under construction on 5 June/97
Ken Young, University of Washington.
It's been long controversial in the rowing community if and how much lift occurs in the sculling stroke. The data has been sparse.
We can consider several factors:
a) consistency of force production with lift versus drag force
In the drag force case, the blade must be moving backwards [relative to still water] in order to generate the thrust. F = A*Cd*1/2 *density*v^2.
b) production of an isolated single major vortex during the beginning of the stroke. Aerodynamic [hydrodynamic] theory requires that a single vortex be produced upon the acceleration of an air foil. The paired vortex remains circulating on the foil until the foil is stopped. You may investigate this further in the lift.html web page
The measured drag force for a racing 1X at racing speed is about 15 lb. In MKS this translates to 68 Newtons and the speed is about 4.8m/sec.
The power requirement is P = F*v = 68*4.8 = 326 watts.
For a sculler with drive/recovery ratio of 1:2, the average force during the drive must be 204 N (45lb). The force perceived by the sculler will be greater than this because the sculler and the oarlocks and oars are part of an accelerated system. [a 1 lb object feels heavier when you are being accelerated upwards. At 1 g upward acceleration, the object will have an apparent weight of 2 lb.] A scull will have an acceleration of about 1 g and the force will not be applied evenly during the stroke. The peak forces measured in the shell may be about 4 times more than the average force as seen by the water. The peak forces as measured in the shell will be about 800 N or 180 lb.
Analysis of a video recording of single stroke of a scull while underway. The scull is a Concept II hatchet blade circa 1991. The sequence of pictures at the top then left to right shows the moment before the catch, the catch, at 45 degrees, 90 degrees and at the release.
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The scull at the catch position. Note the angle of the shaft and the cleanliness of the blade entry. The catch angle is about 30 degrees relative to the long axis of the boat. A catch at this angle will produce a high transverse velocity if the sculler applies a suffieint force on the scull handles. The resulting high dynamic pressure will produce a large thrust. The SINGLE vortex left by the trailing edge of the scull at the catch remains in about the same spot in the water throughout the stroke. The single vortex formation is consistent with a vortex being produced to circulate around the scull blade which is necessary for LIFT. In the 6th frame, the scull has passed and the wake consists of : 1) slow-moving single vortex left by the beginning of the stroke (30 to 80 degrees). 2) slow moving vortex left by the last phase (110 to 135 degrees) and 3) fast moving twin vortex and weak left by the middle part of the stroke (80 to 110 degrees). [The author has tracked these vortices from the complete video record and on several strokes. the images here are typical of the several strokes that have been tracked.]
The red spots show the outer edge of the scull blade for the sequential video pictures spaced apart by 1/30 sec. The green spots show the sequential coordinates of the oarlock at the 1/30 sec intervals.
The displacement of the tip of the blade in the y-direction.. Note that the blade has been displaced forward by about 0.1 meter from catch to release. The blade tip reached forward by as much as 0.4 meters during the drive.
Below, we show you the velocity in the x and y directions. The x is the transverse direction and y is in the direction of motion of the shell. The transverse scull velocity reaches about 3.2 m/sec.
 velocity in the x direction |
velocity in the x direction |
| The velocity of the blade tip in the y direction (longitudinal direction) | |
The velocity of the tip of the blade. Note that the blade has
a backwards velocity when the shaft approaches 90 degrees. at.
The velocity is about -1m/s. The dynamic pressure generated by
the slippage will be approximately:
Dynamic Pressure = 1/2 * 1000* 1^2 = 500 N/m^2.
In contrast, the calculated dynamic pressure for the
transverse motion of the blade in the earlier part of the stroke
is:
v = 3m/s
Dynamic Pressure = .1/2*1000*3^2= 4500 N/m^2
which is 9 times larger than the dynamic pressure for the blade slipping at 90 degrees.
To make an estimation to see how the various segments of the stroke contribute to the thrust in the forward direction, we must take into account the lift and drag coefficients as well as the direction of the lift. The force generated by the blade at 90 degrees is ALL in the forward direction so is very efficient. The blade at the catch direction will produce forces which are partly forward and partly sideways. These factors for a foil which is rapidly changing its pitch and velocity are not available. Further research is necessary to address this question quantitatively. We do know that the thrust generated by lift is sufficient to accelerate both the shell as well as the blade forward.
An estimate for this is consistent with more thrust coming from the part of the s troke before 90 degrees. The velocity of the shell is consistent with this. The shell accelerates to its highest velocity before the scull reaches 90 degrees then is constant then decreases.
Velocity of the oarlock ( the shell) in the y direction.
This figure shows the blade at the catch. The two red spots show the extremes of the vortex position at the catch [left] and the release [right]. It obviously has a small velocity as expected. In contrast, the velocity of the twin vortices made by the stalled scull [at 90 degrees] has a velocity which is about 3 times greater. The reader is reminded that for the same thrust, we must give the same momentum impuse to the water. This can be done with large mass with high velocity or small mass with a large velocity. The smaller mass with large velocity has the larger kinetic energy. [ KE = 1/2 mv^2] This means that the stalled scull condition gives more kinetic energy to the water for the same thrust than the scull with LIFT which gives a larger mass of water with smaller velocity. The conclusion is that using LIFT is more efficient than using RESISTANCE of the stalled scull for providing thrust to the shell .
A careful study of the passage of the scull also reveals a second but smaller slow moving vortex which is produced by the onset of lift during the 2nd half of the stroke. As the blade starts to move inward, the lift process begins anew with the formation of this vortex paired with a vortex around the blade.
You may wish to read a primer on lift and vortex formation.
You may wish to read the measurements of shell acceleration as a function of the phases of the rowing stroke.