Acceleration 101

Some Physics

Rowing Boats do not move at constant velocity. Velocity is constantly changing over the course of a stroke in a rhythmic pattern. The change in velocity is called acceleration.

Acceleration is defined as instantaneous change in velocity of an object. The object of observation in this case is the rowing boat, because that’s what we need to move across the finish line, right Velocity is typically measured in meters per second (m/s). Since acceleration is the instantaneous change in velocity, it’s simply the differential of velocity. The units follow from that: m/s².

Acceleration is directly related to force by Newton’s second law of motion: F = m*a. It’s important to note that F and a are vectors. For the remainder of this article, I will use a simplified physical model of a rowing boat that will only contain two forces: Those that are directed towards the stern and those that are directed towards the bow. Our boat accelerates when forces towards the bow exceed the forces towards the stern. To better match the view of the athlete, we will simplify this model even further by assuming that these two forces originate from stretcher and handle force only. That reduces the number of variables we need to control during the stroke to two: Handle and stretcher force. Easy, right?

Okay, so forces generate acceleration and acceleration results in a change in velocity. Why is velocity interesting again? Because it determines how fast we move towards the finish line. Let’s for a moment assume that we can measure acceleration with infinite precision over the course of a race. When we integrate under the acceleration graph, we get a velocity graph. If we integrate that again, we have a graph showing us the distance travelled vs. time. And that’s what really matters to us rowers: How fast can we travel a pre-defined distance.

The Graph

Rowing in Motion shows the measured boat acceleration over time in a graph. Time is on the x axis and acceleration on the y axis. The middle of the graph (thick black line) represents zero acceleration, which implies constant boat velocity. The two auxillary lines (grey) represent +/- 5m/s² respectively. The graph spans the time it takes to complete a stroke cycle on the x axis and +/- 10m/s² on the y axis.

What’s the deal with the dark and the light curves you ask? The dark red line that is drawn above the light one represents the current stroke, whereas the light red line represents a reference stroke. In this case, the reference stroke is simply the previous stroke.

What’s a stroke?

So how does Rowing in Motion define a stroke? As far as the graph is concerned, a stroke starts when the blades hit the water at the catch.

As we can see, the graph is only showing the reference stroke, the current stroke has just begun. It might be helpful now to watch a short video, to see how this looks in action and getting a feeling for the way acceleration changes over the course of a stroke.

Stroke Phases

Now we know how to read the acceleration graphs. Let’s try to go some stroke phases. We start at the catch.

Catch

With the blades squared and about to enter the water, we are about to reach the forward position. Trying to maintain zero acceleration for by slightly pulling on the stretcher is crucial in this phase. If you push on the stretcher to early, you will observe acceleration become negative early. You need to push on the stretcher at some point, it’s inevitable. But when you do so, we want to have handle force following that as soon as possible to compensate for this negative acceleration. That’s the task of the drive phase.

Drive

The boat is slowest right after the catch. That’s because we have had negative acceleration all the way into the catch and up until the early part of the leg drive. Until the blades have fully caught on stretcher force is exceeding handle force, resulting in negative acceleration. The point that the acceleration crosses the x-axis, marks the point in time when the boat is slowest. It’s crucial to get beyond that point as fast as possible to achieve a high average velocity over the stroke. When handle force exceeds stretcher force, the boat starts accelerating. Positive acceleration should be maintained during the whole drive phase. The area under the drive phase’s acceleration determines the total gain in velocity and should be maximised. A good coordination between leg, upper body and arms is required to achieve a purposeful acceleration curve.

Extraction

Since the boat has been accelerated for the whole drive phase, the boat is fastest when the blades are extracted. After that, we need to prepare for the next stroke.

Recovery

It’s crucial to maintain this high boat speed during recovery. Crews should aim at a constant forward motion supported by cautiously exercising a constant pull on the stretcher.

In the image above, I have marked in green the parts of the stroke where the Boat is decelerating. It’s important to keep them as short as possible and a good recovery phase is vital to achieving this.

Wrapping up

Rowing in Motion helps you thoroughly analyse and improve each part of the rowing stroke. We have tried not to focus on the specific motion sequences that crews should exercise. Varying opinions exist on these topics, but it’s important to realise that each individual adjustment to rowing technique should be validated for its impact on boat velocity. An easy to use measurement tool like Rowing in Motion helps coaches and athletes make scientifically sound decisions to achieve their goals.