Written by: Adam Cord and Adam Caldwell
This is the third chapter in the Introduction to GUT series.
This chapter discusses a few concepts behind “moving with efficiency” on a slalom ski. Top level skiers have the ability to travel extremely fast behind the boat and carry a lot of speed throughout the course. They can do so mainly because they move with good Center of Mass (COM) positioning over the ski, not because they have super human strength.
The Skateboard Example
When riding a skateboard, your COM must be very close to perpendicular, or what is called “normal”, to the board in the fore/aft plane. As long as your COM is positioned somewhere between both feet, you will be fairly well balanced. To turn the board, you simply shift your weight laterally while still maintaining your COM position in that normal fore/aft plane and the board turns and moves with you. However, if you lean mostly on the back leg as you turn, with your COM behind the rear wheels of the skateboard (like the infamous “drop to the tail” style turn on a water ski), the board will shoot out in front as you fall to the ground. In this situation, thanks to gravity, your COM moved in a direction no longer supported by the board.
In slalom, positioning your COM in the normal plane on the ski is the key element to achieving efficiency in the course. Even though it is possible to ski the course with your COM positioned toward the tail, relying excessively on the rope for support, a tremendous amount of effort is wasted. With enough strength it is possible to survive at longer line lengths with a “back seat” COM position, but progressing into shorter line lengths will be extremely challenging.
The Efficient COM Position for Slalom
Let’s look at what an efficient position should look like on a water ski. From a side view, the COM should be positioned over the ski such that a line drawn normal to the ski’s top surface would pass through the skier’s COM, similar to what is described in the skateboard example above. Additionally, this normal axis must intersect the ski at the midpoint between the balls of the front and back feet. The overall attitude of the ski as it rides in the water is primarily controlled by the location of your COM over the ski. Settings can be used to fine tune the ride characteristics, but the COM position is the primary factor. Figure 1 below shows how the ski attitude flattens out as the COM moves from a back position (red circle) to the target normal plane (green circle).
|Figure 1: COM Position and Efficiency|
There are significant disadvantages when the COM is either too far forward or too far back. If the COM moves too far toward the red zones shown in Figure 1, it causes the ski to ride with an inefficient attitude in the water where there will be either not enough lift, or excessive drag.
COM Positioning for Balance
An additional benefit to this position is the improved ability to balance on the ski. Your body senses pressure through the soles of your feet, and uses that information to help you balance. With your COM positioned between the balls of your feet you have effective use of sensory input from both feet, in addition to the ability to articulate both knees and ankles productively, which greatly improves your ability to maintain balance. When your COM shifts too far forward or too far back, your ability to balance diminishes greatly.
TEST: COM positioning can be easily tested on dry land to help understand how much it can benefit or hinder your balance. Stand up tall with your feet in line like you would on a slalom ski. First lean back and put your COM over the heel of your back foot. Notice how your balance feels, and how difficult it is to hold a steady position. Now shift your COM forward, by flexing at the knees and ankles so that your COM is balanced between the balls of your feet. There is a huge difference in fore/aft balance, lateral stability, and your ability to control and change positions athletically when you stand like this. This is the same stable feeling you want on a slalom ski. Just like being too far back, shifting the COM ahead of the ball of the front foot will also reduce balance, and more importantly make you more susceptible to an out-the-front type of fall.
Application on the Water
A real life perspective of the normal plane is represented in light-blue in Figure 2 below. In both pictures, the normal plane is located at the midpoint between the balls of the feet. The red dot in Figure 2: A shows the skier having his COM too far behind the target. In Figure 2: B, the green dot shows the skier’s COM being at the target, which is an extremely efficient position.
|Figure 2: A: “Back” & inefficient – COM over back foot||B: Forward & efficient – COM at midpoint between balls of the feet.|
With his COM further back, the skier in Figure 2: A is putting the ski between his COM and the boat, effectively putting the brakes on. This “back seat” position causes the resulting ski attitude in the water to be tip high, with the tail digging, and the water-break very far back. His effort is being wasted plowing water and fighting the boat. This ultimately reduces cross course acceleration while increasing load.
With his COM significantly further forward and within the target normal plane, the skier in Figure 2: B gains the benefit of having much greater leverage over the ski’s edge, further driving the tip down, rolling the ski to a higher bank angle, and putting more ski in the water. This greatly improves cross-course acceleration and increases the speed at the course center-line. With this high speed and reduced load, he will be able to transition to the turning edge much earlier and more easily, with ample energy to achieve the primary objective of GUT (i.e. taking the handle as high on the boat as possible as fast as possible).
Utilized effectively, ideal COM positioning will help you greatly reduce load, increase cross-course acceleration and improve both fore/aft and lateral balance in the slalom course. Positioning your COM to be near the target plane requires general awareness more than physical effort. Doing so will help the ski carry speed and keep you moving in the direction you want to go, without having to rely so heavily on the rope for support. Being able to enter and finish a turn with the COM in an efficient position will significantly improve your ability to sustain that position into the first wake, and help you accelerate much faster with far less effort.
It is extremely important to keep the COM “normal” to the ski and balanced between the balls of your feet at all times, regardless of the attitude of the ski in the water, where it’s pointing, or where you are in the course. Skiers who habitually lean too far back are in the red area shown on Figure 1, and this is a hard habit to break. Shifting the COM forward to an efficient position within the green zone on Figure 1 can be unnerving and unfamiliar at first. As awareness and confidence improves, having your COM centered over the balls of your feet becomes more natural. Maintaining ideal COM position will help you transfer the tremendous power of the boat directly into speed and acceleration with much less effort, allowing you to carry more energy through the turns and run an early, wide, up-course rhythm.