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Monday
Jan092012

Boat Orientation & Skill Level in Sculling Boats

Constanze Loschner & Margy Galloway - New South Wales Institute of Sport, Sydney, Australia

Provided by: CoachesInfo.com   


The amount of yaw, pitch and roll induced in the boat by a sculler will affect the efficiency of boat propulsion. The purpose of this study was to analyse the movement of a rowing boat (Single Scull) in three dimensions and relate the results to the rowing style of the sculler. The study examines the relationship between the boat orientations and the seat and hand position. In rowing, the pitch of the boat is influenced by the movement of the seat and the rowers' body mass. The roll and yaw of the boat is dependent on the skill level of the athlete. All movement in any of these three directions will influence the boat velocity.

Introduction

The speed of the boat (and therefore the athletes' performance) is very dependent on the stability of the boat. Being able to keep the boat balanced around all axes will decrease the water resistance (hydrodynamic drag) and will be energetically more efficient for the athlete to maintain or increase boat speed. The rower's seat and body mass move along the longitudinal axis of the boat and the system (boat, athlete and sculls) is unstable. The crossing of the handles during the drive and recovery phase adds asymmetrical elements to the rowing motion and so to the roll and the yaw of the hull (Wagner, Bartmus, de Marees,1993). Until now there has only been one study that has examined boat motion in three dimensions. In that paper only example data for two rowers was reported. If boat orientation information was available, it could be linked with aspects of the rower's technique and ultimately lead to improvements.
 
The purpose of this study was to measure boat orientation during single sculling and to relate the results to characteristics of the rower and the rowers' technique and performance.

Materials & Methods

Thirteen single scullers were directed to row at four ascending rating steps (20,24,28 and above 32 strokes per minute (str×min-1)) for 20 strokes each, separated by one minute of light rowing. The athletes were all experienced elite level rowers with the potential to step up into the international level over the next two years. The composition of the testing group was: 6 male (2 heavyweight, 4 lightweight) and 7 female (5 heavyweight, 2 lightweight) rowers.
 
The biomechanical testing boat was set up and adjusted for each athlete incorporating their individual requirements (pins, seat, footstretcher height, pitch and position). The transducers were all calibrated before each test and the data were sampled at 100 Hz and telemetered to the shore.
 
The measurements taken to describe the body movement of the athlete were: as an indication of hand position, the oar angles (electrogoniometer) on both sides mounted over the pin and, as an indication of trunk position, the seat displacement (cable and drum driven potentiometer). The boat angular velocity in all three dimensions was measured with three gyroscopes and the boat linear velocity with a magnetised impeller and coil sensor.
 
The gyroscopes and the velocity sensor were placed in the centre of the longitudinal axis of the boat. The three dimensions measured were determined as: X-axis (Yaw), Y-axis (Pitch), Z-axis (Roll) (Figure 1).
 
Figure 1 .Definition of boat axes and orientation

X-Axis: Yaw
Change of boat direction around the vertical axis of the boat
Y-Axis: Pitch
Change of boat direction around the horizontal axis of the boat
Z-axis: Roll
Change of boat direction around the longitudinal axis of the boat
Negative Value
Bow turns to Bow Side (left side)

Positive Value
Bow turns to Stroke Side (right side)

Negative Value
Bow goes up

Positive Value
Bow goes down

Negative Value
towards Bow Side (left side)

Positive Value
towards stroke side (right side)

 

 

The angular velocity of the three boat directions was integrated to evaluate angular displacement (degrees) with a resolution of 0.1 degrees. The frequency bandwidth was limited to 0.15 - 20 Hz. The whole time series was examined for transient effects of wind gusts, for example, and these sections excluded from the data analysis. Only the within-stroke changes in orientation for each rower were considered. The data was subsequently normalised to percent of stroke and each rower's strokes averaged.

Results

Discussion: The results of this study indicated the variability of the boat movement in all three dimensions throughout the whole test. Although the timing and amplitude of the leg drive (Range = 0.61 m, mean SD = 0.006 m) and arm drive (Range = 111.4 degrees, mean SD = 0.792 degrees) was remarkably similar among all rowers, the boat orientation showed high variability among these athletes. Analysing the three dimensions separately there are some clear differences, which seem to affect the boat run.

Pitch The pattern of the 'Pitch graph' for all subjects showed the same changes throughout the stroke. The range of motion was from 0.3 to 0.5 degrees. There was a moderate correlation of 0.68 between the rowers' mass and the pitch range of motion. Thus about 50% of the variability in pitch motion can be accounted for by the mass of the rower.

There are three significant points that relate to the transfer of the body weight (at the first half of drive phase - peak velocity of leg drive; finish of the stroke - release of blades; first half of recovery phase - Start of leg drive). The change in the pitch correlates with the transfer of the weight of the athlete and the distribution of vertical forces between the seat and the stretcher. The bow reaches the lowest point during the finish of the stroke and the change of direction of motion of the rowers' trunk.

Yaw:This group of subjects produced a yaw ranging from 0.1 to 0.6 degrees. 0.5 degrees correspond to a 2.5 cm movement at the bow of the boat. The changes appeared especially during the first half of the drive phase, where the major forces were applied to the blade and the footstretcher as well as when the oar handles cross over during the drive phase.

Roll The range of direction changes around the longitudinal axis was the highest of all three dimensions being from 0.3 to 2.0 degrees. The 'roll' of the boat started just after the catch. Some athletes were capable of keeping the boat very stable around the longitudinal axis.
The more drastic changes of the boat orientation, the more the boat velocity was affected. This could be one explanation for a lighter athlete being able to row faster over a short distance, even with less force applied to the boat.

Conclusion

The results demonstrate a high relationship between the:

  • Boat orientation and boat run
  • Boat orientation and technique, technique adjustments (skill level, weather conditions)
  • Boat orientation (Pitch) and Weight of the athlete.
  • Information about the boat orientation provides athletes and coaches with another performance indicator that can be applied during training and performance assessment.

References

Wagner,J., Bartmus,U., de Marees,H. (1993). Three Axes Gyro System Quantifying the Specific Balance of Rowing. International Journal of Sports Medicine,14,35-38

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