Oar theories(English version)

Here are three things we do know and would like to tell you about in this presentation: There are performance differences between different blade shapes. These differences may depend on various factors such as rigging, catch angles, power application, "feel", etc. Therefore, crews should determine for themselves what gives them the best performance.

Why talk about theory? Theory gives some direction as to what kinds of changes may be worth testing on the water. Theory may give us some understanding into why and how blade shape, rigging, and technique are interrelated. Let�fs first take a look at the path of the oar through the water. http://courses.washington.edu/ phys208/scull.lift.html This picture is one frame of an overhead video taken from a bridge. The boat is shown on the bottom and is moving from left to right. The red dots mark the tip of the blade at each frame of the video.

Here we've taken the information from this and put it into a CAD program to make it easier to analyse.

For the purpose of discussion and analysis, the motion of the blade can be divided into four phases: The blade moves significantly forward toward the finish line. The blade moves outward, away from the boat. The blade moves backward, toward the starting line. The blade moves inward, toward the boat.

Before looking at each phase close up,
I want to review some definitions so we are all thinking about this the same way.

In this diagram we have a blue object moving through a black fluid from left to right.
As the object moves through the fluid,
the force on the object in the opposite direction of the motion is called drag.
And the force on the object in the direction perpendicular to the motion is called lift.

Now let take a closer look at what happens during each phase.

Phase 1 The blue curved lines show the blade positions at each 1/15th of a second during the first quarter of the stroke. The blade shows significant forward motion toward the finish line. Note, the blade movement is nearly in line with the blade surface. In other words, the blade has a low "angle of attack." Typically, this means that lift will be high relative to drag. Also note the lift is the force that is generating a positive thrust, the force pointing to the right, while drag is contributing a small negative thrust. The goal in phase 1 is to maximize lift and minimize drag.
Phase 2 These are the blade positions during phase 2. The movement is generally outward, away from the boat and the blade surface is at an angle to the motion. The blade is said to have a high angle of attack. Lift is contributing almost all the forward thrust. Drag is not contributing much of anything to thrust either positive or negative. The goal in phase 2 is again to maximize lift and minimize drag.
Phase 3 In phase 3, the movement is perpendicular to the blade surface. Drag is contributing almost all the forward thrust. There is very little lift present. The goal in phase 3 is to maximize drag.
Phase 4 Phase 4 is similar to phase 2 but in the opposite direction. Lift is contributing almost all the forward thrust. The goal in phase 4 is again to maximize lift and minimize drag. You can see that a problem starts to arise by the middle of this phase. The inboard edge of the blade, which is now the leading edge as the blade moves through the water, has a negative angle of attack while the tip of the blade continues to have a positive angle. The water is striking the back side of the blade near the inboard edge.

Possible Improvements Suggested by Theory

Seeing what is happening during each phase can lead to possible ways
of improving the efficiency in that phase.
It may be possible to find analogous situations in other fields
that may also apply to oars. Here are a few ideas we have looked at:

Adjusting the tip angle of the bladechanges the angle of attack in the water.
At the low angles of attack of phase 1, the lift and drag properties are very sensitive to the angle of attack.

Adding vortex generators to the back edge of the blade tip to postpone separation
as the angle of attack increases during the later stages of phase 1 and early in phase 2.
This diagram is from an article at: http://www.avweb.com/
articles/vortexge.html describes how vortex generators on airplane wings can reduce drag
and increase lift as the angle of attack increases.

The Delta Wing Effect:
Again looking at aircraft technology we see that aircraft designed to fly at the higher angles of
attack found in phase 2 use a delta wing. A wing or blade with a tapered leading edges
will form a large vortices along the edges that will increase lift and decrease drag.

Increasing surface area
may be the most important way to improve phase 3 performance.
This situation is similar to sailboats using extra sail area when the wind is from behind.
A blade designed to maximize performance in phases 1, 2, and 3
may not be the best design for phase 4.
This presents a design problem which we do not have any good answers for at this time.
As the power drops off toward the end of phase 4 the loss could be greater then the gain,
so it may be better to end that stroke sooner and go on to the next stroke.

What are some of the potential problems with theory?
Theory is based on steady flow. In rowing, the flow is rapidly changing and
this could make the results quite different.
A positive change to one phase may induce a negative change to another phase.
The overall change could then be negative.
So, the only way to really know what works is to test on the water.