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shepherd
08-13-2005, 12:21 AM
Does anyone know of any tests done to find out the amount of force that is absorbed by a ski rope when a slalom skier is cutting across the wake full bore??? It seems like it would be a simple matter of placing some kind of strain gage in the rope. A non-skiier asked me this once to get an idea of the strength required to ski at a high level.

Any scientists out there with the proper equipment to do the test???

ski_king
08-13-2005, 12:36 AM
A device called a load cell that can read out the force.
(You were right, a load cell has strain gages in it.)

shepherd
08-13-2005, 12:56 AM
A device called a load cell that can read out the force.
(You were right, a load cell has strain gages in it.)

Yeah, that's what we'd need. But it would have to have some way of recording or marking the max force, kind of like a g-meter that tells you the max g's you had...

NatesGr8
08-13-2005, 02:51 AM
You could mount strain guages on your ski pylon to measure the maximum deflection, then from there, there are some equations in which you could use to calculate the force, since the properties of aluminum are readily available. Howevery you'd need some kind of data logger that could keep track of very small voltages since strain guages put out very little signal. Another option would be to make some marks on the rope of a known distance, then measure the change in that distance while the skier was skiing, and then use the maximum deflection and tensile characteristics of the rope to calculate a force.

MasterMason
08-13-2005, 04:25 AM
I would think you could calculate it with speeds. The boat is going away at 36 and your going accross at 80 or so I believe. Too late to break out a pencil but I would bet it wouldn't be that hard to figure out.

Upper Michigan Prostar190
08-13-2005, 10:34 AM
please dont make me break out my old college Physics book........ :uglyhamme

Leroy
08-13-2005, 10:44 AM
Put a scale in line with the rope. If you could find a fish type scale with hooks on each end.

I was reading in the official water ski association how they qualify boats to be "official ski boat" and one criteria was the pylon had to be able to handle 1323 pounds of pull. I would guess a good skier pulls 150-200 pounds at peak.

6ballsisall
08-13-2005, 12:05 PM
Cmon Leroy, you are an engineer. Break out the calculator and get us an answer :D

Hoosier Bob
08-13-2005, 12:10 PM
I would bet higher than 200. MAybe not every day Joe but tourney guys for sure. I will let you know as I have one of those Zebco fish scales and a cousin I call the "Test Monkey!" He will do anything. I think the scale will explode but that is his problem! :uglyhamme Put a scale in line with the rope. If you could find a fish type scale with hooks on each end.

I was reading in the official water ski association how they qualify boats to be "official ski boat" and one criteria was the pylon had to be able to handle 1323 pounds of pull. I would guess a good skier pulls 150-200 pounds at peak.

6ballsisall
08-13-2005, 12:13 PM
I would bet higher than 200. MAybe not every day Joe but tourney guys for sure. I will let you know as I have one of those Zebco fish scales and a cousin I call the "Test Monkey!" He will do anything. I think the scale will explode but that is his problem! :uglyhamme

I can guarantee you there is WAY more pull than 200lbs. Theres more than that pulling them out of the water. If you use a 200lbs+ scale please wrap it up REALLY tight in a towel and put tape around it so when it explodes and springs go flying the driver doesn't end up with a spring hanging out of the side of his head!! :eek:

John B
08-13-2005, 12:16 PM
I think if you contact PerfectPass and ask them, they would know all that info.

Hoosier Bob
08-13-2005, 12:18 PM
I don't think I'll be doin' that to my test monkey! I save him for the really stupid stuff! Cooler skiing, paddle skiing and whatever else you can find on a boat. That would make a good thread. Any saucer, paddle or cooler skiers out there?I can guarantee you there is WAY more pull than 200lbs. Theres more than that pulling them out of the water. If you use a 200lbs+ scale please wrap it up REALLY tight in a towel and put tape around it so when it explodes and springs go flying the driver doesn't end up with a spring hanging out of the side of his head!! :eek:

Leroy
08-13-2005, 12:22 PM
Bob; Definitely a job too simple for Evil Otis! :uglyhamme


JR; You are thinking this through too much, you are going to scare away the test monkeys! :D

I don't even know how to figure this one out with resistance from water. I studied a different kind of fluid mechanics in college that came in 12 oz packages.:friday:

Hoosier Bob
08-13-2005, 12:26 PM
Dat's funny! Did your studies continue after college? I am going to get on the Waterski mag site and see if there are any references. I would guess over #400 and that may not be high enough if you factor in impact shots.Bob; Definitely a job too simple for Evil Otis! :uglyhamme


JR; You are thinking this through too much, you are going to scare away the test monkeys! :D

I don't even know how to figure this one out with resistance from water. I studied a different kind of fluid mechanics in college that came in 12 oz packages.:friday:

6ballsisall
08-13-2005, 12:26 PM
I studied a different kind of fluid mechanics in college that came in 12 oz packages.:friday:


Hey, when I was in College I studied hard on that subject too and always passed with flying colors. I never could find the Dean for that department though......... :uglyhamme

LakePirate
08-13-2005, 12:30 PM
Not that I claim to be an engineer, but wouldn't the weight of the skier factor into this as well. Length of rope woud be a factor, look at a skier with a rope that bounces at 75 feet but stays taught at 15 off.

I do know that a cheap ski rope will not withstand the pull to get a truck stuck in the mud out. :D

Leroy
08-13-2005, 12:39 PM
Funny lakepirate! Didn't logic like that come from a Monty Python movie?

When I ski I feel like I'm stuck in the mud at times!

Leroy
08-13-2005, 12:40 PM
Wasn't the that the Dean behind the counter! :eek:



Hey, when I was in College I studied hard on that subject too and always passed with flying colors. I never could find the Dean for that department though......... :uglyhamme

Hoosier Bob
08-13-2005, 12:46 PM
How do you know she is a Witch? Cause she looks like one!!! :D She turned me into a Newt! Funny lakepirate! Didn't logic like that come from a Monty Python movie?

When I ski I feel like I'm stuck in the mud at times!

Ric
08-13-2005, 12:47 PM
I think if you contact PerfectPass and ask them, they would know all that info.
I'm with JB
take the easy route and call PP
they have already done all this for us!

LakePirate
08-13-2005, 01:00 PM
A newt? Burn Her!

sorry for the t/j

MarkP
08-13-2005, 01:02 PM
Did you guys fig-er it out yet??:popcorn:

bucky
08-13-2005, 01:20 PM
The only comparison I can make is the seated row machine and the deadlift. I don't know about competition slalom, but weight I understand. Getting up only feels like around 140 lbs. The max I pull on the seated row is 200. While professional skiers are terrific athletes, it would be difficult to maintain form with that much.

Leroy
08-13-2005, 01:34 PM
Bucky; Remember you are not pulling, but just holding on stiff arm. I'm with Bob, really good skiers probably over 400 lbs peak.

It could take awhile Mark, unless someone takes JohnB's advise and calls PP! I would guess MC would know also.

Hoosier Bob
08-13-2005, 01:37 PM
You need to change that avatar right meow! I am going back to work meow!A newt? Burn Her!

sorry for the t/j

LakePirate
08-13-2005, 01:39 PM
You need to change that avatar right meow! I am going back to work meow!

Meow? I can say moo.

Once again I apolozige for the t/j.

PendO
08-13-2005, 02:07 PM
what if you had some overhead video, and a radar gun ... then you could do some vector analysis (another beer) ... I'll bet that the rope companies have this all dialed in.

Ric
08-13-2005, 02:12 PM
what if you had some overhead video, and a radar gun ... then you could do some vector analysis (another beer) ... I'll bet that the rope companies have this all dialed in.
t/j
didn't schnitz want to do an overhead survey somethingoranother for a slalom lake for his skiing by coordinates dr jim thing?

wonder why dr jim never ponied up for that idea?

Hoosier Bob
08-13-2005, 02:48 PM
I have only found the minimum resistance a pylon can have to be AWSA approved is 1323 lbs from all angles up to 70 degrees. That's expecting a lot but I am still searching!

JDK
08-13-2005, 04:04 PM
Not that I claim to be an engineer, but wouldn't the weight of the skier factor into this as well (Yes, slalom skiing is all about acceleration and deceleration, and it takes more force to accelerate a heavier body to a given speed over a fixed length). Length of rope woud be a factor, look at a skier with a rope that bounces at 75 feet but stays taught at 15 off.(No, even though the weight of the longer line is very small, the bounce is acting over very small angles, which provides a lot of mechanical advantage.)

I do know that a cheap ski rope will not withstand the pull to get a truck stuck in the mud out. :D

My bet is that the hardest pull experienced by any skier is when getting up.

MarkP
08-13-2005, 04:26 PM
My bet is that the hardest pull experienced by any skier is when getting up.I disagree with that..

Hoosier Bob
08-13-2005, 04:44 PM
I would have to say the slalom cut is the hardest. I have seen a newbie getting up launch a pretty nice rope though! Falling/wipeouts have to give a nice reading, especially for those who don't know how to let go! I pulled up a 250lb friend of mine and I think he went right to the bottom of the lake! I know he was putting up some numbers! This will all be answered soon and we can get back to skiing!I disagree with that..

Hoosier Bob
08-13-2005, 04:45 PM
Mark how about a green Barney? :banana: I disagree with that..

propstruk
08-13-2005, 05:53 PM
You could measure the deflection of the pylon during the highest loaded portion of the pull. Then all you would need is the measurements of the pylon and a few of those engineering formulas that helped to put me on academic probation.

Upper Michigan Prostar190
08-13-2005, 06:03 PM
I agree with mark and hoosier. Coming out of the corners is definetly the strongest pull on the rope. Being from an engineering background, and not "fluid" mechanics either ;), I have to say the easiest solution to this problem is already been offered here. Simply Put a device in the line to measure force, for example a good scale capable of up to 400 to 500 lbs. pull. TO use physics, and math it would be almost impossible to calculate this force. there are SO many factors to bring into play: the static coefficient of friction of the ski bottom material with respect to the water. weight of the skier, moment of inertia of the skier, the surface area of the ski that is submerged in the water at the given time of the hardest pull( how can you measure that?). then there is the rope stretching too. that will mucky up your results. To honestly measure this, you should be pulling with a rigid cable that WILL NOT stretch, otherwise your losing some of the force in the rope stretch, thus giving you a false reading on your scale.

I think the angle you PP guys are thinking is calculating a factor of time that the skier slows the boat temporarily while he cuts his corner and then calculate the force exerted from that. I dont know if that would work as the force is not constant. it peaks for a split second and is over. the amount of time you have to measure the change in speed is so small. I cant think of a way to do it accurately. Physics is SO much more complicated than Statics. once you put motion into the picture, the whole ball game changes. Its much more difficult to figure out. I was a master at statics problems, but I sure wasnt a master at physics problems.

Now I am not Albert Einstein or anything. I am just offering my thoughts on the idea. I may be wrong on all counts. Its been a long time since I graduated college and actually had a working knowledge of this stuff. I havent cracked a physics book in 10 years. Just my :twocents:

anybody else got any ideas?

Upper Michigan Prostar190
08-13-2005, 06:08 PM
You could measure the deflection of the pylon during the highest loaded portion of the pull. Then all you would need is the measurements of the pylon and a few of those engineering formulas that helped to put me on academic probation.

this would work, but my question is how would you accurately measure the deflection of the pylon? :confused:

I still think a scale with a rigid "rope" is the easiest way. I am not taking credit for that scale idea either, someone else posted it here way before me. I just think its the best idea so far, and by FAR the most practical.

propstruk
08-13-2005, 06:15 PM
this would work, but my question is how would you accurately measure the deflection of the pylon? :confused:

I still think a scale with a rigid "rope" is the easiest way. I am not taking credit for that scale idea either, someone else posted it here way before me. I just think its the best idea so far, and by FAR the most practical.

You are right. The easiest way IS to measure the force on the rope.

To do what I was talking about you would have to get a fixture to hold a runout gauge at the top of the pole. Not very difficult but the scale would be a lot cheaper.

MarkP
08-13-2005, 06:40 PM
Mark how about a green Barney? :banana:
Iíll find the right one soon enough.. ;)

shepherd
08-13-2005, 07:06 PM
I do know that a cheap ski rope will not withstand the pull to get a truck stuck in the mud out. :D

It won't pull a stump out of the ground either :o

shepherd
08-13-2005, 07:11 PM
The only comparison I can make is the seated row machine and the deadlift. I don't know about competition slalom, but weight I understand. Getting up only feels like around 140 lbs. The max I pull on the seated row is 200. While professional skiers are terrific athletes, it would be difficult to maintain form with that much.

That's what made me think of it years ago when I was hitting the gym as hard as I was skiing. Doing the seated rows, I'd hold a short bar like I'd hold the slalom handle when I did my set. I was pulling around 160 on the seated row and it felt comparable to a slalom pull.
I wouldn't go by the rope strength or pylon strength requirements because I'm sure they safety-factor the hell out of those things -- like 4 or 5 times or more...

Like we've been saying here, we need some kind of load cell in-line with the rope. Anybody have one???? ;) I wonder where we can get one. Sounds like it's time for a google search.

Leroy
08-13-2005, 08:02 PM
Here you go Sheperd! Group buy and we can pass around to see what you can "pull"!


http://topnotchscales.com/s/index.html

6ballsisall
08-13-2005, 08:15 PM
Group buy

http://topnotchscales.com/s/index.html


Leroy you said the naughty words on here! You're gonna get grounded by the powers that be dude!! :eek:

MarkP
08-13-2005, 09:32 PM
Here you go Sheperd! Group buy and we can pass around to see what you can "pull"!


http://topnotchscales.com/s/index.html
Thatís actually a cool Idea.. I didnít have a chance to check out that link too far but I will after dinner:headbang:

MarkP
08-13-2005, 09:59 PM
Here you go Sheperd! Group buy and we can pass around to see what you can "pull"!


http://topnotchscales.com/s/index.html
Leroy,

Did you see one at that site that you think would work?? I didnít. Well, I did see a nice one for my kitchen.

Thank you for that..

Workin' 4 Toys
08-13-2005, 11:28 PM
Fish scale? One know the ones they weigh the sharks with.

Hoosier Bob
08-13-2005, 11:36 PM
That would work but you would have to have someone constantly monitoring the scale. It would be hard to register a wipeout, deepwater start or I guess 5-6 people on a tube. That thing would be jumping around like crazy. Leroy can hold 600 pounds static so next time we go out I am going to put the rope around his neck! That should give us an idea! :friday: Fish scale? One know the ones they weigh the sharks with.

mlange
08-14-2005, 12:33 AM
The PP Switch is triggered at around 250 lbs of force and the PP engagement underscore between the RPM's and Speed on the display changes to a dash when the switch is on.

Obviously, the pros are pulling/creating over 250 lbs to engage the switch on every pull. I cannot consistently trigger it, but am only working on 22-off.

Mike

Leroy
08-14-2005, 01:56 AM
There are some nice scales on there. This one should work for most of us!



Leroy,

Did you see one at that site that you think would work?? I didnít. Well, I did see a nice one for my kitchen.

Thank you for that..

MarkP
08-14-2005, 06:55 AM
There are some nice scales on there. This one should work for most of us!I see it now..........

bucky
08-14-2005, 08:22 AM
We may be overcomplicating this. Some of the engineers out there can calculate rope tension based on lean angle and skier weight. Not as exciting as buying test equipment, but close enough.

shepherd
08-14-2005, 09:49 AM
Actually, this is what I had in mind but it's too expensive:

http://www.transducertechniques.com/TLL-Load-Cell.cfm

You'd also need to buy the electronics for it too work. Looks like the industrial scale Leroy found would be most practical as long as you have an observer who can watch the scale.

I work at a Navy lab. Maybe I'll ask one of our engineers if they have any ideas/surplus equipment.

If I get one, I'll be happy to pass it around to anyone willing to pay the shipping and promising to post his/her results here...

jrandol, are "group buy" really naughty words? why?

ski_king
08-14-2005, 09:59 AM
Actually, this is what I had in mind but it's too expensive:

http://www.transducertechniques.com/TLL-Load-Cell.cfm

That is what I was thinking also. I use them at work alot.
I was thinking of a 1000 lb. one and put an eye bolt in each end and put in put it in a rope about a foot from the pylon. Problem is the electronics require 115 VAC. Also, they are easily damaged and my boss may not be too crazy about the idea.......

JDK
08-14-2005, 01:49 PM
We may be overcomplicating this. Some of the engineers out there can calculate rope tension based on lean angle and skier weight.
Nope, not unless the skiers rate of acceleration is also factored in. Determining this accutately using only calculations is very complicated.

bucky
08-14-2005, 02:06 PM
The skier's distance from the boat is fixed, and the boat speed is constant. There is no acceleration relative to the boat. Just line force balanced by gravity, with a balance point somewhere between the skier's feet as the fulcrum.

edit: there would be some centripetal (centrifugal?) force, but that would come in as speed in an arc around the boat increased, and at best equal the initial hard pull.

Nope, not unless the skiers rate of acceleration is also factored in. Determining this accutately using only calculations is very complicated.

Upper Michigan Prostar190
08-14-2005, 10:34 PM
you can contemplate this till the cows come home. JDK said it best, this is VERY complicated,actually its nearly impossible. there are so many factors that you cant measure. I mean how are you going to measure how much ski is in the water during the turn? how are you going to measure the skiers angle? and how then are you going to measure the moment of inertia of the skier?? Riddle me that one Batman!!! :) This is beyond engineering all the way to sophisticated physics. YOu would need a serious physicist to be able to calculate this with math. Just throw a scale in the non stretchable line, and be done with it. If anyone can REALLY figure this out ACCURATELY with math, then I highly suggest you submit a resume to NASA because you have a future buddy. :D

and bucky, there certainly is acceleration in respect to the boat, you get slack in the line right? that means you traveled faster than the boat to produce the slack line, thusly you have accelerated. the skier isnt going 36 mph on the same axis as the boat when he turns at the bouy, and then when he comes out of the corner he is gaining speed to be in effect "catching up" to the boat. Remember, anytime you increase in speed, you accelerate.

Jorski
08-14-2005, 10:52 PM
Guys,

It has already been done. in the eighties Dave Benzel invented LISA a device that went between the rope and the pylon and recorded load and angle (and I believe they also measured skier speed with a radar gun) several times per second during the skiers path.

Schnitz is trying to get a group together to design LISA II...my recollection was that pro skiers peaked out at 55 mph and 500 lbs of load. Remember, that is a peak momentary load, it does not mean that all pro skiers could dead lift that amount.

email schnitz or post the question at the nichols site...guarantee a quick answer to your question.

Upper Michigan Prostar190
08-14-2005, 10:55 PM
Thank you Jorski!!!! :toast: :wavey: Good info!! :)

erkoehler
08-14-2005, 10:57 PM
Good info, thanks!

JDK
08-15-2005, 01:53 AM
The skier's distance from the boat is fixed,agreed and the boat speed is constant.agreed There is no acceleration relative to the boat. disagreed, when the skier is in the process of CHANGING direction, he/she is accelerating or decelerating and increasing or decreasing the rope pull Just line force balanced by gravity, with a balance point somewhere between the skier's feet as the fulcrum.
Think of it this way; there's only 2 ways the line force can increase (1) the skier turns or (2) the boat accelerates.

edit: there would be some centripetal (centrifugal?) force, but that would come in as speed in an arc around the boat increased, and at best equal the initial hard pull.

I've got to think about the certifugal force thing for a while :confused: :o

Bruce Carr
08-15-2005, 11:05 AM
In the early 90's, I built a "LISA" machine myself. The system measured line tension with a load cell, rope angle at the pylon (with a angular potentiometer), and boat speed (through a pressure tranducer connected to one of the pitots).

The signal from the pitot scrolled a moving map of the course and overlaid the theoretical handle position onto the map. The line tension was displayed as a bar graph along the edge of the moving map showing the tension at any point along the course.

To answer your specific question about the line loads, we routinely recorded 800-900 lb spike loads just after the turn (usually caused from a slack rope hit). Smooth skiers usually maxed out at 400-500 lbs right behind the boat.

Bruce

ski_king
08-15-2005, 11:11 AM
Thanks Bruce.
Do you still have LISA?

mlange
08-16-2005, 04:01 PM
Jorski is correct. WSM did an article on this sometime within the last year or two and even showed differences in skier speed vs. line length. There was a bump in speed from 35 - 38-off as I remember, which I believe Wade Cox attributed to being one of the possible reasons that many amatuers can run 35-off but can't hit 38-off.

Anyway, Wade, Jamie and others are all right around 55mph when running 41-off at 36mph.

Mike

shepherd
08-16-2005, 04:08 PM
You see, I figured that somebody must have looked into this before. And I knew that this was the place to ask... :)

shepherd
06-03-2007, 12:18 AM
To answer your specific question about the line loads, we routinely recorded 800-900 lb spike loads just after the turn (usually caused from a slack rope hit). Smooth skiers usually maxed out at 400-500 lbs right behind the boat.

Bruce

Just watched the West Coast Slalom DVD. They had a line load measurement device while Terry Winter and Marcus Brown were skiing. Peak "spike" loads were around 700 lbs coming out of the turns, and they were holding fairly steady at around 400-500 lbs across the wake.

Forces of similar magnitude were measured at 28 off. No wonder my back is kiiling me! :(

tex
06-03-2007, 12:26 AM
220 or 222.

Leroy
06-03-2007, 02:22 AM
That is good info, think about dead lifting 400 pounds.

In the early 90's, I built a "LISA" machine myself. The system measured line tension with a load cell, rope angle at the pylon (with a angular potentiometer), and boat speed (through a pressure tranducer connected to one of the pitots).

The signal from the pitot scrolled a moving map of the course and overlaid the theoretical handle position onto the map. The line tension was displayed as a bar graph along the edge of the moving map showing the tension at any point along the course.

To answer your specific question about the line loads, we routinely recorded 800-900 lb spike loads just after the turn (usually caused from a slack rope hit). Smooth skiers usually maxed out at 400-500 lbs right behind the boat.

Bruce

88 PS190
06-03-2007, 05:33 AM
There have been pylons pulled from boats.

Marcus Brown had at one point a blurb about pulling over a 1000 pounds on a strain gauge in shortline.

Go figure.

Watch footage of Jamie Buschesne running shortline, particularly his ugly passes and then try to say he's not holding back atleast 800 in part of that crossing.

JimN
06-03-2007, 10:48 AM
"I can say moo."

Moo. Pasteurization is good for milk, bad for beer. Moo.

JimN
06-03-2007, 10:53 AM
Bucky- "There is no acceleration relative to the boat"

You don't think the skier pulling as hard to the sides or backward when they're crossing the wake is changing the boat's speed or vector? They wouldn't need tracking fins if there was no lateral forces that "re-aim" the boat. The skier's speed and acceleration are all relative to the boat.

JimN
06-03-2007, 10:58 AM
I'm surprised none of you car guys chimed in about this. Anyone remember the G Force Analyzer? It mounts is a car and is used for tracking G forces when on a race course, skid pad or road. The pulling force on a ski rope will be completely dependent on the mass of the skier. The G forces, if different skiers take exactly the same path, will be the same. Impossible to reproduce but in the same path, the same. Find the G forces, multiply by the mass and you have the tension in the rope.

UMP- How old could your physics books be- 3-5 years? Geez- mine are probably older than you.

stevenm2
06-10-2007, 10:19 AM
I thought LISA was the inflatable from the CPR class I took!
this is an interesting thread,however the thought of ALL the varibles is giving me a headache.

TMCNo1
06-10-2007, 10:55 AM
I'm surprised none of you car guys chimed in about this. Anyone remember the G Force Analyzer? It mounts is a car and is used for tracking G forces when on a race course, skid pad or road. The pulling force on a ski rope will be completely dependent on the mass of the skier. The G forces, if different skiers take exactly the same path, will be the same. Impossible to reproduce but in the same path, the same. Find the G forces, multiply by the mass and you have the tension in the rope.
UMP- How old could your physics books be- 3-5 years? Geez- mine are probably older than you.



I'm looking for a G String Analyzer,
Find the G string, multiply it to the masses and you have tension!!!!!!!!!!!!:cool: :rolleyes: :D

bucky
06-10-2007, 10:49 PM
Yeah, I've been wrong before. I don't remember what I was thinking, since it was 2 years ago.
Bucky- "There is no acceleration relative to the boat"

You don't think the skier pulling as hard to the sides or backward when they're crossing the wake is changing the boat's speed or vector? They wouldn't need tracking fins if there was no lateral forces that "re-aim" the boat. The skier's speed and acceleration are all relative to the boat.

boxman3
06-10-2007, 10:58 PM
Sounds like a problem for "Myth Busters!!!" I really do want to see the math on the speed crossing the wakes... Come on guy's!!!!