Forces, fluid dynamics, rowing
observing the average wattage output of the boat, it can be compared to previous winning crews to determine perceived rankings against competition. For example, as seen above, the figure average is 323 watts, at least 17 watts below the gold medal time calculated from the year before.
In the second graph, data is presented on a gate force versus normalized time graph. This gives an alternate view into key timings within the drive of a rower’s stroke. Normal ized time is the percentage of each rower’s time taken for each stroke . So, regardless of how long it takes for each rower to take a stroke (ideally the same), it benchmarks power output against a comparable percentage. Finally, the telemetry also calculates the slip within each stroke, as shown by the effective length of each rower. Slip is the measure of the length of stroke spent putting down force which is too little to move the boat. This happens at the start and end of the stroke, so minimizing this section can increase the total effective length a rower achieves. This slip can occur if a rower ‘rows’ the blade in, meaning they are driving their legs before the blade is in the water. This is inefficient as air is about 830 times less dense than water. Comparing this with the equation for drag, all power exerted not connected to the water is useless. Using the data, coaches and rowers can analyse the difference between stroke length and effective length, with the goal to get the longest effective length, to apply force to the boat for the longest period of time, with F=ma, that means the boat will be under acceleration for as long as possible. By using telemetry, a coach can apply the data collected from the boat and work with the rowers to analyse their weak points, for example, their length, smoothness, sequencing or overall power output, meaning the insight gained can often be the seconds needed to determine a race over 2000m.
Drag forces (oar)
The power a rower exerts in each of their strokes is not fully converted into useful energy driving the boat forwards; some is lost to slip, for example. Propulsion is generated by the oar and its interaction with the water. During a stroke, the flow field around the oar blade during the drive phase is often measured and several flow phenomena such as the generation of leading and trailing edge vortices are linked to the generation of lift and drag, which both contribute to rowing propulsion.
As seen in the figure on the left, the boat is travelling in the x- direction, with the force Fx therefore being the propulsive force. The component Fx and Fy are both bound to x,y respective. Fn and Ft are the normal and tangential forces from the end of the blade. The lift and drag forces FL and FD are similarly based on the tip of the rowing blade. Drag wants to be maximized, as it is the opposing force to blade motion. By getting the largest drag on the blade, the most power can be exerted by the rower and therefore move the boat the fastest. Looking at the equation of drag, 𝐹 𝐷 = 𝜌𝑣 2 𝐶 𝐷 𝐴 , drag can be maximized by the variables of fluid density, velocity Figure 2: Blade direction and forces during a stroke
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