Available online at www.sciencedirect.com Procedia Engineering 13 (2011) 501 506 5 th Asia-Pacific Congress on Sports Technology (APCST) Instrumentation of a kayak paddle to investigate blade/water interactions R.J.N. Helmer a, A. Farouil a, J Baker b, and I. Blanchonette a* a CSIRO Materials Science and Engineering, Belmont, VIC 3216, Australia b Australian Institute of Sport, Belconnen, ACT, 2616, Australia Received 31 March 2011; revised 3 May 2011; accepted 4 May 2011 Abstract This study investigated the hydrodynamic pressure experienced by a point on a paddle during kayaking training sessions to assess paddling technique and effectiveness. A force sensor was mounted on the bottom of each blade and waterproofed, with minimal change to the blade shape. Additionally, an accelerometer was externally mounted on the kayak to measure the acceleration of the boat and an e-textile with strain sensors on the elbows was worn to monitor arm technique. These devices were synchronised in a common wireless data acquisition system. Paddler technique data such as stroke rate, stroke duration, depth of blade immersion and stroke symmetry were measured. The paddle blade pressure sensors were unable to directly measure stroke force but provided a useful synchronised measure of stroke pull time that enabled a useful technique characterisation. 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of RMIT University Keywords: Kayak; paddle; pressure; acceleration; symmetry 1. Introduction Prior to the mid-1980s sprint kayaking paddlers typically used a flat blade and a wooden kayak. Since then various kayak and paddle designs have gained favour as kayakers strive to improve their performance [1]. For example, the preferred kayak paddle has changed from a wing blade, which had a typical airfoil appearance, to the blades generally now used by international competitors which are more propeller like in shape. Mathematical models have suggested that different blades have different efficiencies, e.g. the wing blade is reported to have a 15% higher efficiency than the conventional flat * Corresponding author. Tel.: +61 3 5246 4000; fax: +61 3 5246 4057. E-mail address: ian.blanchonette@csiro.au. 1877 7058 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. doi:10.1016/j.proeng.2011.05.121
502 R.J.N. Helmer et al. / Procedia Engineering 13 (2011) 501 506 blade (wing blade 89%; flat blade 74%) [2]. The on-going design changes to kayaking equipment has led to changes in paddler technique. Characterizing paddler technique has often required specialised test environments and procedures. As a consequence of the equipment changes, and the challenges of field measurement, the biomechanics and efficiency of individual paddling technique is not easily characterised. This has led to some uncertainty as to what coaching strategies are best employed [3]. The aim of this study was to design a submersible force sensor that could be mounted on a paddle blade and explore its ability to measure paddle depth, stroke pull time (i.e. time in water), and stroke force in field settings in conjunction with other performance measurement devices. 2. Experimental The complexity of kayak paddle blade designs and blade and water interactions meant that it was not feasible to explicitly specify the requirements for the design of a submersible force sensor. Elite kayakers are reported to produce average peak pulling forces of up to 400N and impulses of 100N.s. over 1000m [4]. Peak forces during starts can reach 800N for a single stroke [5]. Hence, for a blade with a nominal area of say 400mm x 250mm, a peak pressure of up to 8kPa might be expected. The hydrostatic head at the tip of a fully immersed paddle, i.e. at a depth of approximately 400mm, could be expected to be approximately 4kPa (for fresh water at 20ºC). Based on these nominal calculations, and the need for minimal disruption to the paddle, a Flexiforce model A201-01, 1lb-force (4.4N) force sensor was selected as it has a very low profile and is flexible, and so can conform to the shape of the blade. The paddle blade pressure sensors were formed by locating Flexiforce 1lb-f sensors between a precision machined disk and a reference substrate (e.g. paddle blade or ruler). In this way the sensors were prepared to have a defined area as has been done in other studies [6]. The modified Flexiforce sensors were calibrated for depth using a dip test that involved mounting sensors with a 19mm diameter disc onto a plastic ruler and immersing this assembly into constant temperature water to a known depth (all inside a plastic bag). The sensors temperature stability was also tested through immersion into different temperature baths within a range of 15 30ºC. A small number of sensors, those with similar performance, were specifically selected for mounting onto the kayak blade for use in field trials. Different locations and sensor areas were chosen to explore the sensitivity of the modified sensor during real paddling. As it was desired that the blade depth, stroke pull time and stroke force be measurable, initially a combination of two sensors positioned in different locations was adopted to explore sensor response. For example, one sensor was located toward the lower edge of the blade and the other closer to the shaft with diameters of 18mm and 50mm for the tip and shaft respectively. The sensor was made waterproof by encapsulation in a plastic membrane and bus leads extended up the paddle shaft to a central position where the sensors were connected to a customised Bluetooth wireless unit with circuitry configured to suit the needs of the Flexiforce sensor with 8-bit sampling at 250Hz. Typical assembly on a conventional paddle blade is shown in Figure 1. Fig 1. Paddle blades with pressure sensors.
R.J.N. Helmer et al. / Procedia Engineering 13 (2011) 501 506 503 The modified paddles were used in a series of on-water tests with an amateur and an elite kayaker. The on-water logging system included a three axis accelerometer (+/- 3g) mounted on the deck of the kayak and an e-textile shirt with strain sensors located on the elbows similar to that used in other studies [7]. The mobile monitoring system used in field testing was configured so that the three measurement systems were connected via Bluetooth to an onboard UMPC (Viliv XC70) with customized logging software developed using National Instruments LabVIEW suite running in Windows XP similar to that discussed elsewhere [8]. All equipment was contained within plastic press seal bags to be splash proof. The kayakers were asked to paddle at a pace that was consistent with their normal training. The on-water system is shown during field use in Figure 2. Fig 2. On-water monitoring system in use with amateur kayaker. 3. Results and Discussion 3.1. Calibration of sensors The calibration of the Flexiforce model A201-01 with hydrostatic pressure (depth) revealed some significant differences at low pressure (shallow depths) between sensors and so it was necessary to select particular sensors for on-water trials, (see Figure 3(a)). The method of waterproofing the sensors also affected their response, perhaps due to changes in air pressure within the housing. Following various attempts a method was achieved that was robust in the field and stable across a temperature range generally consistent with a range that might be expected in Australian waters (Figure 3(b)). (a) (b) Fig 3. (a) Sensor calibration in water, (b) Sensor stability with water temperature and depth.
504 R.J.N. Helmer et al. / Procedia Engineering 13 (2011) 501 506 3.2. Paddle depth during field trials An on-water calibration was performed at the start of each field test by vertically dipping each paddle blade in prior to starting a session. Figure 4(a) shows a plot of paddle pressure sensor variation (depth, mm) during a typical field test with a novice paddling at approximately 84 strokes per minute. The results suggested that hydrostatic pressure was dominant and further development is required to distinguish hydrodynamic pressure variations. The kayakers were observed to generally fully submerse paddle blades (approximately 400mm) and the stroke to stroke variation in paddle pressure is attributed to variations in the angle of blade immersion and perhaps some contribution from actual stroke force. Overall it was not possible to directly compare the magnitude of paddle pressure between consecutive strokes, left or right side, across sessions, or between kayakers. (a) (b) Fig 4. (a) Paddle pressure (depth, mm) detected using a pressure sensor mounted on paddle blades, (b).left (LS) and right (RS) stroke pull times for a novice paddler (K1). The in-water time detected with the paddle blade pressure sensor was considered representative and enabled a useful characterisation of left and right side technique. For example, Figure 4(b) shows the asymmetry of a novice paddler s (K1) left (LS) and right (RS) stroke pull times and the variation on each side. 3.3. Synchronisation with other performance measures Figure 5 shows a plot of typical field data from the on-water logging system. The paddle blade pressure sensor s pull time measure was very useful for identifying key performance metrics derived from other synchronized performance measures. In particular, ready identification of left and right side stroke metrics. Appendix A (Figure 6) shows a comparison of an amateur and elite kayaker technique from which key performance measures, similar to those reported elsewhere [3], can be obtained.
R.J.N. Helmer et al. / Procedia Engineering 13 (2011) 501 506 505 Fig 5. Typical field data from the on-water logging system: e-textile shirt with strain sensors located on the elbows, three axis accelerometer (+/- 3g) mounted on the deck of the kayak, and paddle blade pressure sensors for an amateur kayaker paddling at ~84 strokes per minute. 4. Conclusion The paddle blade pressure sensors were unable to directly measure stroke force but provided a useful measure of stroke pull time that enabled useful technique characterisation when synchronized with other performance measures. Acknowledgements We would like to thank Nigel Hoschke and Tim Head for their assistance with the on-water trials. References [1] Sanders R, Baker J. Evolution of technique in flat-water kayaking. In Issurin V, editor, Science and Practice of Canoe/Kayak High-Performance Training,. Tel-Aviv, Israel.1998, 67-81. [2] Jackson P. Performance prediction for Olympic kayaks. Sports Sci. 1995;13: 239-245. [3] Baker J, Rath D, Sanders R, Kelly B. A three dimensional analysis of male and female elite sprint kayak paddlers. Proc 17 th Int Symp Biomech Sports 1999:49-52. [4] Sperlich, J., & Baker, J. (2000) Biomechanical testing in elite canoeing. In Proceedings of the XXth International Symposium on Biomechanics in Sports (edited by Gianikellis, K), Cacares, Spain [5] Baker, J. (1998) Evaluation of biomechanic performance related factors with on-water tests. In International Seminar on Kayak-Canoe Coaching and Science (edited by Vrijens, J.), pp 50-66. Gent, Belgium [6] McLaren J., Helmer RJN, Horne SL, and Blanchonette I. Preliminary Development of a Wearable Device for Dynamic Pressure Measurement in Garments. Proc Eng 2, 2010;2:3041-3046. [7] Helmer R, Mestrovic M, Farrow D, Lucas S, Spratford W. Smart Textiles: Position and Motion Sensing for Sport, Entertainment and Rehabilitation. Adv Sci Tech, 2008;60:144-153. [8] Helmer R, Mestrovic M, Taylor K, Philpot B, Wilde D, and Farrow D. Physiological Tracking, Wearable Interactive Systems, and Human Performance. In Proc. 20 th ICAT, 2010.
506 R.J.N. Helmer et al. / Procedia Engineering 13 (2011) 501 506 Appendix A. Comparison of Elite and Experienced Novice (a). (b). (c). Fig 6. Comparison of average of 10 strokes at normalized stroke time (a) Average left (-L) and right (-R) elbow angles during right strokes (RS) and left strokes (LS) (b) Right stroke average paddle depth, and (c) Average boat acceleration from right strokes (RS) and left strokes (LS) for Left elite kayaker (K2) at 70SPM, and, Right experienced novice kayaker (K1) at 84SPM.