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@Horton Just use the equation, that's why I posted it. 2.500 x 6.850 SA = (2.697 * 6.850) + (4.841 * 2.500) - 18.924 = 11.653 in^2 2.510 x 6.850 SA = (2.697 * 6.850) + (4.841 * 2.510) - 18.924 = 11.701 in^2 Therefore 2.510 x 6.850 is 0.4% larger than 2.500 x 6.850. You can also rearrange the equation to solve for a depth or a length given a desired SA and the other known variable.

I posted this last night, but it appears to have vanished. It's never as good the second time around... I generated a fin surface area equation based on depth and length measurements. I used a 2016 Radar fin since I had one laying around and it seemed like a good industry standard shape. The equation obviously only holds for this fin shape, but it should hold good for the trends for any similarly shaped fin. I digitized the fin and used CAD to make the length and depth adjustments. The fin holes are accounted for. The equation is based on a simple linear regression model, which fit the measured data extremely well. Surface Area = (2.697 * Length) + (4.841 * Depth) - 18.924 The equation fits the measured data with an R-Squared value of 0.999. Here's the data out of CAD: Here's the same data in graphical form along with the predicted values from the equation. The measured values are the points, the predicted values are the lines.

@Horton I started a new discussion last night, when I submitted it I got a message saying it needed to be approved, it has since disappeared. Is this a new thing?

I've measured skiers anywhere from 400 to 700 lbs. force across a pretty good range of skill level. In general, the better the skier the smoother and higher the load. Some previously posted data.

My opinion, assuming the shape is identical you're pretty much just paying for flex and weight, or lack thereof. It's pretty difficult to take the exact same shape, swap out carbon for glass and still end up with the same flex. The only way to actually make that work is to add a fair bit of thickness to the ski, and therefore no longer the same shape. Shape is primary, flex is secondary, light weight is a distant third slightly above graphics, nice to have but not critical, within reason of course. So, if shape and flex are truly the same, save some money. But if you can get a better flex pattern for a few extra bucks and drop a couple ounces as an extra bonus then probably worth the extra cash. Flex = shape under load, very important. As a side note, there is much much more to laminate design other than fiber selection... far too complex for a late night internet post. @Horton "rebound rate"... you should consider a self administered panda. I'll do the math for you sometime if I really get bored. @vtmecheng the strain rate dependicies you typically see in a shock analysis don't really apply here, the effects are negligible, it's very reasonable to just assume linear elastic behavior. I almost never say this, but I think you're over thinking it... this coming from a guy who has strain gauges glued to his ski. Just buy the better ski and don't look back.

1. At the risk of showing my ignorance... what or who is an LOC? I keep waiting for someone to actually spell this out but it hasn't happened. 2. I don't really care to much since the scores will wash out pretty soon anyways, but if you are a M2 skier with scores from class C events that are carried into M3, would it not make sense to treat those scores as an "opt up" to 36mph? The current system for the rankings list knocks all of your M2 scores down by 6 as if they were scored at 34mph.

@marktueffers I have the data and would be happy to run the numbers for you if you're curious. I don't have my laptop with me at the moment, so it will be a day or two. For the sake of discussion however, I can't say I agree with the premise. To start with, lets say you're absolutely right, how would knowing the minimum average load help you ski better? Unless you typically ski with a load cell hooked up you would have no idea whether you are currently above or below your target. Secondly, and more importantly, here's a counter premise for you. The more load you can sustain the better. I'm not a believer of "light on the line". The faster you are cross course the more time and margin you have at the ball. I've measured a few skiers now, most of which are way better than me, and i can tell you most load the line harder than I do. The trick is they are able to sustain the loads without getting pulled up or breaking form. My belief is to be fast cross course you have to have good exit speed out of the turns and have good enough body position in the pull to be able to sustain the loads necessary to be early to the next ball. This all said, there is a difference between pulling efficiently and pulling against the boat. This line of thought assumes you are already more towards the efficient side of life. The better body position you have the more load you can sustain, the faster you are cross course, the more time and margin you have at the ball, the better position you can establish for the next pull, repeat... I suspect the 'light on the line' concept has more to do with good exit speed out of the turns so that the line load builds progressively as opposed to a slam and go style. Just a guess though, would love to hear the meaning from whoever started the concept.

@DefectiveDave my system is not exactly elegant, but it is quite functional. The setup is reasonably straight forward. A little bit of soldering, a little bit of fabrication, and a lot of time playing with Excel (our your number crunching software of choice). I'm not a software guy, so I generally stick to what I know. The whole system really hinges on the use of a data acquisition system (DAQ). There's several companies that make these that provide commercially off-the-shelf introduction models for the home user. There's nothing special about the collection side of the software, whatever comes with the DAQ is probably sufficient. Your restrictions with the DAQ are sampling rate and finding one with a built-in instrumentation amp. Load cells function using a wheatstone bridge, hence the need for the amp. If this is all making sense then you're probably good to go, if not then I'd suggest doing a little more homework before throwing any money at it. If you're really up for giving it a go send me a message and I'll fill in some of the details. Expect to throw away a few hundred bucks, several nights of computer time, a couple nights of garage fabrication, and lots of strange looks as you show up to the dock with a laptop and a mess of wires. As far as the load cell goes, make sure you're using one with sufficient load rating including a decent safety factor. I've measured skiers near 700 lbf of load (although he's a bit of a freak). I'm using a simple S-type load cell with some beefy load rated eye bolts at each end. I've made it the same length as the 41' off loop, so I just take that loop out of my rope. I'm not concerned about the tight radius of the rope at the connection since this is typical of how ropes are connected from one loop to the next. Load tested everything, did great.

@Mark_Matis precisely. @disland I definitely tend to drift in as I approach the gates (despite my best efforts), so I'm not too surprised that I'm slowing down quite a bit. @Triplett @ScottScott yeah I agree, there's definitely some imbalance there. Here's a graph of the -28/36mph pass with the handle path inverted and shifted one ball to the right. You might have to zoom in a bit, but there's definitely a deference from one side to the other. Here's the same handle path (-28/36mph) but with the rope load plotted along with it. I think it's really interesting to see the difference in how I'm working the boat in my heel side vs. toe side pull. It almost looks like I double pull at the wakes from 1-to-2, 3-to-4, and 5-to-6. It's crazy how consistent the imbalance is. It's almost like a flashing neon light identifying something that needs to be fixed. I just need to figure out how to fix it.

@Deke exactly... well mostly anyhow. I actually did it two different ways to see which was cleaner and as a means to double check my own work. The first was the angular velocity approach... convert angular velocity at each time step to a Cartesian velocity vector (i.e. centripetal velocity), then add the velocity vector of the boat to change the reference frame from the boat to the lake. The alternative approach was to map out the Cartesian coordinates of the skier (handle) at each time step with reference to the lake, then take the discrete derivative per each time step to get the magnitude of the velocity vector. The results from the two methods overlaid pretty well, but the low pass filter seemed to do a better job with the first approach. The filter just takes out some of the noise in the data. The trick is knowing how much and when to apply. The reality is it smooths out some of the peaks, so when the charts shows 48 mph as a peak velocity, the unfiltered data actually showed 51 mph. However, that 51 was such a finite peak that it was most likely noise. And if it happened to be real it certainly wasn't sustained for any duration worth mentioning. I think I just made that sound much more complex than it really is. All you really need is one physics equation and some vector math.

So this is kind of interesting... I measured a 28 off pass at 34 mph and 36 mph in back to back passes. I overlaid the handle path and the speed of both. The handle path is nearly identical, at least without zooming way in. I had to double check to make sure I was pulling the right set of data. So at least for me, I run a pretty similar path just at higher speeds. Peak speed at -28/36mph was 48 miles per hour.

@DefectiveDave No means for donations, but thanks for the offer. Correct, 2 handed gate with a glide. As far as my ability, I'm just an average skier but on a good day I can scrap through 32 off at 36 mph. I fully intended to video the passes that I measured, but somehow completely overlooked the obvious and didn't get the camera out. So this is not the measured pass, but from a set the week before, an opening pass at 28off/34mph. My bad habits are ever present, so it's probably a reasonable representation of what was measured.

Another night, another graph. This time I've added Skier/Handle Speed overlaid on top of handle path through the course. Speed is absolute speed relative to the surface of the earth in miles per hour. Personally I like thinking in terms of angular speed about the pylon much better, but I hear this question asked quite a bit... so here it is. Speed is the red dotted line and uses the scale on the right hand side of the graph. I seem to peak right about 45 mph somewhere around the whitewash on the way out to buoy line. A point of interest, your angular speed peaks much sooner, quite a bit before center line if things are going well.

Another night, another graph. This time I've added Handle/Skier Speed in mph. The speed is absolute speed relative to the surface of the earth. Speed is the dotted red line overlaid on top of the handle path through the course. For this pass I seem to peak right about 45 mph somewhere around the whitewash on the way out to buoy line. Personally I don't think absolute speed tells you much of anything useful. It's simply too dominated by the transnational speed of moving down course. But... I hear people ask the question quite a bit, so here it is. I get much more use out of angular speed relative to the boat. And no, they do not peak in the same place. Angular speed peaks much sooner, well before center line if things are going well.

The skier happens to be me this time. Right foot forward.

It's been about six months since I've made any progress with pylon tracking data, so it seemed like a good time to make my ski partners suffer through another day on the lake with computers, wires, and sensors (they are very kind! @Gloersen ). The approach I took before was to put strain gauges directly on the pylon and then extract load magnitude and load direction from pylon deflection. It worked pretty well, except when the load was too low the signal to noise ratio got pretty rough. So, I modified the setup and am now taking load data directly in-line with the rope and rope angle directly as well with a little swivel tracker. I'm pretty happy with the results. The swivel tracker had a few degrees of mechanical play in it, so I need to snug that up, but the data seems good enough to share. Here's some data graphed out (slightly low pass filtered). There is a lot you can do with this data; I find the hardest thing is to figure out how best to present it. It seems like a good place to start is simply to plot out the physical handle path, so here you go... Keep in mind the few degrees of slop in the swivel hardware, which explains why it seems to come up slightly short at the turn balls. I tried to scale the graph to the correct aspect ratio to make it visually correct. It's pretty much a saw tooth pattern. It will be interesting to overlay different line lengths and speeds. I had a few requests the last time around for a copy of the raw data, which I pretty much ignored (sorry about that). The data was just not clean enough at the time to really let it out. It's still not where I want it, however it's probably good enough that if anyone wants a copy just send me a message. I'd be happy to contribute to the wasting of free time.

Nice! The term you're looking for is galvanic corrosion. It's the interaction of the carbon, aluminum, and salt water that's making the magic happen. In this case the carbon skins are the cathode and the aluminum honeycomb core is acting as the rapidly corroding anode. Basically you turned your skis into batteries. It's the same reason aluminum fins and fin blocks get all chalky where they contact the carbon skins of a slalom ski. The process still happens in fresh water, just at a much slower rate that no one really cares. Salt water is a way better electrolyte.

Yeah... they have a small following of gullible test dummies, myself included. If you give this mod a go ensure you leave enough edge distance on the plastic shell for the bolts or the first time you release you'll shear the bolts clean out. Good stuff though.

Is that 36 mph??? I timed it three times, keep getting times much closer to 16.08 than 16.95. Awesome.

@So_I_Ski Here's some more data for you to ponder. This is a 34 mph skier running -32, -35 and -38 (a skier that is very capable of running 39 off). The gate shot is nearly identical for all three passes in terms of angular position versus time. If your angular speed is held constant, but your rope length shortens, your "speed across land" actually gets slower do to the geometry. The gate speeds (speed across land) below are 44.5 mph @ 32 off, 42.2 mph @ 35 off, and 41.6 mph @ 38 off. The angular speed can be visualized on the graph by the slope of the angular position over time. I have no idea if this is a purposeful technique, or simply scatter, i.e. I wonder if the results would still be the same if I had measurements from 100 passes at the same line length and averaged them. Maybe at 34 mph you make the speed jump at -39..?..?.. Then again, maybe 15 off through 35 off were so easy for Wade that he really didn't have to get after it until he got to 38 off. What I find even more interesting is that if we assume all this data is even close to accurate then we may actually be looking at a snap shot of perfect pass vs. zero off technique... or possibly just the different style between two different skiers. I did a quick look at some additional data from another open level skier running 32 off and 39 off at 36 mph. Interestingly enough his gate speeds were 42.4 mph @ 32 off and 45.8 mph @ 39 off. Pretty crazy hugh! I will say that this individual is very purposeful in maintaining a consistent gate regardless of rope length. Personally, I'll hold my conclusions until we have a lot more data from a lot more skiers. But... it is fun to think about.

@So_I_Ski that's pretty cool. Does the article by chance say how the measurements were taken? Or if they were repeatable at the different line lengths?

A little bit of thinking out loud, as in non-vetted thoughts... @Horton As far as giving away a membership with each new boat or high end ski purchase I think you're talking metaphorical pennies in return. For real change you need to think bigger and more long term. I think any idea for real change needs to be grounded with real data. I just tallied all the skiers off the current slalom ranking list (yes I only did slalom because I don't have hours of free time). I suspect many people have done this from time to time, but you can't dismiss the visual point it makes. If you want to grow the sport you need to go where the energy is, COLLEGIATE is the growth engine. @JeffSurdej I guess I could see a membership tier focused towards the recreational skier, but I have no idea what that would look like. Sanctioning of tournaments, governing of rules, and maintaining the rankings is the core of the value AWSA provides. I'd be impressed (pleasantly) if you can find something of real value that the non-tournament skier would pay for. A 20% increase in membership within one year is certainly a lofty goal. I have my doubts, but then again a few big ideas like collegiate alumni regional's (awesome event by the way) and you never know. However, I don't think flash moments of growth is true sustainable long term growth. Seems to me that AWSA should either focus all their efforts on retention of the collegiate group (said many times before, but the point is better presented with the data), or simply growing the collegiate group. I have a narrow view, but I basically only see three means that people start tournament skiing: their parents make them when they're kids, they join a college team, or they get a wild hair sometime later in life when they can afford the high cost of entry. Out of these three, growing and retaining the collegiate stream seams like the best target. I tend to think that there is a critical mass to retaining collegiate skiers beyond college. The main reason I pay to ski tournaments is to compete against the friends I made back in college and that list is getting pretty slim, and that is only possible through the rankings list. As far as actual ideas (the hard part): - Identify the top ten largest colleges/universities that don't currently have a team/club but have a body of water within 30 miles suitable for skiing. Post fliers and offer support to any student willing to start a team. Help find water access, help find a boat, help structure a club that can grow on it's own. - Waive the entry fees for any collegiate skier who shows up to any non-collegiate tournament (I think this is huge because it encourages the retention into regular tournament skiing). I never heard of competitive skiing until I went to college, and I certainly never skied an AWSA tournament until I was almost out of college. - Use AWSA membership money to offer financial support to any private ski lake willing to host a collegiate team as their practice or tournament site. - Offer on-site support (send out a tournament director with lots of experience) to any site willing to host their first collegiate event. - Very lofty idea... start digging lakes near major universities that don't currently have a team/club. My collegiate days are far behind me. It just seems if you want to grow the sport you have to go where the energy is. I pretty ignorant about how AWSA functions, I honestly have no idea where NCWSA ends and AWSA begins... is NCWSA a subset of AWSA? Regardless, collegiate is what fills the ranks.

No no @Wish ... just happened to randomly be in Charleston... nothing to see here. :wink: If you think those guys don't take advantage of every bit of data out there then you clearly don't comprehend their level of obsession.

@Horton Hardware all up is cheaper than a new ski. No plans to commercialize, at least not a hardware solution. The reality is everything is readily available off the shelf, the barrier is in the knowledge to make it all work and interperting the data. If anything, I think it makes a lot more sense as a service than a product. With the time it takes to review and analyze the data I can't see anyone doing it every set out. Most people don't even video all the time. Now if a ski school or other setting of the nature was offering it I think that could be pretty cool. Ski all day, review data at night. Once you know where your problems are it still takes some time to improve your technique. For example, I've looked enough at my own data now to know that I don't get across course near as fast on my toe side pull as I do heal side. I now have a specific area of focus for improvement, but it's still gonna take me months to become a better skier. Once I get the bugs worked out it'd be interesting to see if anyone wanted to be measured. Since it's so portable there's definitely some options available. The biggest value I see is in comparison to "gold standards". If you could have your data overlaid with a pro skier whose style you were trying to emulate, that would be sweet.

@Gloersen I'm pretty sure he was right foot foward (same as me), no idea on letter/number. @Jody_Seal I had heard there was some cool work done in that regard. Does any of that data still exist, and available for disclosure? It'd be cool to compare. I debated on how best to attach the insturmentation, we found a means to make the gauges exteremly portable so I can jump from boat to boat with minimal setup. The possibilities are endless, once the base acquisition system is setup it's pretty easy to keep tying in other sensors. We talked about tying into the cam or crank sensor... maybe some day we'll get around to it. @disland We video'd most of it, just need some more time to go through it all. @scotchipman There's a reason his pylon is always coming lose. The peak load is cool but somewhat expected if you've ever seen that guy ski. What was way more interesting is the speed he carries through the swing and how early he gets up on the boat. Once you have angular position plotted you inherently have angular velocity as well. For now it's just an experiment in what's possible, and to see if it holds any merit as a traing tool. In that regard I think we're onto something.