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How Moving Water Affects Your SpeedCoach:

How Moving Water Affects Your SpeedCoach

A common misconception of the SpeedCoach system is that the measurements will be affected by moving water. Actually, the SpeedCoach meter was designed so that it is completely unaffected by currents and tides. Understanding this can help you train more effectively.

Let's start with a basic analogy for rowing on moving water. Imagine using the moving walkway between terminals at an airport. Before getting on the walkway, assume you are walking at a speed of 3 mph. When you hop on the walkway, you continue to walk at 3 mph, but the walkway adds another 2 mph. To the observer standing at a gate, it looks like you are traveling at 5 mph. If you were training for the Airport Terminal Olympics, you would want to know your walking speed of 3 mph, not the observed 5 mph.

Now imagine that you've arrived at your gate, only to find that the gate for your flight has been changed. You get back on the same moving walkway, but this time walking in the opposite direction. You continue to walk at 3 mph on the walkway, which is moving at 2 mph in the other direction. But to the observer standing the gate, you are moving 1 mph. (In fact, he thinks about calling security.)

The speed that is most important to your rowing is the same as the 3 mph walking speed. Likewise, you want to know how fast your boat is moving through the water, NOT how fast your boat is moving compared to land. It doesn't matter if your boat is traveling upstream or downstream, your speed through the water will be the same (assuming the pressure is the same).

Let's use an example of two 1000 meter pieces on a racecourse, one upstream and one downstream. You finish the downstream piece in 3:30, drink some water, and get a couple minutes of rest. Now you turn around do the upriver piece in 3:52. We all know that you can't compare the times for these pieces, because one was downstream and one was upstream. The SpeedCoach meter, however, would have displayed the same speed in either direction.

There is a huge benefit to using a speedometer when training. If you use the distance measured by the SpeedCoach meter, as opposed to the land marked distance, you can compare pieces in either direction. Furthermore, you can compare pieces from day to day, even if the current or tide has changed! Please note that the wind WILL affect the measured speed. This is a much different phenomenon than moving water, because the wind is actually pushing against the boat. Sorry. But we are working on that. (No, not really.)

One more way to think about this is to imagine rowing upstream at exactly the same speed as the current. Your boat will appear to be standing still to the person on shore, even though you're working pretty hard to keep your boat from moving backwards. Fortunately, the SpeedCoach meter will continue to show your boat speed and count off the meters rowed through the water. (By the way, a GPS would indicate no speed and no distance. It has no idea if you are on land, water, or a moving walkway at the airport.)

Why we recommend placing the impeller between five to six metres from the bow.

Laminar vs. Turbulent Flow: On a rowing shell, two different kinds of flow are apparent. The smooth laminar flow at the bow (depending on the boat and speed, this can extend up to three metres from bow ball) and turbulent flow, all the little vortices along the hull that ultimately create the lovely sound of a fast moving boat. The laminar flow is very important to boat speed – laminar flow creates significantly less resistance. Therefore, everything has to be done to maintain laminar flow. Even the smallest disruption on the hull will create turbulent flow right away and create an undesired effect on the boat. Hence it’s particularly important to keep the bow clean at all times and free of dents, scratches and tape. For this reason we avoid placing the impeller in the laminar flow.

Accessibility: A wireless pick-up of impeller measurement is a must to avoid drilling holes through the hull. Hence, a sensor needs to be placed as closely as possible to the spinning impeller. In most rowing shells, there is an easy placement for both around five to six metres from the bow (typically around the two-seat). Since the sensor should be somewhat protected from outside influence, we recommend to place it under the foot stretcher. Under no circumstances should it ever be necessary to drill holes into bulkheads or have wires dangling on bow decks. 

Boat pitch: The vertical movement of the boat during each stroke needs to be accounted for, too. The impeller needs to be ideally placed where the flow conditions are consistent. Through the pitch, boundary layer thickness changes close to the bow. With our impeller position we have more stability and therefore more accuracy.

Thickness of Turbulent Layer: The layer of turbulent flow increases along the hull and also with increasing speed. Additional minor inaccuracies and differences in hull designs affect the boundary layer. To maintain consistent conditions that allow precise calibration, placement of the impeller between five and six metres from the bow works best for most accurate distance and speed measurement.

Effect on steering: Five to six metres from the bow, the impeller is relatively close to the turning point of a rowing shell, even an eight. If mounted in the bow, however, an impeller will act as a fin in front of the turning point and could have an adverse effect on steering of a shell.
How much drag does the impeller create, and will it slow me down? The hydrodynamic design of the NK impeller mount actually creates a fraction of the drag of a standard skeg. Laboratory tests were performed on the hull mount impeller to determine the drag effects on a rowing shell. For single sculls, the drag from the hull mount impeller is approximately 0.1% of the TOTAL boat drag, and proportionately smaller for larger boats. A larger size impeller does not equal more drag since it is placed in the turbulent flow, however, even the smallest object in or near the laminar flow will have a significant negative effect on boat speed (with no gains).

Impeller Size

Recently a smaller impeller was brought to my attention, that according to the manufacturer has a much better drag effect than our impeller. Always wanting to improve, we are of course interested. However, having seen it I was rather disappointed, smaller isn’t always better. Just like with coxswain minimum weights, there are limits to everything. We had similar experiences in our Kestrel Pocket Weather Meter side of Nielsen-Kellerman, where smaller impellers entered the market to measure wind speed. However, the significantly reduced size of the impeller fins (that makes it spin) are also prone to increased slippage and therefore more likely to produce inconsistent and inaccurate readings. In addition to the fin size other aspects are to be considered: Is there a high quality ball bearing inside the spinning part to allow consistent spinning? How is the finish of the impeller, is the entire surface perfectly smooth? Is the entire fin streamlined and optimized for laminar flow?

Another issue is where the impeller is placed. Since we absolutely want to avoid putting anything into the laminar flow of the boat near the bow, we are, in respect to drag, much better off in the turbulent layer, where the drag effect is less. However, in order to be able to measure the flow, the spinning part cannot be too close the hull either, as the turbulent flow changes drastically with the speed changes a rowing shell produces during a single stroke. And of course, the change of the profile of the boundary layer with increasing boat speed also needs to be accounted for, since it’s by no means a linear relationship.

Since we spend so much time training (see above) and accurate feedback is important, I cannot stress enough how a visual perception, for example size, can fool us when it comes to performance, accuracy and benefit.

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