The Swing

[Stevie Boy]

So, does the motion of a childs swing, resemble in any way the swing through the back of the boat, can you achieve the same pendulum effect of a childs swing, with the added forward movement of the boat or are the dynamics totally different ?

 

[Ed_Johnson]

You will hear many say there is no similarity..I DISAGREE !!!

 

To keep it really simple, without quantum physics explanations, just think of it this way...Both rely on acceleration, to create momentum, to carry them to an apex.

 

Apex is defined as change in direction, the slowest point, and the point where acceleration can begin, to again, create the momentum to carry out to the next apex.

 

The greater the acceleration, the greater the momentum.

 

I believe that is where the similarity resides.

 

 

 

[Wish]

http://ballofspray.vanillaforums.com/discussion/12801/is-staying-connected-or-keep-the-line-tight-misleading-or-misunderstood-terms#latest

[Stevie Boy]

@Wish thank you, interesting, just one thing though, on a childs swing you can gain momentum and increase your swing from a small start, surely a difficult thing to achieve once you have entered the course.

[bbruzzese]

Rossi and JB used to talk about this a lot. You work from the top of the swing to the bottom then ride the momentum to the top...much like working from buoy to centerline and riding it from centerline out to the buoy. I wish it were that easy...

[Ed_Johnson]

For Nate, he does make it seem that easy !!!

[skibumm]

You need to gain acceleration in both instances from the point of direction change. The difference is that you have the acceleration of gravity to aid you in the swing example and you do not have that advantage or aid in the course. That is what makes gaining angle so crucial to success

[mwfillmore]

Skiing the course is exactly like a pendulum or a baby swing... with rockets attached to the swing

[dstone124]

You may not have gravity @skibumm , but you do have the added "benefit" of the boat pull which could be considered an amplified gravity source. Like @bbruzzese said, JB used to preach about this concept quite a bit. Kids on a swing generate height or swing higher, by pumping their legs forward and backwards. You'll notice this pump happens at the bottom of the swing closest to the ground. Same on the slalom line. In my opinion, you don't gain width on the course by pulling from the buoy line to the boat guides. Rather by working more efficiently, stacked, and with good angle, behind the boat or as close to mid-line both sides as possible. This is why finishing the turn and skiing back to the handle is so imperative.

Therefore I think there are similarities that that can be transferred in concept from the playground to the course. To me, the best of which is when to work your ass off, and where to enjoy the ride.

[DefectiveDave]

@Stevie Boy ,

 

While it might be a helpful analogy for some, I don't believe the swing concept tells the whole story. The boat's pull should not function as gravity in the analogy, but is probably better representative of a fixed pivot point. For example, if the skier moved out to the side of the boat into a position with no relative velocity or acceleration and all the water/air were suddenly removed, the skier would remain stationary relative to the boat so long as the boat did not change speeds. Gravity would rather be best represented by the water's drag as it pulls you towards the point of least potential energy (directly behind the boat). The higher on the boat you are, the more potential energy you have. The goal of the skier would then be to increase the gravity heading into centerline and decrease it heading outbound towards the ball. This would allow one to extract energy from the water, so when we consider the skier as a system we are not considering it as a conservative system.

 

However, this is overly simplified as the water also functions as a viscous medium through which we are trying to travel. The gravity analogy only accounts for one component of the acceleration on the skier. The other component is that cross-course drag which must be minimized in order to build and maintain speed. I've diagrammed it all below going with this analogy.

 

 

It should be noted that once you are under the pull of the boat it is not possible for the ski itself to generate net positive acceleration in the cross-course direction (before centerline), therefore there is always drag which must be minimized by efficient ski position. In fact, on it's own the ski is always net decelerating and it is only possible to accelerate in a given direction by changing direction which increases net deceleration. So the ski only accelerates across course for a very brief period during the turn around the buoy, afterwards all acceleration across course before centerline comes from the boat. After centerline, all acceleration must come from the ski/skier and you don't want this.

 

As a caveat, after centerline it is actually possible to get negative drag due to the relative angle of the ski. Under some conditions this makes it actually possible to accelerate outbound after centerline. This is illustrated by the fact that one can accelerate away from centerline in order to prepare for the gates. However, there is a trade-off as this negative drag increases the "gravity" component. In general, this trade-off is only a positive gain when the relative angle of the rope from centerline is less than 10 degrees and the skier's velocity is relatively slow in course, but that is too involved for this post. At short line lengths you really don't have error margin to be able to get velocity after the centerline unless you are traveling slow, in which case you are probably screwed anyway.

 

So from the diagram above, it is best to stay connected and stop pulling against the boat in the vast majority of scenarios. If you are in trouble maybe you can keep pulling, but I wouldn't get your hopes up. Finally, if you don't stay connected you get a more drastic edge change which actually pushes you further up on the boat but decreases your width and ensuring you have less potential energy for the trip back to the other side, assuming of course you get to the ball in the first place. This is because when the boat catches up to you (along with the slack hit) you are not going to be as high on the boat, decreasing your potential energy and forcing you to accelerate from a slower initial speed due to the time it took the boat to "catch up".

 

Moving on, the goal of the skier is to get wide and high on the boat, in order to do so efficiently using this analogy under nominal conditions the skier must:

maximize the "gravity" component and minimize the drag component going into the centerline

minimize the "gravity" component and minimize the drag component going out to the buoy

 

Obviously we can play with these two principles or try to maximize the potential energy or "height" we get on the boat, but in reality the "gravity" and drag of the water are complex functions of each other, body position, ski shape, settings, etc. Also, we can't make a discrete switch from one edge to the other at centerline and must deal with a transition edge-change phase which could potentially be optimized or played around with. It's easiest to just generalize them into concepts which are approximately true and functional:

Good stack and body position going into centerline will maximize "gravity" and minimize drag

Edge change at centerline (or at least close to it) in order to reduce "gravity" going outbound

Minimize the extent of the edge change after centerline in order to minimize drag going outbound

• i.e. minimize the moment of the body to prevent an excessive, early edge change

• i.e. stay connected to the handle through the CG of the body

 

Someone else may very well draw different conclusions, but using this analogy I effectively reach the same conclusions that everyone discusses all the time. That's a mouthful and I don't know if it's all clear to others, but I think the ideas are all there.

[jlittle]

This thread sort of reminds me of... does the water move the sailboat or does the wind move it?

[6balls]

The swing in skiing is not the same as the swing point on a swing is fixed. Our attachment point is moving down course and the lines are not quite the same. Having said that, I believe many concepts are the same in terms of energy management in order to get high and thus wide as the line shortens.

 

 

[OldboyII]

It is a pendulum attached to baby swing ))

[gregy]

@6balls depends on the frame of reference. You could consider the boat fixed and the water moving.

 

There's also the Centripetal Force (CF) which is what I think gives the casting out feel after the edge change. Centripetal force is the force required to keep your COM moving in a arc. This force is provided by a combination the rope and Friction of water on the ski.

 

 

 

 

The less "connected" you are to the handle the more of the force will have to be provided by the ski/friction. This will cause the ski to slow and shut down, where as if you rely more on the rope to provide the CF you will swing more freely up on the boat. Which more resembles a pendulum.

 

As you shorten the rope the your radius ® decreases and your Linear Velocity (v) increases. r has indirect effect on the CF and v has a squaring effect on CF, so both increase CF. Velocity more so. When your at 15off and 22off its hard to get that casting out feeling I think because your CF is much lower and can easily be provided by the ski/friction relationship.

 

But I think this may all starts changing starting at 38off were the skier has a short cast out then has to kinda parallel the path of the boat to the buoy. When I watch Nate's videos at 39 and 41 he appears the be skiing a perfect line to manage slack vs. staying really connected. Which may be what @wish is eluding to on the other thread.

 

Unfortunately for me this is just theory because I'm a really bad skier.