is, if there is enough of it, and you try to push the nose down, you end up creating suction at the tail and that creates a lot of drag as well, (also a scientifically proven source of "phantom rollers") It is a fact that the higher the AOA of the ski the tighter it can turn. (to a point) I have no argument with your statement; The sign of a skilled slalom skier is a more constant and tip down ski attitude. Big fast turns ending with the tip up equal loss of water speed and are basically a stall. If you stall or wheelie, you then have to re-create your water speed from scratch. The boat does not wait for you. The fact remains, the ski turns tighter as you raise the tip. But that doesn't equate to a better way around the ball for exactly the reason you stated above. You will stall out because you don't have enough inertia to maintain the ski at such a high (inefficient) AOA. It's the same as when you turn the boat really tight, you have to add power big time to keep the boat at speed. "If all your thinking is biased the idea that you want the less ski in the water at the apex - you need to start over. " Dude, if you saw me ski, that's exactly what you'd say! In fact that's what my instructor said the first time he saw me ski. But I'm getting better. I know that keeping the tip low through the turn is important because you are coasting pretty much from the edge change to the exit of the turn, so you must balance carrying speed with turning tight. But I wasn't saying that you should try to get as little ski in the water as possible at the apex,I was just stating the way the ski works. There are basic laws of physics that cannot be ignored. The ski turns because of the lift created. When the ski is not on edge, all the lift is simply holding the ski (and skier)up. Once it goes on edge the lift is both pushing the ski sideways and holding it up. More overall lift is required. The only way to get more lift, is to either increase the AOA, increase the surface area (more ski in the water), or increase the speed. Any time you alter one condition another must change to maintain equilibrium. My point about a flat ski being able to corner and accelerate better than a rockered one was prefaced by the statement "Assuming you had the skill to move your weight around correctly" you don't. (don't take that personally, nobody does) It may be possible to design a good rockerless ski, but I doubt it. As I said, rocker helps to stabilize the trim of the ski. I'm going to throw this one out there, I don't know, I'm just supposin', a low rocker, smaller than normal ski in the hands of the -41crowd can really perform. Stick the same ski in the hands of someone like me or even someone in the -28 group and it will hinder our progress. I'm just guessing, but would you guys agree? @Skijay, I'm with Eric on the spray thing. Spray is not an indication that the ski is sliding, it is an indication that the ski is planing. The amount of spray is related to how heavy you are, and your AOA. At the tightest part of the turn we are at our heaviest because of the g's created at the turn, and we have a higher angle of attack Thus tonnes of spray. Low AOA = small spray. I'm not saying that the ski doesn't slide, just that spray is not an indication of it. But those photos are a wealth of information, thanks for putting them up! What I'm really keen on is the apex photo, there is a small trail of turbulence coming off the fin. That little trail (often 2 spaced about a fin length apart) is in every single video and photo I've examined where the camera angle and resolution is right to see it. @eleeski, how do you play with rocker? whew! That took me three days to write. Chris

The measurements have been taken. (sort of) http://www.soe.uoguelph.ca/webfiles/jruncima/Waterskiing%20top_files/Water%20Skiing%20Biomechanics-%20a%20study%20of%20intermediate%20skiers,%20non-publisher%20release.pdf Unfortunately with less than ideal skiers, and with an eye toward lightening load for recreational skiiers. None the less, these are solid numbers and we can assume better skiers would generate more than the peak numbers achieved by these skiers. Max recorded speed was 1.51Xboat speed. Max rope load not including deep water start, was less than 1.5X weight. The author of the subject is a prof at the University where I work. I've been meaning to introduce myself to him. Maybe we could use his gear and measure some short line guys in the course.

So that's what those nets are on some guys handles. I had no idea what they were for. @Klundel I'm not sure I understand which type of crash you would throw the handle away on.

@richarddoane. I was going to ask exactly that. Being new, I need to develop good habits. My first hard learned lesson is to back out if you don't have good position. My driver tried to save me, but I went OTF at the second wake, and got a sprain bad enough to end my season because I tried to pull when I didn't' have good position. Watching this, I thought, exactly what you said, but I don't know if there's a good reason not to.

Chris Young here. I skied a lot on my late teens but never in a course and never with any idea of what I was supposed to do. I'm 49 now and decided to get some instruction and start skiing the course in late August of this year. I'd like to say I've got 3@15 but Cobber (Hi Geordie) is one of my drivers, at McClintocks in southern Ontario, and he knows better. I went OTF on my last pass of the season and sprained my ankle. I'm also on a CDX, (Hi Omland). Got hooked so fast I went out and bought a Supra for the cottage. It's no Nautique, but it'll give me something to practice behind. I've been a serious windsurfer since I was 18, and still am. Ski when it's calm, windsurf when it's windy.

holds water he he that's a good one. Our misunderstanding comes from which angle of attack we're discussing. When I'm talking about the planing surface, I'm talking about the ski. As the tip of the ski rises, the ski is able to turn tighter.(your lever analogy applies well here because the centre of lift moves aft as the angle of attack increases) Originally I thought that the fin was operating at a high angle of attack as well, but now I'm convinced that's not the case. As you and Eric have postulated, the fin is really only keeping the ski pointed in the right direction so it's angle of attack cross course is likely only a degree or so. In the turn however, we're putting a lot of pressure (you guys much more than me) on the fin. According to Rick (Maui Ultra Fins) a foiled fin is required for the slow speeds more than the high speeds. Apparently at the low angles of attack that the fin experiences cross course, a flat plate will perform nearly as well as a foil. It's the slow speed, high load part around the buoy where a foiled fin could earn it's keep. I had read the "ski is a lever" article before, but only quickly. The tuning of fin and binding placement is consistent with windsurfing and surfing (no surprise there) but a few of the assumptions are a bit off (if I may be so bold) I think the ski as a lever concept is sound, but the conclusions as to why are not quite right. As the ski's attitude (angle of attack) flattens, believe it or not, the drag goes down. That is until you get below around 4 degrees, at which point it starts to go up again. Additionally, it's not the drag from more fin in the water that brings the tip down when you add more depth or move it aft, it's the increase in lift on the fin (pushing the tail deeper) and where that is on the lever. If we disregard the brief moment of edge change, the fin is always pushing the tail of the ski deeper, the less fin you have, or the further forward it is, the less it is able to overcome the skiier and the higher the tip will be. The problem is balancing a tight turn by a high angle of attack with too high an angle of attack and having the planing surface stall. Once that happens, we have to take too much rope load to accelerate. At this point we need to get weight over the front of the ski to get it to a more desirable angle of attack, but as we all know too well, you do that and the boat will just yank you into a face plant. A high performance ski is a finicky beast, just move your weight a bit one way or the other and it'll make you pay. That's the reason why there's so much rocker in a ski. Rocker tends to stabilize the angle of attack. Assuming you had the skill to move your weight around correctly, flat ski would actually accelerate and corner better than a rockered one, but boy, you think your teeter totter is unstable now... There is a misconception that a rockered board is "looser" in surfing and windsurfing because the board follows the curve of the rocker. The truth is, it's all about the angle of attack of the planing surface (see how I brought this all back to angle of attack? pretty cleaver eh?) The rocker stabilizes the planing surface and the board/ski rides at about the same angle of attack a bit less dependent on the rider's weight distribution. Now, after all this discussion, extra curricular reading, and sleepless nights, I'm beginning to think a better fin design may have very little effect on our skiing. Go figure. Being the animal I am though, I'm going to have to experiment anyways. Too bad my skiing sucks so bad, I'll hardly be able to offer much of an informed opinion. I am, right now, however, heading out to the garage to remove the wing. I'll put it back on when I know I'm heading into the ball too fast. Probably never.

@Horton, I thought that was a bit vague when I wrote it. A higher angle of attack of the planing surface has more lift, but also more drag (L/D continues to decrease above 5 degrees). Also,the higher your angle of attack, the tighter the turn. The problem therefore with a higher angle of attack is that the ski will be slower cross course. So what I was saying was that we might be able to keep the same overall drag by reducing drag on the fin and then capitalizing on the lower drag by getting the tip up a bit more so as to turn tighter. What I meant by higher drag putting you further down course at the finish of your turn, was misstated. What I meant to say was that the higher drag ski (assuming all other things such as angle and exit speed are equal) would loose more ground down course than a lower drag counterpart. We're trying to convert as much of the force from the rope to speed as possible. So a more efficient (higher L/D) ski will do that.

First of all, I've said it before, but I want to make sure everyone knows, as a slalom skier I'm really really new. I'll let you know when I finish a pass at -15. So I'm coming at this with a windsurfing design background, and an outsiders view of slalom ski design. I totally agree Eric. A very narrow planing surface, high angle of attack, copious rocker, concave, are all design aspects that kill the L/D. These design aspects are the necessary evils required to make a ski that can corner really aggressively. What I suspect, is that a lower drag fin will allow the ski to accelerate out of the turn better, and while it's all well and good to say that the power is unlimited due to boat speed, the more overall drag on the ski, the further down course you go at the finish of your turn as the ski accelerates. The problem now becomes, when you get to the ball, you want a tight corner, and the slower you go the more feasible that becomes. Hmmm. What if you had a slightly higher angle of attack? It kills your cross course speed but makes the ski turn tighter. So we get a bit of cross course speed back by taking out some drag on the fin. I think that's why beginners like me should take the wing off, but the 36mpg guys need it. At slower speeds, the wing just makes it harder to get the ski up to speed, but at higher speeds, you'll never make the turn without slowing down. The holy grail is a super slick ski going behind the boat that the will decelerate as soon as you start to lean into the turn. A lower drag fin would need to be accompanied by a new rocker line and binding placement. Not a simple thing methinks. On the cavitation front, I've been scrutinizing all the video I can find, and it appears at first inspection that the fin begins to cavitate about 3/4 through the turn and lasts only a fraction of a second. If the only thing we get out of a foiled fin is eliminating that little bit of loss of lift, it might allow a slightly more agressive turn. Sorry for the essay, but this stuff really grabs my brain. Chris

Absolutely a barn door will generate lift. But it's L/D ratio sucks. I've been having a side conversation with the designer at Maui Ultra fins, (Dietrich Hanke, who has made significant change to windsurfing fins by applying the science from a pretty well informed background) his input and DW's has got me rethinking again. I think I had assumed way more load on the fin, and a higher angle of attack than it actually experiences. So we're back to an attached flow on the low pressure surface, which, then means if we pick a good foil section, we can reduce drag. How much is another question completely. Chris

Cool, nice to see some quality analysis! I'm still leaning towards the cavitation theory though. We're dealing with a flat plate at a pretty high angle of attack here. (I think) From what I've read, (and I'm still reading because this 'hole' discussion has really got me thinking) is that even really well designed foils start to cavitate at around 50mph. A flat plate is hardly a well designed foil and I'm sure short line skiers are hitting well above 50 at the highest rope load part of the pass. I've been mulling over some simple methods to get some real numbers as to cross course speed and angle of attack, and when I get a chance I'll see what I can come up with. If anybody has some real data, I'd love to see it! Either way, if we're not operating in cavitation, then a proper foil would be the answer to a much improved fin. If we are, then a symmetrical supercavitating foil would be the answer. @Skijay, even the fins on an arrow (or rocket, or tail of an airplane) are creating lift in order to create the stability you're describing. That's how it works, as soon as the fin is not 100% in line with the flow (water or air) it creates lift that wants to push it back in line. Chris

@AB thermostats are very simple animals. Any that have failed on me have been open, but it can easily stick closed or halfway. Chris

@SkiJay, I agree with pretty much everything you said in the last post. But note, I'm not saying that the thinner leading edge has less drag simply because it's thinner, I'm saying that a thinner leading edge promotes cavitation, and thus brings the fin to a lower drag condition quicker. Same goes for the holes Until I manage to get some proper numbers though, all this is just conjecture. We really need to know the angle of attack of the fin at all points through the pass, and as hard as I try to visualize it, I can't really make a decent guess. I'm envisioning me drilling some holes and making a special measuring thingy for this summer... Cheers, Chris

@SkiJay I had considered that, but felt that if you optimised the foil for the pull section of the pass it would work well enough for the rest of pass, to end up with a considerable net gain over the current flat plate with holes. Then last night I started running some basic calculations based on assumptions of what the angle of attack would be and it seemed impossible for the fin to be producing any lift at all. (in the traditional sense with laminar flow over the upper surface) Which of course we know isn't true so what gives? I've been looking at the problem all wrong. I'm convinced now that the fin is operating in cavitation. That's why the flat plate fin works so well, it's actually better suited to operate in this region than a foil would be! The holes facilitate the cavitation. So through the turn, the speeds and angles of attack are in the region where a traditional foil would be better, but as we hook up and accelerate, the fin transitions into cavitation, and the holes actually encourage this. It's just a theory, but I'm pretty sure this is what's going on. This also explains Horton's observations that a thinner fin feels faster, the sharper the leading edge, the better the fin will cavitate. As a result the drag will be lower. Chris

thager, I think you hit the nail on the head, if you foil it, you make it so much more effective that it would need to be much smaller. What you did in essence is put a much bigger wing on the fin. That's my thought with the main fin as well, I think we could make it much smaller if it was correctly designed. I think if it was done correctly the ski would accelerate better and hit higher top speeds, which of course would mean earlier to the ball with less effort. But where's the fun in that? Chris

@ DanE The surgeon took a nickel out of your knee? How the hell did that get in there ;-)

@Horton, lift is the generic term used in aerodynamics and by extension, hydrodynamics to describe the pressure differential. In this case lift can be in any direction, not just up. The fin has 2 main functions on a slalom ski: keeping the nose of the ski pointed forward, and keeping the tail from sliding out in turns. For the first, every time the fin is at any angle to the direction of travel it will begin to produce lift which naturally pulls the tail back in line. The second case is really the same as the first, except the turn is exerting a lot more force. I completely agree with DanE when he says This is, however a by-product of the main function of the fin, yet I think it may be the most important aspect of the fins function to study. I don't agree however, that an overly big fin will blow out because it creates too much lift during a turn. As I said early on in this thread, I suspect the shape of the fin works similar to a delta wing in that it never really stalls (total loss of lift due to overloading the fin) A delta wing stalls progressively, whereas a normal wing (windsurfing or surfing fin) stalls all at once. This may in fact be desirable in order to allow the tail of the ski to slide a bit, but I'm not so sure. Back to the question that started all this. I'm pretty sure that holes effectively make the fin act like a smaller thicker fin with no holes. The holes will "vent" pressure from the high (pressure) side to the low side, which will create turbulence and kill lift. What may be going on here is a kind of "stall proofing" where the turbulence created by the holes are creating some kind of flow pattern that keeps the flow attached. The fact that you guys , (I'm way too inexperienced to feel any of these tiny tweeks) can feel a .005" difference in thickness may have to do more with flex than with drag, but this so complicated that without someone who really understands the fluid dynamics and can do the math it's all just supposin'. I am 100% confident though that a far better fin shape awaits, starting with a foil. Food for thought! I think I'm full! Chris

With all due respect, I think you guys are missing the point here. There are two basic types of drag, parasitic and induced drag. The induced drag is a result of the fin producing lift, and the parasitic drag is the friction of the fluid moving across the surface. You're focusing on the parasitic drag but the big number in drag for us is the induced drag. The only point in your pass where the fin is not producing considerable lift (and thus considerable induced drag) is at the neutral point in your edge change. Therefore, having a more efficient foil will have a pretty large effect on the overall drag being produced by the fin.

@ Brady, that stings. But the funny thing is I started watching it and now I can't stop. he he. I won't be watching football or eating turkey today, all there is is NFL and I'm working anyways, thanksgiving was almost a month ago. The proper game is on Sunday when the Argos meet the Stampeders in the Grey Cup. @ DanE based on the fact that a brand new D3 blade is advertised for 12 bucks and a decent windsufing fin is over $100 you're right about extra cost.

I kinda figured it was something like that. Still pretty cool though.

@ DanE,The fin, whether foiled or not, will create lift. It's job is to stop the tail of the ski from sliding sideways. As soon as the ski is pointed in a different direction than it's direction of travel, the fin starts "lifting" in the opposite direction. The difference between a foiled shape and an unfoiled one is that of efficiency. The ratio of lift to drag improves as foil shape improves. If we were to foil our fins, they could become smaller, and have less drag. The burning question is though, is this desirable? I'd love to know what happened with the Goode experiments.

LOL @Brady, note the 'very' in brackets. Lots of smart folk here, not sure I qualify though. When I was dating my wife at her University in the '80s I looked up and photo copied everything I could find on hydrodynamics pertaining to planing surfaces in the engineering library. I was shaping windsurfers (still am) and wanted to be a bit more methodical than the trial and error method. Unfortunately for me, I never went further than high school math, so i never got the full picture. It's pretty dry stuff but I'm re reading a lot of it with an eye to water skis now. I have some ideas that I'd like to try, shaping a ski or two should be a lot of fun. But back on topic, I'm a huge fan of the guys at Maui Ultra Fins, and I've been threatening to talk to them about ski fins. BTW, love the hole story at Maharaja, doesn't matter if it's true or not, it's cool.

Cool, no relation, WileCoyote is a kindred spirit, I've been using the alias for years. I'm always building some contraption or other and throwing caution to the wind. I just bought a boat and it's likely to be christened SupergeniusArt Shcoll IIRC was the guy with the Super Chipmunk, I remember that shot of him from the cowl facing backwards on the intro for Wild World of Sports I think. I wanted to fly for as long as I can remember and I flew light singles for a short time in my teens, but ended up as an Avionics Technician. You guys (and those nut cases air racing) the were super cool when I was a kid, nice to run into you on the net. So that flight through the hangar, in the Bond film, was that for real?

Coming from a windsurfing background and as a (very) casual hydrodynamics student, I find ski fins quite perplexing. The two things that make no sense to me are the lack of foil shape and the holes. I'm quite certain that this is something that has evolved over decades of trial and error. I'm still struggling from a hydrodynamic standpoint to understand exactly what we want a slalom ski to do. My initial suspicion as far as the fin goes is that it is a variation of a delta wing, which never stalls, it just keeps loosing lift as the angle of attack is increased. I'm no where near a good enough skier to have any kind of opinion on whether we actually want the ski tail to slide or not, due to diminishing fin lift but, it is certainly better than having it suddenly loose all it's lift, at which point we will most definitely fall. (in windsurfing this is known as spin out) I'm also very surprised that the very small adjustments to the fin can make such a big difference to how the ski performs. If we're using callipers to make fore and aft adjustments, adding or moving a hole should have massive consequences, yet you see all kinds of funky hole patterns. I dunno, my head hurts.