GUT-102: The Speed and Load Relationship

[Horton]

denaliskis.com

http://www.ballofspray.com/images/2016/Denali_Logo_Black.orig.png

 This is the second chapter in the Introduction to GUT series.

Thinking about skiing in terms of extreme limits helps us to understand the underlying physics. Let’s compare two different scenarios at the centerline (CL) of the course to help demonstrate the unique relationship between speed and load.

For the first extreme, imagine approaching the gates, but instead of pulling out to the left, you stay behind the boat and start right on top the left wake. The instant you reach the gates you slam the ski on edge and pull extremely hard to head toward one ball. For this example, let’s assume that you are able to reach 5mph of cross-course speed by the CL, with your ski angle 50° to the course. In a highly loaded position, you are moving much faster straight down the lake (36mph) than across it (5mph).

Now imagine the opposite extreme. Let’s say you are again coming into the gates, but this time you pull out to the left and get very high and wide, nearly passing the windshield on the boat. With a wide and early turn in for the gates you build a lot of speed and angle progressively, allowing you to reach a 40mph cross-course speed by CL, and like the first extreme, with your ski angled 50° to the course. This time you are actually moving faster across course (40mph) than down course (36mph).

Figure 1 illustrates the two extremes we discussed above. As you can see for Scenario 2, the 40mph cross-course speed translates into a 53.8mph resultant speed, while in Scenario 1, the 5mph cross-course speed translates into a resultant speed of only 36.3mph, almost the same as the boat. What’s important to understand is that even though the physical angle of the ski relative to the course is exactly 50° in both cases, the load acting on the ski will be completely different due to the speed and direction of water flow relative to the bottom of the ski.

Figure 1:Scenario 1: 5mph Cross-course Speed, Ski at 50°http://www.ballofspray.com/images/2016/GUT_102_Figure_1_Scenario_1.pngFigure 1:Scenario 2: 40mph Cross-Course Speed, Ski at 50°http://www.ballofspray.com/images/2016/GUT_102_Figure_1_Scenario_2.png

Figure 2 below illustrates how the water flow changes from one scenario to the next as a result of the increased cross-course speed of the ski/skier traveling through CL. In this graphic, the reference has changed to focus strictly on the ski rather than the course and the flow of water relative to it. When the skier has low cross-course speed, the water flows across the ski at a high angle of attack (42° in Scenario 1). However, as cross-course speed increases, the direction of the flow changes significantly even though the ski is still positioned 50° to the course. With the cross-course speed reaching 40mph at CL (Scenario 2), we can calculate that the water flows only 2° relative to the ski - despite the ski’s 50° orientation in the course!

Given this perspective, it is easy to understand why the load is greatly reduced as cross-course speed is increased. The faster you are moving cross-course at CL, the lower the ski’s relative angle of attack with the water and the lower the overall load on the skier. The speed and direction of the water flow across the bottom surface of the ski dictates the magnitude of the lift and drag forces acting on the ski, and consequently how much load the skier must overcome.

Figure 2: Scenario 1: 5mph Cross-course Speed, Ski at 50°http://www.ballofspray.com/images/2016/GUT_102_Figure_2_Scenario_1.pngFigure 2: Scenario 2: 40mph Cross-Course Speed, Ski at 50°http://www.ballofspray.com/images/2016/GUT_102_Figure_2_Scenario_2.png

Summary
Sustaining speed through the turns and developing high cross-course speed before CL is essential to maximizing your efforts in the pull and successfully running short-line passes. The better you are at generating and managing speed, the less unproductive load you will have, and the shorter line lengths you will be able to run. With a better understanding of the speed/load relationship and the importance of cross-course speed, you can begin to learn more about how to better control your position on the ski to further improve efficiency in the course.

[6balls]

Ok, so light on the line is not all wrong, correct? If done correctly...proper angle with high CL speed load should be lighter and efficiency increased.

 

[Wish]

I ski more passes with less effort like never before. Light on the line is a byproduct of the technique.

[Horton]

@Mark_Matis I think everyone at a high level agrees that you should finished the turn before loading. I do not if future GUT articles will exactly address this but what Chet is saying and what Adam & Adam are saying do not conflict

[ozski]

Yeah I have to say one should compliment the other when executed correctly.

[AdamCord]

@6balls this article is just explaining why having more speed will mean that there is less load on the ski. The faster you are going through the wakes, the less you are being dragged sideways down the lake by the boat.

 

I personally don't like the term "light on the line" because people tend to assume it should be applied everywhere in the course. The more speed you have, the less unproductive load you will have, but just not pulling hard won't get you the speed you need. Someone may drive for Nate Smith and say "he's feels so light on the line", and that's because he's so fast, not because he isn't pulling his brains out. The important question is "how do I get fast?"

 

Now to and answer your question more directly...yes, there is a time and place to try and be light. We will get a lot more into this in later articles that we're working on but the gist of it is that dropping the hammer and killing all your speed at the turn is counter productive. That will cause the load to spike, but more importantly it will make you slow behind the boat.

 

Remember that the objective is to get the handle as high on the boat as possible, as fast as possible. Everything we do needs to work toward that goal. We are working to get as many of these introductory articles that lay the foundation done as we can in the next few months so that we can get into the really cool stuff B)

[Than_Bogan]

This is great! I think you've come to a clearer and simpler explanation of this concept than the early draft that I was priveleged to read.

 

Important note to the audience: A lot of the GUT foundation material is really interesting and insightful, but doesn't directly tell you anything to do to ski better. Trust me that that is coming! But Denali feels, and I think they are probably right, that the "what to do" part only really makes sense after you understand some of the science behind it.

[Wish]

To add personal experiance to what @Than_Bogan said: Whats been frustrating for me over the yrs is to be told by a coach (payed at times) to do X or Z on the water and my skiing should improve. The why is often left out. So it's almost easy for coaches (not all) to contradict each other and themselves at times. Having been exposed to GUT allows me to take any suggestion from any coach and answer the question why this suggestion may or may not work and how it fits in with the underling goals expressed in the GUT science. Not everyone needs the why answered but for most, having a tool such as GUT allows for a go to philosophy that can help break down what's solid advice and what could just as well be left on the dock.

[FLeboeuf]

Are there precise numbers of maximum angle of ski vs water (or amount of lean) that you dont want to exeed depending on how fast you are going throughout the course in order to not overload and obtain maximum speed? Has it been figured out yet or is it not relevant?

[Adam Caldwell]

@FLeboeuf - We know based on geometry exactly what the minimum cross-course speed is to run the course - based on a straight line path (shortest) distance point to point. We can use that information to then understand what the minimum cross-course speed is at CL. Then, we can use known velocity data captured via radar-gun of the top pros to understand what cross course speed (at CL) it takes to run 41off, and compare that to the known minimum. That means we know the upper and lower limits of cross course speed. The interesting thing is the edge change is dependent primarily cross course speed and ski angle, so we can use resulting velocity data compared against the skis angle of attack to understand the actual edge change, and not only where it can happen, but WHY it happens. This gets very very interesting.....

 

All this goes into helping you understand what the challenges and objectives are on the water so you can focus on evolving your confidence, knowledge and technique overcome them.

 

The next sections of GUT will get into more and more detail. Stay tuned!

[Adam Caldwell]

@Mark_Matis - The graphics used were to show how much cross-course speed impacts the actual flow of water relative to the skis position. Speeds and ski angle used in the GUT-102 graphic are very possible at 39-41 off @ 36mph.

[Horton]

@Mark_Matis Your old HO is like a 1985 Porsche 911 - Cool but WAY outmatched by the current stuff. If you want to divert the conversation off GUT and onto new skis please make a new thread so we can keep this one mostly on topic.

[Horton]

@AdamCord and I have talked about this concept before but when it really clicked is when I related it to jumping. (I was a national competitive jumper in the early 90s)

 

When I took ¾ cuts I always had a huge amount more load and less speed than when I took a full double cut. With the full double cuts I was starting my turn at the pylon and making a ton of speed but my loads were too light at the bottom of the ramp.

 

Once I moved from doubles to 3/4 cuts my technically best and furthest jumps were from a wide ¾. The guys that were going a lot farther than I was had more of both and were doing it from a full double. They were pros and I was not.

 

The first time I tried to transition back from a full double cut to a ¾ I took the worst out the front of my career. The boat seemed to have so much more power and it pulled me right out of position at the bottom of the ramp. At the time I had no idea why. Today it is totally obvious. I had the angle I was used to but a lot less speed so I had much more load than I expected.

 

This is not really what this GUT article is about but to me it is a parallel and helps it make sense.

 

[FLeboeuf]

@adamcaldwell awesome! Waiting enthousiastically and impatientely

[Ed_Johnson]

One of the reasons I loved skiing with Chet was the fact that he would let you know the "WHY."

If your head didn't explode you would really LEARN a lot.

 

What the "ADAMS" are doing here is GREAT STUFF !!! They are really on the right track, and I can't wait for future editions...Thanks Guys !!!!

 

 

[FLeboeuf]

Ed_ johnson, agreed! I also really appreciate the scientific and objective approach to the sport.

[FLeboeuf]

@adamcaldwell, just to be sure, "skis angle of attack", is the angle of the ski vs the surface of the water ( or angle of lean if the body is perfectly perpendicular to the width of the ski)? Not ski vs CL, right?

[Adam Caldwell]

The angle of attack is the ski's angle (or position) relative to the CL of the course.

 

What you are referring to is called bank-angle, or roll angle of the ski. For simplicity, in or our discussion of GUT-102, the bank-angle of the ski is not discussed, but is obviously relevant in real-world conditions.

[AdamCord]

@Horton you are on the right track. All things being equal, a 3/4 cut will result in you being slower behind the boat and also having more load on the skis when compared to a properly executed double cut. This is because you are starting with a shorter runway, and thus you have less time and space to build speed. This applies directly to the gate in slalom. A properly executed high/wide gate always beats a narrow one for the same reason.

 

[ALPJr]

"A properly executed high/wide gate always beats a narrow one..." simple concept.

[Wish]

@ALPJr yes but I can be wide/high and slow. I can be narrow and fast and have a better ball one. So a bit more to it then just how high on the boat.

[AdamCord]

@Wish it's only timing and body position, how hard could it be?

[ALPJr]

@Wish agreed, much, much more to it. And have enjoyed following the GUT discussions. The quote I wrote from @AdamCord summerized it best for me - the importance or result of the "why".

[Wish]

@ALPJr haha... ya, completly missread your post and missed that it was a quote. Was to late to edit it by time I realized it. Ooops

[Razorskier1]

This is terrific! @6balls -- one of the concepts you have to wrap your head around is speed, the race to the buoy, and your relationship of acceleration/deceleration relative to the boat.

 

For example, last fall I was in the water talking with the Adam's about speed and shortline skiing. In the past I often felt that my problem at shorter lines was too much speed (at the ball). In fact, my problem was too little speed at CL. It takes a lot of speed to catch up to the boat and win the race to the buoy, so you need more speed, but at the right place! Said differently, at CL you will be going very, very fast, and that speed will let you win the race to the buoy. To achieve that you need to be accelerating from turn in to CL. On the other side of the wake (after CL), you will still be much faster than the boat, but will be slowing rather than accelerating. This will (a) give you the feeling of having tons of time at the ball and, (b) will mean that at your desired turn-in point, the boat will be moving away from you, providing you the line support needed to immediately begin accelerating again. When you get this cadence right, it almost feels like you could ski with a blindfold, because the buoys are just exactly in the right place to maintain the rhythm you established with your very first move up on the boat for your gate.

[Razorskier1]

Oh, in addition, at high speed you can actually pull quite hard without creating load. Pulling hard in the right places creates more speed than load.

[mbabiash]

With all of this in mind. How does zero off letters (A B C) come into play. Should those settings be what feels better to the skier or should we try to ski on a specific letter and change our skiing to a specific letter in order to get max speed at CL.

I'm not trying to start a zero off thread and it may have nothing to do with this theory but I thought I'd ask.

 

[ozski]

The answer=A2

[Horton]

@ozski No. Yes. Maybe. Depends on the boat and the prop & the skier skill level

[Wish]

@mbabiash so I have not run any letter other then C1 for yrs. Then I got on the Denali and while getting to know the GUT philosophy/science, I started to wonder the same. So for a couple weeks, I ran every letter and every number for multiple passes and sometime revisiting settings just to make sure. I did this without knowing what letter other Denali skiers were using and for the most part had forgotten what the letters and numbers were supposed to do (been on C1 for too long). The letters and numbers were chosen at random..sometimes my driver would just pick for me. So for me, A2 was the letter. Found out later that, and I may be wrong, most Denali skiers are using this same setting. What stayed constant for each test was conditions, site, boat, driver, line length (32) and ski settings. So I ended up ranking the letter and number from most consistent passes to worst.

 

A2

B1

C2

B2

C1

A1

A3

B3

C3

 

What was clear is the 3s seemed to be the worst..brutal actually..C3 about killed me. Letters were somewhat mixed as well as the 1s and 2s. I'm staying with A2. But I think there is room for folks to still find a letter they like. As you get into the nuts and bolts of what you need to do as a skier in terms of the race to the ball, I think it would be very plausible that your preferred letter may change.

 

 

[Than_Bogan]

Actually @mbabiash, I think that's an insightful question. I've always skied B1, but I found myself liking A2 at least as much when I was really trying to do this stuff (including stuff that hasn't been formally presented yet). Next season, I'll for sure be "starting over" in picking a ZO setting, because the way it interacts with this approach is different.

[mwetskier]

i haven't had time to read this entire thread yet so some one may have already addressed this. The reason that having faster speed behind the boat feels ' lighter on the line ' is because the boat's speed is a constant ratio of time over distance. at 36 mph the boat travels 52.8 feet per second downcourse so if your ski is turned 90 degrees to the wake and you're just standing there you will feel the entire load necessary to drag your side ways ski down course at 52.8 ft per sec. which will be such a big load on your body that you cannot possibly maintain your ski angle -you *will* be yanked out of your bindings or your ski will instantly be forced to rotate back to a zero angle straight down the lake. this is because the entire pull of the boat will be experienced in one big tug.

 

On the other hand if you are crossing the wake at high speed the boat's pulling force will be spread out across the course of your side ways travel so only you will only experience the entire load during the very brief moment you are behind the boat. Most people can sustain a very high load if it is for a very short amount of time which is why you can jump off a 2 foot high park bench onto concrete and not get broken legs. the impact will generate greater than 3.5 gs but our resilience allows us to absorb that momentary force -just not for a sustained period of time.

 

The amount of time a skier crossing the wake at 40 mph is behind the boat is less than 1/10th of a second. we can take a high load for that tiny amount of time if we try which is why carrying more speed is better and also why the boat driver *feels* what seems to be less load on the rope. result = ' light on the line '.

[Horton]

[wolfgang]

Not starting a new thread @ Horton , but back in the day I had a 1959 Porsche, it was a 356. The 911 came later. It was slow with surprising handling but very cool. Couldn't build up much speed in the straights so you never wanted to slow down for the corners!

[Razorskier1]

@mbabiash - i have always skied C1 or C2. After working with the Adam's, I tried A2, then A1. I never thought that would work for me. However, A1 was much better than C1. Why? Because with an A setting the boat is moving away from me at the ball, whereas with a C setting it is slowing down at that point and "waiting" for me. I want the boat moving away, so that when I connect again I've got immediate line tension and swing. This was counter-intuitive to me, but makes perfect sense now.

[mwetskier]

@Horton -i would have expected you being a trick skier would understand what i'm ' talking bout '. When you're behind the boat on a trick ski and you snap into a side slide all the water passing under your ski is putting pressure on the entire back edge of the ski and all the water passing under you is going straight behind you. that's a lot of pressure and that pressure gets relieved by the ski popping to the surface so the back edge is no longer submerged and fighting the load of the boat. So how come your trick ski doesn't do that same thing to relieve the load when you're cutting hard across the wake? the answer is because all the gallons of water per second that are pushing against the bottom of the ski are flowing off the tail of the ski as you pass behind the boat. That effectively dissipates the pressure before it can build up enough to pop your cutting edge out of the water.

 

the number of gallons per second that are rushing by under the ski is a constant of sorts but the pressure generated against the bottom of the ski is less at any given moment if the ski is constantly bleeding that pressure off the tail. less build up of pressure than there is against a slow moving or even *static* ski. so angular speed essentially equals ' light on the line '.

[Horton]

@mwetskier I believe that you believe that makes sense but I have no idea what it means.

 

Please reread the original article. Do you think the Denali guys got it wrong or are you just trying to rephrase what they said?

[AdamCord]

@mwetskier pressure is not a measure that has time in it. Pressure can be high, low, whatever, but time is not a factor with an in-compressible fluid like water.

 

I'm not sure if you read the GUT 102 article at the beginning of this thread, but it is attempting to explain that the direction of flow is a major factor in the load that the skier feels. There will be a great deal more drag when the ski is sliding sideways than when it is pointed in the direction it is going. The faster a skier is moving across the lake, the less the ski will be sliding sideways.

 

Does that make sense?

[mwetskier]

@AdamCord -i understand that pressure *alone* does not contain a time component. The time component i was attempting to involve in the discussion is the *length of time* a human body can withstand a given amount of pressure. Almost any human can withstand the momentary 3.5 gs generated from jumping off a park bench. Very few can withstand 3.5 gs for a sustained period of time.

 

Since the highest load against the skier's pull generally occurs right behind the boat it stands to reason that the shorter amount of time the skier is exposed to that load the better the skier will be able to withstand it -i.e. not get stood up with his arms pulled out. And the faster the skier is moving across the wake the less time he will spend in that high pressure zone. in itself the pressure does not have a time component, but the skiers ability to resist that pressure certainly does.

 

I *did* read the gut 102 article, and the way i understand it relates to flow rate and newton's third law. what i tried -and obviously failed -to explain above is that a certain amount of water is going to flow under the ski over a given period of time. with a ski turned side ways to the wake but *not* moving side ways the majority of that flow will be off the trailing edge of the ski -i.e. a side slide. but if the ski is moving across course then *some* of that flow is going off the tail of the ski and less is coming off the side edge.

 

The faster the ski is moving side ways the more water flows off the tail and the less flows off the side. that's where newton's third law comes in to play -' for every action there is an equal and opposite reaction '. I see the ' action ' as water flowing off the tail of the ski and the ' reaction ' as the ski moving faster across course. by directing the flow of the water under the ski the skier can convert the energy of the boat dragging him down course into the ski moving side ways across course. the more efficiently the skier converts that boat pull into cross course speed *when its easy to do* -such as out near the ball -the less energy he will have to exert to maintain that speed *when its hard to do* -such as directly behind the boat.

 

I'm not an engineer -in fact i've never even been on a train. so i can only try to state my understanding of the principles in terms that make sense to me -and often i can't even do that. please accept my apology for that short coming.

[Fehlindra]

Would like to see a skilled skier 16-13m perform a side by side video what we have learned from the begin wait until finishing the turn to get the best angle against the boat and this new pulling phase direct from the buoy (skilled in da matter so its possible to clearly see some difference)

[Adam Caldwell]

Sometimes a picture is easier to understand.

 

Here is a series of images to help fill in the gaps between the "extreme scenarios" discussed in "GUT-102 - The Speed/Load Relationship". This is just a more detailed perspective showing what happens to the direction of water flow (relative the the ski) as cross-course speed increases. The ski's angle (relative to the course) is fixed at 45 degrees, and the frame of reference is of the ski at the instant it crosses CL, where the skiers down-course speed is equal to the boats speed, and we can focus on cross-course speed as the only changing variable.

 

We will get into much more detail in the next few sections of GUT.

 

[Horton]

Slightly off topic but still in this line of thinking... In the last few years @AdamCord has helped me with fin settings a few times. The thing that Adam taught me (that seems so obvious now) is that the amount of fin you have in the water at the center line has a huge impact on load.

 

When we talk about angle and speed and load you have to remember the for the ski to move at the same speed & direction as the boat at the center line the ski has to be sliding. If you have too much fin in the water the ski will resist the slide so much that it will pull the handle off your hip. Cord calls this getting "peeled apart". I had to mess with this a lot for both the Warp and the Helix 2 reviews. This is when I learned that fin turning is important for wake crossings as well everywhere else.