Rowing Power

Several scientific articles about rowing power were recently published by Dutch researchers (1, 2), which placed doubts upon the standard definitions of this important indicator of performance. In this newsletter, I will be reviewing this topic, which I last discussed in RBN 2004/06 (3,4).

Power is a process of energy transfer. It must always have an object which produces energy, and a subject to which the power is applied. The basic equation of mechanical power P is:

P = F v cos(α)                                                   (1)

where force F and velocity v must be between the object and the subject, not relative to something else, and α is the angle between the force and velocity vectors. Quite a common mistake in rowing publications is an incorrect model of power transfer, which mismatches forces and velocities and creates erroneous results. This mistake has been made in the starting equation of the articles 1, 2:

   (2)

where F_o,r / F_f,r are the forces, which the object (rower) applies to the subjects (oar handle / stretcher), but v_h/w - v_b/w are velocities of the subjects relative to “something else” –the “world” reference frame. However, the outside world does not apply the force and power to the handle and stretcher – the rower does this, moving at some velocity relative t the outside world, so we must refer to the rower’s centre of mass (CM), not to the “world”. According to the erroneous equation 2, the rower’s power depends on other external factors: a rower applying the same handle force and velocity, would have higher power in bigger/faster boats or in a tailwind, and in small/slower boats or in a headwind the power would be significantly lower (up to 20-30% difference), which should not be the case. Moreover, the “world” reference frame was related to the shore, not to the water: in this way, the rower’s power depends on the speed of the river current, which is completely untrue.

Previously, I have outlined three main methods of rowing power calculation (3, 4), and now, we’ll discuss them in more detail. The methods differ by the choice of the reference system:

1.      The “traditional” method is based on the boat frame;

2.      “Propulsive-waste” power is related to the mass of water surrounding the boat (not to the “Earth”);

3.      “Rower’s power” bound to the rower’s CM.

In the “traditional” method, I noticed that the boat is not an inertial reference frame, because it moves with acceleration, and this was confirmed in the referred articles. This means, say, at a 2g boat acceleration, moving a 10kg mass would require 20kg of force, similar to lifting up a weight inside an accelerating rocket (Fig.1). However, if the force and velocity are measured directly, the gauge will show 20kg of force, so the power calculations are correct.

The power created between the pin and the oar handle can be called the “Net power”, which is transferred through the oar shaft to the spoon, so this is the only source of energy which moves the rower-boat system forward. This power is measured directly and correctly with our BioRowTel system and the NK EmPower oarlock.

The 2^nd power definition describes the Net power transformation at the blade, where it is divided between the Propulsive power, which propels the rower-boat system and Waste power lost in blade slippage. Both components are difficult to measure directly, so this method is not practical.

The 3^rd power definition is in the rower’s reference frame and could be called “Gross power”, which is applied to both the handle and the stretcher in the ratio of about 60%/40%. Gross power is higher than Net power by 4-7% of inertial energy losses spent on the relative movements of the rower and boat (RBN 2010/05). Gross power is also not very practical to determine: measurement of the horizontal stretcher force is complicated, and the velocity of the rower’s CM is quite hard to determine. Also, definition of the Gross power could be extended by the energy that the rower spends on the vertical movements of the oars, friction on the slides, etc., so this method is quite uncertain.

Fig. 2 schematically shows the process of energy transformation in the rower-boat system. The rower consumes Metabolic energy P_m and applies mechanical Gross power P_g to the oar handle and stretcher (24-28% of P_m), while 72-76% of P_m is heating the rower’s body. About 6% of P_g is spent on inertial losses P_i and 94% goes to Net power P_n delivered to the blade, where it is spend on the Propulsive P_p (about 80%) and Waste P_w (20%) powers. About 6% of P_p is lost due to variation of the boat velocity (P_v) and the rest is effective propulsive power P_e.

In conclusion, the Net power transferred from a rower to the external environment is the most important indicator for the evaluation of a rower’s performance. It is measured correctly and reliably with BioRowTel and NK EmPower systems.

References

1.       Hofmijster M., Lintmeijer L., Beek P., van Soest K. (2018) Mechanical power output in rowing should not be determined from oar forces and oar motion alone, Journal of Sports Sciences, 36:18, 2147-2153

2.       Lintmeijer L., Hofmijster M., Fischedick G., Zijlstra P., Van Soest A. (2018) Improved determination of mechanical power output in rowing: Experimental results, Journal of Sports Sciences, 36:18, 2138-2146

3.       Kleshnev V. (2000) Power in rowing. Proceedings of XVIII Congress of ISBS, (2) Chinese University of Hong Kong, 662-666

4.       Kleshnev V. (2016) The Biomechanics of Rowing. Crowood Press. 190 p. ISBN 978 1 78500 133 8.

©2018 Dr. Valery Kleshnev www.biorow.com