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Interpretation of the Blade Work variables

Interpretation of the Blade Work variables

In this newsletter we will try to summarise previous data about blade work, derive some benchmarks and make practical conclusions.

The blade Drag Factor DFb (Fig.1,a) goes to infinity at two critical points (catch T0c and finish T0f), where the blade velocity in the water crosses zero, which makes it difficult to analyse. We therefore decided to derive an inversed variable: Blade Slip Factor SFb (Fig.1,b):

SFb = Vbb * abs(Vbb) / Fb               (1)

where Fb is the normal blade force, and Vbb is the normal component of the blade velocity in the water, including the oar bend. In this way, SFb = 1 / DFb when the blade velocity Vbb is positive, but when Vbb becomes negative at the catch and finish, SFb keeps its sign and becomes negative as well. If DFb shows resistance to the movement of the blade in the water, SFb indicates its slippage, similar to electrical resistance and conductance.

Similarly, Blade Inefficiency Nb (Fig.1,d) could be derived as a reversed variable to Blade Efficiency Eb :

Nb = Pw / P                                     (2)

Where P is rowing power applied to the oar, Pw is the waste power of the blade slippage. As Nbl = 1 - Eb, then Nb crosses zero and becomes negative, when Eb becomes higher than 100% at critical points. If Eb is part of the rowing power spent on propulsion, Nb is the share of energy lost in the slippage of the blade in the water.

As the blade is only partially submerged before and after the critical points T0c and T0f (RBN 2018/06), we decided to analyse only the part of the drive between them. Average blade Drag Factor DFb.a was derived:

DFb.a = Fb.a / Vb.a2 = Fb.a / (Lb / Tc-f)2            (3)

where Vb.a is the average blade velocity and Fb.a is the average blade force between the critical points, Lb is the displacement (slippage) of the blade, time Tc-f = T0f - T0c.

A large data set measured with the BioRow system (n=28252) was analysed using the above definitions of the blade work. High positive correlation r=0.72 was found between Blade Efficiency Eb and Drag Factor DFb, and the same r between Nb and SFb, which means the 51% variation of Eb is explained by the variation of DFb and vice versa. The remaining variation is related to an effect of the stroke rate SR and average boat speed Vb, where higher values lead to a higher Eb (r=0.22 with SR and r=0.28 with Vb), but lower DFb (r=-0.13 with SR and r=-0.18 with Vb). Correlations with Nb and SFb were inversed to the above values. The interpretation of this is as follows:

· Blade Drag Factor is the measure of ABSOLUTE resistance of the blade in the water, which slightly decreases (blade slippage increases) at higher stroke rates and boat speeds, because the blade slips more at higher force and power.

· Blade Efficiency is the measure of propulsive power as a RELATIVE share in the total power, Efficiency increases at higher rates and speeds. because the total power increases more dramatically than the blade slippage.

Average DFb of each sculling blade was about 85% of one sweep DFb, so the total blade DFb per sculler was more than 70% higher than one per rower (Table 1). However, the blade efficiency Ebl was only 1% higher in sculling boats than in sweep boats. Bigger boats had a slightly higher Ebl and DFb.

In crew boats, rowers applying relatively higher force and power usually have slightly lower Ebl (r=-0.17 with relative power), and DFb (r=-0.06).

Fig.1 clearly shows that blade efficiency Ebl and DFb are the lowest during the middle of the drive, (the highest Inefficiency Nbl and Slip Factor SFb). If the duration of the propulsive phase from T0c to T0f is divided into three equal periods, then average values of Ebl are related as: 105.8%-96.2%-97.9%, and DFb: 107.5%-83.7%-108.8% from the average values for the whole phase. This means, the beginning and end of the drive are more efficient zones for blade propulsion, and most of the blade slippage energy is wasted in the middle of the drive.

©2018 Dr. Valery Kleshnev www.biorow.com