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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:
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°
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.
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