Volume 9 No 94 Rowing Biomechanics Newsletter January 2009
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Volume 9 No 94 Rowing Biomechanics Newsletter January 2009 Vhandle, which was significantly lower during the Q&A first two strokes. This means the rower had to work Q: Aj Harper, a coach of the high performance in a slow, heavy mode, which decreased the boat regional program in New Zealand asked: “Have acceleration and muscle efficiency. you any information on the best possible racing We made the hypothesis that maintaining a start for a boat/types? Most people I speak to tend more even actual gearing may increase the effi- to use the basic, full, half, three quater, full sce- ciency of the start. In simpler words, it means that nario. However I was speaking to someone the the catch angle should increase proportionally with other day who suggested just full strokes only is the the boat speed during the start. What sort of angle way to go.” should a given crew use in this case? A: Definitely, doing full strokes is not the best The chart below shows the measured angles way to do starts for the following reasons: from the previous chart and modeled angles, which • The gear ratio is higher (heavier) with a long are proportional to the boat speed (Vcatch): catch angle (RBN 2007/03), which makes row- 100% 70 Catch Angle (%) ers work in a slow, static and inefficient mode. 90% Catch 63 • The hydro-lift effect doesn’t work at low boat Angle speeds (RBN 2007/12), so pushing the blade 80% (deg) 56 Measured outwards at the catch increases its slippage 70% 49 through the water and amount of energy 60% Model 42 wasted. Strokes To evaluate correlation of the catch angle with 50% 35 the boat velocity we have analysed a start of an in- 0 2468 10 12 At the catch of the first stroke the boat speed is ternational level crew: zero, so we assumed its angle would be equal to the 100% V, A (%) 60 angle at the second catch. The optimal sequence is 50 st nd 80% the following: the 1 and 2 strokes – 62% of the rd th Rate 40 full catch angle, the 3 – 73%, the 4 – 90% and 60% th Vaver (%) then gradually increasing to 100% at the 9 stroke. 30 Vcatch (%) To give you some numbers in degrees we put them 40% Catch Angle (%) 20 on the right Y axis, assuming the full catch angle is Vhandle (%) 20% 70 deg. What should these angles look like in a Rate (1/min) 10 boat? The picture below illustrates them: Strokes th 0% 0 Full 4 rd 02 4 6 8 10 12 3 The average boat velocity over the stroke cycle 1st & Vaver achieved 90% of its maximal value at the 5th nd stroke, 98% at the 7th and 100% at the 9th stroke. The boat velocity at the catch Vcatch is not the same as Vaver because the speed varies during the stroke cycle. The most significant difference was after the first stroke, because there was the highest variation of the boat velocity from the stationary The first two catches should be made with the position. We used Vcatch for further calculations handle position above the toes of the stretcher, so- rd because it defines the interaction of the blade with called “half slide”; the 3 catch – “three quarters th the water at the catch. slide”, the 4 – about 10cm shorter than the full th The catch angles were approximately 80% of length, which should be achieved by the 9 stroke. the maximal value during the first three strokes. The hypothesis needs to be proved in practise Then the length increased gradually and reached its by means of analysis of various start techniques. maximum in the same stroke (the 9th) as the boat Also, blade slippage and hydro-lift effect should be speed. considered. We hope to do it in the future. When we divided Vcatch by the actual gearing Contact Us: ratio derived using the catch angle (RBN 2007/03), ©2008: Dr. Valery Kleshnev, we obtained the corresponding handle speed, [email protected] , www.biorow.com Volume 9 No 95 Rowing Biomechanics Newsletter February 2009 We modelled the third component in a similar way Q&A and found that each 1kg of extra dead weight per rower Q: Roger Moore a senior boys coach at Pem- would make the boat speed smoother and increase its broke School, Adelaide, South Australia asked: average value by 0.11% or 0.37s over a 2k race. We “Do you have any figures regarding coxswains think this is a maximal value; if, with poor technique, weight and the effect on boat speed, in particular the rowers rush the recovery increasing fluctuations in an the eight? I have an important race in a few hull velocity, then this value will be reduced (RBN weeks and my coxswain is 11kg overweight. Our 2007/10). recent races have been decided by only 1-2s.” The table below summarises these values, assuming that rowing technique is good: Another coach asked a similar question: 1kg per rower Speed 2k race 2k race “Would it be possible to use the formula for deter- extra DW losses (%) in 5:20 in 7:10 mining the effect of dead weight on boat speed … Drag factor -0.061% +0.20s +0.26s to calculate the expected time for a coxed four Inertial losses -0.240% +0.77s +1.03s based on their 2000m time in the coxless fours. In Speed fluc- other words: Is a coxed four nothing more than a tuations +0.110% -0.35s -0.47s coxless four with 55 kg of excess baggage in terms Sum -0.191% +0.61s +0.82s of drag factor?” Every 1kg of extra dead weight per rower A: There are three components of the influence can decrease the boat speed by 0.19% or about of extra dead weight on the speed, which affect it in 0.7s slower over a 2k race in 6:00. different directions: If we refer to the second question about coxed 1. Higher drag resistance force caused by and coxless fours, we find that 55kg of extra dead higher mass of the system and consequently greater weight (EDW) in a four (13.75 kg per rower) water displacement; would make the boat 9.5s slower over a 2k race in 2. Higher inertial losses, which decrease pro- 6:00. Similar analysis for a pair (27.5kg of EDW pulsive power because the rowers have to move a per rower) gives us 21.3s slower over 2k in 6:40. heavier mass back and forth; We compared these values with the results of 3. Lower energy losses caused by reduced fluc- Olympics-92, where the coxed four and pair events tuations of the hull velocity in the water. were last contested. The difference in results be- The first component can be estimated using empirical tween M4- and M4+ for the winners was 4.3s and equations for the dependence of the drag factor on the the average for the finalists was 6.4s, which is rower’s mass (RBN 2007/07). The drag factor depends lower than the above value. In M2+ and M2- these on the amount of water displaced by the hull, which differences were 22.1s and 20.5s respectively, equal to the total mass of the system. Therefore, we can which is very close to the predicted value. add the dead weight to the rower’s mass. We need to Biomechanical conditions are also quite differ- calculate two values (DF1 and DF2) for the drag factor for each mass (without and with deadweight) using ent; in heavier coxed boats, it is more difficult to equations in Table 1 of RBN 2007/07. Then using the transfer power through the stretcher (RBN equation P = DF * V 3 and assuming that power produc- 2008/12). The leg drive is slower and more load is tion P is constant we can derive the equation for the ra- applied to the upper body, and so rowing in coxed tio of the speeds: boats more closely resembles ergo rowing. In cox- 1/3 V1 / V2 = (DF1 / DF2) less boats, a fast leg drive and work though the Drag caused by 1kg of extra dead weight per rower stretcher are more important. decreases the boat speed by 0.061% or 0.21s over a 2k Our summed factor -0.19% corresponds quite race in a time of 5min 40s. well with findings of other authors (1, 2). However, The second component (inertial losses) can be de- they analysed only the drag factor component, rived using mathematical modelling with sinusoidal which represents only 30% of the total value in our movement of two known masses relative to each other. We found that at a rate 36 str/min and relative dis- analysis. Probably, a further discussion is required. placement 0.6m, each 1kg of extra dead weight per References 1. Atkinson B. 2001. The Effect of Deadweight. rower decreased the boat speed by 0.33% or 1.13s over http://www.atkinsopht.com/row/deadwght.htm a 2k race. This value depends on rowing technique and 2. Dudhia A. 2008. Effect of Weight in Rowing. http://www- can be decreased by means of transferring kinetic en- atm.atm.ox.ac.uk/rowing/physics/weight.html#section7 ergy to blade propulsion at the finish of the drive (RBN Contact Us: 2006/10). Using the measured data analysis we took it ©2008: Dr. Valery Kleshnev, as 0.24% or 0.81s over a 2k race. [email protected] , www.biorow.com Volume 8 No 96 Rowing Biomechanics Newsletter March 2009 used a solid frame connecting the stretcher with the swivel Coach and Biomechanics: Jurgen Grobler (similar to the sliding rigger, of later years), and they were able Jurgen Grobler is one of to measure the propulsive force of each rower.