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Calibration of the SpeedCoach:

Calibration of your SpeedCoach

There are many misconceptions surrounding calibration when it comes to using a SpeedCoach. Here is some background information that will hopefully help you set up your SpeedCoach for your individual needs.

All SpeedCoaches (Red, Gold and XL) have a "calibration" value that helps the unit compensate for different impeller locations and to perform accurately on that particular boat. All boats are shaped differently and hull integrity can be jeopardized due to damage. Additionally, due to sealed bulkheads and the boat's internal structure, sometimes it is impossible to mount the impeller & blue speed sensor exactly 5 meters from the bow. To compensate for these inconsistencies from boat to boat, you can adjust your SpeedCoach's calibration value to reflect your particular boat.

This individual calibration is NOT necessary for the SpeedCoach to be a useful training tool. When the impeller is mounted 5 meters (16' 3") from the bow, the factory calibration value of 1.000 will provide readings that are very close to accurate. And even if your unit is not calibrated, you will always see relative changes in boat speed.

For example, if Monday your average split was 1:57 and Tuesday your average split is 1:55, then you are going faster on Tuesday than you were on Monday. Additionally, if you are using the SpeedCoach to seat race athletes, and the average speed for the lineup with Joe is 1:45, and the average speed for the lineup with Sam is 1:42, then Sam's lineup is moving the boat faster.

So why calibrate? Many teams use the SpeedCoach to monitor their speed based on a time standard that they are trying to achieve. The SpeedCoach reading absolutely accurately is important in this instance, and calibration would be an extra step to ensure this.

For example, if your crew is preparing for their championship race and you know that typically, crews that advance to the final go around/under 8:00, then you can build a powerful training plan based on knowing that you need to average a 2:00 per 500 meter split. You can build in target splits for pretty much every workout all season, and by gauging your daily progress compared to the targets, you'll be able to anticipate how your crew will stack up on race day.

Whether to calibrate your unit or just go with the factory calibration of 1.000 is an individual decision based on your training needs. If you decide that you want to calibrate your SpeedCoach, the formula is at the end of this post. (The SpeedCoach XL's feature a nifty calibration routine that steps you through the process and does the math for you.)

A few parting notes:
* Calibration is boat and impeller location specific. If the unit is moved to a new boat, or the location of the impeller is modified, it is recommended that the unit be recalibrated.

* If you are in a team setting with mulitple boats and SpeedCoaches, if you are going to calibrate, try to keep the same unit with the same boat. This way, you won't need to change the calibration value on the SpeedCoach unit every day. Additionally, once you calibrate a unit to a particular boat, consider marking the impeller location with permanent marker and jotting down the calibration value somewhere on the boat so it's easy to remember.

* It is recommended that regardless of whether you plan to calibrate or not, the impeller should be placed as close to 5 meters from the bow as possible. Every boat has a turbulent boundary layer of water underneath the shell, and as you move to the stern, the layer gets larger. Placing the impeller closer to the stern increases the chance that the water will be too turbulent for the impeller to spin consistently. NK has wiring harnesses for every size boat so that the blue speed sensor can be mounted 5 meters from the bow.

* 5 meters from the bow may seem like a random number, but on most singles, that's typically an accessible area. In most sweep boats, that's typically right around 2 seat, also an easily accessible area.

Calibration Routine
To calibrate the SpeedCoach, row over a measured distance on still water (on flowing water, row both up and down stream and average the results shown on the display). Then, use the following formula to obtain the new calibration value:

(Old Cal Value) (True Distance)/Displayed Distance= New Cal Value

For example, if the known rowed distance is 1 mile, but the display shows 0.92, the calibration value would be:
(1.00) (1.00)/0.92 = 1.086

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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