|Propulsive Efficiency of Rowing|
An important task of sport biomechanics is estimating the mechanical efficiency of competitive sports. Rowing efficiency could be divided into two main parts: internal (muscle) and external (propulsive) efficiencies. Energy applied to the oar handle is the dividing point between these two energy transformation processes.
The internal or muscle efficiency is determined mainly by the effectiveness of muscle contraction and is estimated for rowing to be in the range of 14-27% (Fukunaga et al., 1986; Lisiecki and Rychlewski, 1987). The external or propulsive efficiency connected with hydrodynamics of the boat shell and oar blade is estimated to be in the range of 60-80% (Sanderson and Martindale, 1986, Affeld et al., 1993). The propulsive efficiency of rowing will be the focus of this paper. Two main types of energy waste affect propulsive efficiency of rowing (Nolte, 1991, Smith and Spinks, 1995). The first one is connected with boat velocity fluctuations that increases drag force losses due to the non-linear character of velocity-resistance relationship during movements in liquids and gases.
The second source is determined by characteristics of oar blade work in the water and could be defined as a function of hydrodynamic drag and lift forces (Zatsiorsky and Yakunin, 1991, Affeld et al, 1993).
Different approaches towards optimisation of rowing propulsive efficiency exist. Sanderson and Martindale (1986) suggest modifying the rower acceleration during recovery and increasing the oar blade area. They found boat velocity efficiency in a range 93.5 - 95.5% for a single scull. Nolte (1991) recommends increasing of the stroke length and minimising displacement of the rower's centre of mass. Schwanitz (1991) believes that force emphasis on the first part of drive, especially before the 90-degree position, could give some advantage.
The measurement was conducted during on-water rowing in competitive boats (Sykes Racing) using a radio telemetry system. The angle between oar and boat in horizontal dimension was measured using a servo potentiometer. The force applied to the oar handle was measured by detecting the oar strain using an inductive proximity sensor. Boat shell acceleration along horizontal axis was measured using a piezoresistive accelerometer. An electromagnetic sensor (Nielsen-Kellerman Co.) measured boat velocity. The A total number of 71 rowers in 21 crews was measured.
Every crew performed a set of three test trials of one minute each, with unlimited recovery time. The stroke rates were 23.3, ±1.9 min-1 in the first piece, 29.6, ±1.7 min-1 in the second one and 35.8, ±2.5 min-1 in the third one.
The data was collected and stored in a PC and then processed using special software. Typical patterns of the biomechanical parameters of the athlete's cyclic movements were produced. Then the patterns of derived parameters and the average patterns of the crew were calculated and used for analysis.
Calculation of energy waste. The following two assumptions were made:
The force applied to the oar blade (Fb) was calculated using the measured handle force and oar gearing. The track of oar blade during the stroke cycle was determined using oar angle and boat velocity data (Figure1) and, thus, blade velocity (vb) was derived. The wasted power in the blade's passage through the water (Pwb) was calculated as a vector sum of blade force and velocity:
The total instantaneous power applied to the handle (Ph) was calculated as a product of handle force torque and oar angular velocity. Propulsive efficiency of the blade (eb) was derived as a ratio of the handle power (Ph) to the propulsive instantaneous power (Pp):
The drag coefficient k was calculated for each test trial using instantaneous blade force (Fb) and boat velocity (vi) in the equation:
Wasted energy due to boat shell velocity fluctuation (Pws) was calculated using the equation:
where v is an the average shell velocity during the stroke cycle.
Efficiency of boat shell propulsion (es) was calculated using propulsive power (Pp) and its waste in shell velocity fluctuation (Pws). Overall mechanical efficiency of rowing propulsion (e) was calculated as the product of blade and shell efficiencies.
Although it is useful information for researchers, mechanical efficiency says little from a practical point, because it does not show gain or loss of boat velocity. Therefore, another definition of efficiency was derived as a ratio of actual boat velocity (Vreal) to a maximal one (Vmax) that could be available in terms of whole produced power spent on boat propulsion. We call this parameter "Propulsive Effectiveness" and derived it for shell propulsion (fs):
and the same way for blade propulsion:
Overall velocity effectiveness of rowing (f) was calculated as a product of blade and shell parameters.
Factors influencing the blade efficiency. The main biomechanical parameter influencing blade efficiency was boat velocity (Figure 2a). Therefore, blade efficiency was different in distinct boat types and some differences were found between sculling and sweep rowing. No significant differences were found in blade efficiency between male, female and lightweight, heavyweight rowers.
A significant relationship between Ratio of Average to Maximal Forces and blade efficiency was found (r = 0.48, p<0.01) that shows the importance of this parameter for a rowers' technique evaluation. This parameter slightly depended on stroke rate and did not depend on the rower's sex and weight or on boat type. The average value of this parameter for the whole sample was 53.8±3.3%.
Factors Influencing on Boat Efficiency. The first factor influencing boat efficiency was stroke rate. Increasing the rate led to an increase in velocity variation and loss of efficiency in every crew (Figure 2b). On average, about 1.4% of velocity was lost at rate 20 min-1 because of this factor and about 2.4% at 40 min-1.
The second important factor was the ratio of drive time to stroke time. The correlation between these parameters was significant (r = -0.73, p<0.001), but it could be partly explained by rate influence, because both of them were rate-dependent. Therefore, the deviations of both drive/stroke ratio and boat efficiency from their rate-based trends were taken. The result was a significant correlation (r = -0.69, p<0.001) that means a gain of boat efficiency by decreasing the drive/stroke ratio.
The oar angle parameters (catch and release angles) did not influence boat efficiency and neither did handle force application parameters.
Overall Efficiency. Overall efficiency of rowing was significantly different in boat types. On average, propulsion of the boat-rowers system consumes only 77.6±5.6% of mechanical energy applied to oar handle. The main reason of the 22.4% energy waste is the water shift by the oar blade (17.8%) and the less significant one is the boat velocity fluctuation (5.6%).
Both Boat and Blade Efficiencies were higher in bigger boats. This affected significant differences in Overall Efficiency between small and big boats. Statistical analysis did not show significant differences of efficiency parameters between male, female and lightweight, heavyweight rowers' groups.
Overall Rowing Effectiveness was boat-type-dependent as well. On average, rowing results could be the 8.2% better if the energy waste was absolutely removed. The main reason for velocity loss is the Blade Efficiency (6.4%) and the less significant one is the boat velocity fluctuation (1.9%).
Table 1 - Mechanical Efficiency of rowing in different boat types (mean and SD).
The values of propulsive rowing efficiency found in this study were slightly higher than the data in previous research. Future experiments which include determination of the drag and lift components of the blade force would provide more accurate results.
The presented rowing efficiency data allowed us to produce the following recommendations for practitioners: