(Presented by Pete and Dick Dreissigacker at the XXIX FISA Coaches Conference, Sevilla, Spain 2000)

Over the last 25 years we have done a lot of thinking about oars.  We've also done a lot of on water testing of oars.  We have come to the conclusion that the more we learn, the more we realize how little we know.  The function of oars is a very complex topic.

Here are three things we do know:

• 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 us 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 related.

Let's first take a look at the path of the oar through the water.

This picture is one frame from an overhead video taken from a bridge.  The boat is shown on the bottom and is moving from right to left.  The red dots mark the tip of the blade at each frame of the video.

Blade Path

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

CAD Blade Path

For the purpose of this discussion and analysis, the motion of the blade can be divided into four phases:

• The blade moves significantly 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.

Of course, in reality, all the phases blend together smoothly from one to another.

Before looking at each phase close up, it's important to review some definitions:

In this diagram, we have a blue object moving through a black flud 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.

Lift and Drag

Now let's take a closer look at what happens during each phase:

Phase 1 The blue curved line shows the blade positions at each 1/15th of a second duing the first quarter of a stroke.  The blade shows significant forward motion towards 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 genearting positive thrust, the force pointing to the right, while drag is contributing a small negative thrust.  The goal in phase 1 is to maximise lift and minimise drag.	Phase 1
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 of attack.  Lift is contributing almost all the foward thrust.  Drag is not contributing much of anything to thrust either positive or negative.  The goal in phase 2 is again to maximise lift and minimise drag.	Phase 2
Phase 3 In phase 2, 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 maximise drag	Phase 3
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.	Phase 4

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 look at:

Adjusting the tip of the blade changes 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 stage 2.  This diagram is from an article at: http://www.avweb.com/news/reviews/182564-1.html describes how vortex generators on airplane wings can reduce drag and increase lift as the angle of attack increases.

Vortex Generators

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 edge will form large vortices along the edges that will increase lift and decrease drag.

Delta Wing Effect

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.

Continue reading for more information about Oar Testing.