The most exciting or most disappointing upboxing ever

[SkiJay]

What the handle does is interesting as far as acceleration, the connection, timing, and rhythm are are concerned, but what the ski is doing would provide feedback on ski setup and skier inputs, which would really be fascinating. Both would rock, but either one or the other would still be a big step forward.

[bbirlew]

I was imagining the trace (or xensr) sensors would allow for a variety of placement, and are inexpensive enough to have multiple sensor for the same run (one on the ski, another on the handle, etc).

I was designing my app with that in mind.

 

 

[Horton]

Let the games begin

 

[Skoot1123]

Curios question I have: since the accelerometer will not always be in the "up/down" position (meaning it is angled whenever the ski is on edge) how will that affect the data transformations?

 

Ideally in the testing world I would have those accelerometers placed in a location that wouldn't have a pitch or yaw change. Would be neat to come up with a mounting device for the tracer sytstem that would always keep the sensor in an orientation that was XYZ only. To measure the angle or tilt of the ski with the water surface could be done with a "modified" bending beam (strain gage on the bending beam) setup.

 

Very interesting and challenging "problem" for sure.

[Horton]

@shoot1123 you lost me XYZ only?

[Skoot1123]

@horton - if the sensor was kept level (normal to the earth's surface). Sorry - I should have worded it differently.

[disland]

axis and direction would be nice, but believe velocity and acceleration alone would be very valuable

[gator1]

Well, in the "28 off from a heli" video I posted, for example, my buddy jim's handle is going 29.52 mph as the tail of his ski clears 2 ball, and 39.667 mph as the nose of his ski touches the near whitewater. And we know position, velocity and and acceleration at every 10th of a second of the pass.

 

Seems to be worthwhile data.

 

For those of you keeping track, he did PB this summer after about 15 years at 34 mph, and earned his lap dance two sets in a row. But I'm not sure he ever collected.

[Horton]

@gator1‌ how do you know the speeds?

[Horton]

First Trace test = fail

[bbirlew]

@horton, what went down?

 

[Horton]

Not sure it ever got the GPS signal. I had it in the open for more than 10 minutes. I was able to get GPS (all blue lights) a few day ago at home.

 

I gave up and set it to record and went skiing. When I got home it said to was downloading to my phone but nothing. I do not know if the data was there or if I misunderstand the UI or what.

 

Will try again tomorrow. Fingers Crossed.

 

[bbirlew]

Hmmm... Ya. I think we are so spoiled these days with our phones using the Agps (assisted gps - basically uses cell towers and known wifi locations to very quickly get a rough position and assist in finding the satellites - at least from my limited understanding, I think that is how it works!). I remember some of the old receivers taking like 10min to get a lock... Perhaps that is a bit of what you are fighting with this device?

[gator1]

Video protractor shows me rope angle vs time. Trig and calculus in pics does the rest.

[rayn]

@horton I have a much more interesting and useful project for @Than_Bogan‌'s intern that we have been speaking about in the boat, please do not distract him from that. If I can just keep his mind spinning on that he might actually solve the remaining issues.

[Horton]

@rayn a robot with 6 boobs?

[mwetskier]

' @rayn a robot with 6 boobs? ' @Horton -besides you and ray who would the other 4 guys be with the robot?

[Horton]

I captured a bunch of data tonight and will make it available to everyone tomorrow

[Horton]

Well this is a start.

 

https://www.dropbox.com/s/vh93elo93vqwcir/data-141102-00-20-16-000.gpx?dl=0

https://www.dropbox.com/s/vkk0kk9qybq625d/data-141102-00-20-16-001.gpx?dl=0

https://www.dropbox.com/s/t9dwgkzc8rmytdk/data-141102-00-20-16-002.gpx?dl=0

https://www.dropbox.com/s/qbgx288dnx579fi/data-141102-00-20-16-003.gpx?dl=0

 

[Horton]

Captured more today. I will be in contact with the guys at Alpine Replay. The data so far is no bueno

[gator1]

While you guys are waiting for GPS to get a lot better, this is the kind of data we can get real time. In addition we can get position mapped onto the course and acceleration mapped onto the course.

[Horton]

Real time with a protractor and slide rule?

[gator1]

No. Real time with an iPad and an app.

[Skoot1123]

Interesting graph @gator1 - I am suspecting that the handle speed and position could help us determine where the optimum position to start pulling would be. I'm not surprised that boat speed and handle speed "match" at the buoy, but I am suprised the handle speed slows down so much. HMMMMMM Overlay or match that with the video and it'll be pretty neat - if not useful!

[Horton]

The handle is going slower than the boat for about .9 seconds between the ball and the wakes? I am going to need an explanation @gator1‌ ?

[tbrenchley]

@Horton‌ I assume the handle's speed is reduced because you are reducing the width you created on the boat at the ball and allowing the handle to get back to the center of the wakes where the most force and speed would be applied.

[gator1]

@Skoot1123‌ Jim has gotten slowed down to boat speed at the ball. That's good, means he won't take much of a hit. A tenth of a second later, he kicks the tail, pivots the ski. Makes a wall of water. That wall is the evidence of him slowing down. As we've seen on other posts, maybe 2gs of deceleration. That's a lot of slow down.

 

He won't speed up until the rope goes tight. But now the ski is pointing at the boat. No pull on the rope. So he is still slowing down. He's a good skier, so he rips his arms into his core as/after he pivots. Creates a little load on the rope, so he starts to accelerate a little bit. As he feeds his arms out, he gets up to about 30 mph. Finally, about 2/3 from ball to whitewater, his arms are finally straight and the rope is getting rapidly loaded, so rapid acceleration begins.

 

@Horton, look at the video, with the perspective that you won't speed up after the ball until the rope goes tight. You'll see it doesn't get tight until 2/3 to the white water.

 

When I look at that graph I can feel each part of the decal/coast/accel, and to me its right where the graph says it should be.

 

Wouldn't it be cool to see what this chart looked like for Regina or JR at 39.5?

[Horton]

I guess your speed parallel to the boat would be slower than the boat before the wakes and faster after the wakes no matter what you do.

 

I do not see how a skiers water speed could be less that the speed of the boat for nearly a second unless the ski was basically pointing at the boat.

[Horton]

Oooo well I assume a tight line.

[gator1]

Replace wake with centerline and you'd be right. Even at 43 mph right before he crosses the cneterline jim's speed parrellel to boat (Vx) is less than boat speed.

 

This is what I love about data: forces us to think about what happens vs what it looks like.

 

That or my math is wrong. @Skoot1123‌ you're one of the tribe.....is my trig right?

[thager]

Is it Spring yet?

[Skoot1123]

@gator1 - I'll try and take a look tonight - after I get the kids to bed! All the equations are there on the sheet - so I'll do a double check.

[gator1]

@tbrenchley‌ Yes. Relative to the boat, you are moving backwards until you get to the centerline. Just like a pendulum is moving down until it gets to the bottom of its swing.

[gator1]

@Skoot1123‌ Thanks! Don't want to tell the galactic leader he's wrong if I've got a math error.

[Horton]

You can tell me I am wrong all day long and use good math. Does not mean I believe the results are not wacky

[gator1]

I'm out. If you ever get your GPS to work (I'm betting you won't while I'm young enough to care, but if you do) I'll look forward to the results.

[E_T]

@horton ship that thing to me ill gladly try and get it working for you

[TylerR]

@Horton‌ I second @E_T‌ 's comment I bet he can fix it in no time flat.

[Horton]

So there is good news. I have been exchanging emails with one of the founders of Alpine Relay -owners of Trace. He is surprised by data I have gotten and wants me to do some testing. (lets be honest that first data set is CRAP)

 

Cross your fingers - perhaps I got a bad unit. I am still semi optimistic that there is something there for us.

 

[Skoot1123]

I am by no means a math "whiz" but I did look at the numbers and calcs that @gator1 provided/shared. The calculations as I have reviewed them are correct. Basically @gator1 combined the different components (vectors) of the handle into one direction using trigonometry. At a rope length such as 28 off the description of the HANDLE path is more like a pendulum. At shorter rope lengths the path has a more parallel component to it (as in watching skiers at 38 off and shorter) relative to the boat. Even if there was error in measurement with the protractor the graph does give us some interesting information. I don't know if Gator was evaluating the direction (or angulation) of the ski relative to the boat but we have to remember that the "extension" component (the reach of the arm, smear/turn of the ski, BEND of ski (so it is more like a bannana instead of a straight line)) is hard to account for (not impossible but a difficult component to evaluate).

 

@horton - lets say the speed and graph are correct? Wouldn't that suggest that at 28 off we have a LOT more time than what we think we do? It could also indicate that the skier carried his speed efficiently through the turn and indeed carried him back to the white water/wakes. With this being the case it suggests that unless the skier has a tight line connecting him to the boat he/she is always slowing down. It also shows the importance of being efficient - the interesting thing is WHERE to be efficient.

 

Good stuff @gator1 - this is fun. Lets keep at it!

[Horton]

@Skoot1123‌ the way I read that data it is simply impossible unless you rewrite the laws of physics.

[gator1]

Thanks @Skoot1123‌ next step is a pot or a rotary encoder at the pylon with an arm like one of the camera mounts. Simple data logger stores time stamped angle. Mark with beep from ZO. Dump into lookup table does the trig.

[Waternut]

Funny thing about math...right or wrong, if the people you try to convince don't understand it, it's useless. It's extremely frustrating too so I typically reserve math for the workplace. Glad someone is trying to quantify it though. Keep plugging away @gator1.

[gator1]

Nah. On careful, Double Bastard Ale fueled reflection, I'm done plugging away on this one. I've got the data I need. It'd be nice to get the same data set on Regina running 39, but that isn't gonna happen. It'd be even nicer to get GPS data real time, but going up to a satellite to ask it where it is in relation to something here on the ground 10 times a second is a long shot if you need to be +/- a foot or so. There's a reason nobody is doing auto drive in the course.

 

Not trying to save anybody's achilles with this deal. No dog in this hunt. Between the heli video, this data, responses to my posts on conservation of angular velocity, the "straight back leg" debate, some "pm" ing with the tech heads on this site,and too many hours of video watching I've debunked (in my own pea sized brain) the harmful teachings that I thought I couldn't understand because I wasn't a gifted athlete. Turns out they were just utter bull shit. "light on the line" for god's sake! "trailing arm pressure", and "open to the boat"/closed to the boat, "COM this" and COM that and on and on.

 

If you doubt the utter bull shit line, go back and read a magazine article on technique from 10 years ago. And I tried to make all that crap work. And the people who wrote it believed it just as earnestly as the people who write it today.

 

And while its kind of fun to get horton so exasperated he gives me Pandas and starts talking about his 45 days on this earth and the laws of physics, its not that fun. Now, if I can get him to bet me a new BOS tshirt against the one everybody dog cussed, then I'd get motivated to prove I'm right.

 

So, anyway, speaking of dogs and hunts its PHEASANT SEASON.

 

Anybody that wants to make a tracker like I described above, I'd be happy to give you a download of my thoughts. But they're probably worth less than what I'd charge you, which is a beer.

 

@Skoot1123‌ if you're down in springfield or out to spokane and want a pull, be sure to call.

[Moggie]

I find it hard to believe that a GPS based system is ever going to have the resolution required to accurately dissect the dynamics of water skiing.

 

To the engineers; is there not a small scale GPS-equivalent system out there where you’d only require two signal senders in the boat and a receiver on the ski to triangulate the receiver position. I don’t know what type of signal you’d use, infrared, ultrasound, radio wave, laser….but surely in this day and age there’s something that can provide an accurate (resolution of a few cm), dynamic, wireless triangulation within say 25m radius?

[Horton]

Sub 1 foot GPS is out there and is not super expensive. The guys working on auto steering are using it. Auto drive is said to be 18 to 24 months out. Testing units are already out there.

[Horton]

http://youtu.be/OQ4ckYbXYnY

[Moggie]

Just thinking of another way to skin the cat. Instead relying on a signal which has travelled 20,000km to the receiver, would it be more accurate and more reliable if it only had to travel 20m?

[Horton]

Geosynchronous Dones?

[Moggie]

That looks interesting.....