Effect of blade rotation in the water

Effect of blade rotation in the water

Previously, in RBN 2021/10, it was hypothesised that the rotation of the blade in the water during the drive may consume energy and reduce the propulsive force and power within the rower-boat system, and here we provide further analysis of this effect.

New BioRow developments allowed us to measure some variables with different sensors and cross-validate the methods. E.g., the normal handle force was measured simultaneously using the method of two torques on the inboard Fh(M) (RBN 2020/10) and directly with the 4D sensor inside the handle grip Fh(F) (RBN 2021/06).

Data comparison (Fig.1, M1x at 34.5spm, Fat2 blades) showed nearly identical force curves and very similar average values (±0.5% difference), which confirms the validity and accuracy of both methods.

Similar cross-validation was made for the normal blade force: it was measured with two torques on the outboard Fb(M) and as the difference Fb(F) between the gate Fg and handle normal forces Fh(F):

Fb(F) = Fg - Fh(F)                                   (1)

The data comparison produced very different results (Fig.2,a): the blade force determined using oar bend was significantly higher than with the gate-handle sensors, especially during the middle of the drive, with less differences after the catch and before the finish. Comparing port-starboard oars, it looks like force losses from the blade to the gate-handle system are related to the depth of the blade in the water (b-c): the starboard blade was deeper after the catch and its losses were higher there (1), portside blade was deeper before the finish – with more losses there (2). Averaged over the drive, the difference was 19% on port side and 21% - on star board.

To explain this phenomenon, let’s assume the spoon rotates in the water around its geometric centre, which remains stationary (Fig.3). Then, the two halves of the blade produce equal-oppositely directed reaction forces: forward force Fforw on its outer half and backwards force Fback on the inner half, so the total propulsive force applied to the rower-boat system would be zero. However, Fforw has longer outboard lever than the lever of Fback, which means Fforw creates more torque at the pin than Fback (torque is the product of the force by its lever). Consequently, the total torque at the blade is not zero, resulting in a bend of the shaft – this could be interpreted as some blade propulsive force, while the real force moving the system is zero.

Of course, in real rowing, the blade centre never remains stationary, but the effect is similar. Usually, from the -40o to +20o oar angle, the pivot point is located on the blade (Fig.4), so its outer parts move backwards, creating a forward reaction force and the inner parts move forward creating a backwards force. Over the drive, inner blade parts move forward a longer distance then the outer parts (Sinner>Souter) creating more drag, but they have shorter leverage and lower input into the outboard torque. Also, parts of the shaft move even faster forward and may create significant drag under the water, but their leverage even shorter. This explains why the blade force measured through torques was overestimated and the centre of pressure was shifted towards the outer blade edge (RBN 2020/10). Also, it solves the puzzle in RBN 2014/02, where ratio of the handle/gate forces was found variable through the drive and for various blade submersion profiles.

The above discourse is related to the drag propulsion component only. Lift force can create propulsion even while the blade parts move forward. This is why the difference on Fig.2,b is the highest during the middle of the drive, when drag propulsion dominates, but it is smaller after the catch, when the lift force is significant, and even found negative before finish (3).

Concluding, for the first time, we were able to estimate experimentally the losses of force/power caused by the blade rotation in the water. Still more work required to decide if ~20% of force difference above is real losses, or it is just overestimation of the blade force measured with torques method. There will be further comprehensive evaluation of the blade efficiency and comparison of various blades and techniques in the future…

Acknowledgements: Thanks to Dick Dreissigacker and Alex Dunne of Concept2 Inc. for their support of this study.

©2021 Dr. Valery Kleshnev www.biorow.com