GPS vs. IMPELLER

NK has now released the SpeedCoach GPS Model 2 that uses a 3 axis accelerometer & a 5 Hz GPS (Garmin & others are only 1 Hz). This now addresses the issue of slow updates of boat speed. The problem of moving current still remains: you are measuring the speed relative to land. The NK SpeedCoach overcomes this by being able to be switch to Impeller mode, therefore having your true speed relative to the moving water.

With emerging technology and affordability of GPS, coaches and rowers often ask why NK's speed measurements still rely on an impeller. Below is a recent article written by Dr. Volker Nolte, an expert on biomechanics as well as an internationally accomplished coach and rower, comparing GPS and Impeller based speed measurements:

Thanks to the improvements in technology, it is now possible to give rowers invaluable feedback about their performance in the boat. Electronics developed at a pace that was unthinkable a few years ago measures time and distance with minuscule sensors and computer chips. The computer then calculates related quantities like stroke rate and speed, displays it on monitors, and still is small and light enough to be used in a racing shell without any measurable influence on the performance of the rower. Therefore, it is understandable that more and more rowers utilize such equipment.

Rowers use electronic feedback equipment to direct their training towards very specific, but different goals. High performance athletes want to control their training intensity to gain the desired fitness improvements in the most effective way and look for feedback during their races. Recreational rowers need information to direct their power output for health reasons. Researchers use training and race data to analyze performances for physiological, biomechanical, and strategy studies.

For each of these tasks, it is critically important that the feedback one receives from the equipment is correct. The electronics provide indicators that potentially have very serious and important consequences for the rower. In case of incorrect data feedback, athletes may choose inappropriate intensities that do not lead to improvements, or even put the athletes' health at risk. Obviously, researchers need proper data to interpret their findings correctly.

Therefore, it is very important for any user to know exactly which data the respective equipment provides. In general, two different methods are currently utilized to measure the quantities mentioned above: Impeller and GPS.

Impeller measurement is based on the principle that the water sets it in a motion that corresponds with the speed of the boat that it is attached to. The motion of the impeller is directly dependent from the flow of the water relative to the boat. This means that the movement of the boat relative to the water is measured. Sensors in the boat record the spinning of the impeller to calculate the required data.

Global Position Systems (GPS) measure the position of the receiver as a place on the earth's surface. If the receiver is connected with the boat, GPS is able to track its movement and can calculate various data from that. This means that the movement of the boat relative to the ground is measured. Below, you find a comparison of the two measurement systems.

General Information
System Impeller GPS*
Necessary Equipment Monitor, impeller, wires, magnet on sliding seat if possible, but not necessary Monitor, receiver
Necessary Preparation Impeller and wire installation, calibration None
Measurement Method Impeller spins with moving boat relative to water; computer counts turns of impeller; calculates distance traveled relative to water, boat velocity relative to water, stroke rate Computer receives signals from satellites to determine position on earth every 1-3 seconds; calculates distance traveled over ground and boat velocity over ground
Calibration Through rowing of a known distance, the calibration factor is found and set on monitor Automatically done by computer
Accuracy Distance travled and velocity always relative to water: Less than 2% for any measurement with or without current Distance traveled and velocity always over ground:
~1-15m for position or any distance;
This means that without current, the distance or velocity measurement for one single stroke could be off by more than 10%, but accuracy improves dramatically with overall distance and the calculation of average velocity over longer time; Measurements of actually traveled distance and boat velocity on water with current are off depending on the relation of the current speed to average speed of the boat relative to the water
Energy consumption Low Relatively high
  Impeller GPS*
Without current With current Without current With current
Actual total distance rowed Positives
- very good if properly calibrated
- Accuracy 1-2%

Negatives
- calibration necessary
Positives
- very good if properly calibrated
- Accuracy 1-2%

Negatives
- calibration necessary
Positives
- very good for large distances
- Accurate by 1-15m

Negatives
- possibly inaccurate for small distances
Positives
- measurement is over ground


Negatives
- extremely inaccurate
Speed per stroke Positives
- very good if properly calibrated
- Accuracy 1-2%

Negatives
- calibration necessary
Positives
- very good if properly calibrated
- Accuracy 1-2%

Negatives
- calibration necessary
Positives


Negatives
- measurements vary widely from stroke to stroke
Positives


Negatives
- extremely inaccurate
Average pace Positives
- very good if properly calibrated
- Accuracy 1-2%

Negatives
- calibration necessary
Positives
- very good if properly calibrated
- Accuracy 1-2%

Negatives
- calibration necessary
Positives
- very good through accuracy of travelled distance
- improvements through smoothing programs
Positives


Negatives
- extremely inaccurate

Despite its very simple usage, GPS systems have to be operated with care. The information generated by GPS is potentially extremely inaccurate, especially when used on a body of water with current. Used without consideration of this fact, the training feedback could harm an athlete's development or even health.

The impeller system, however, clearly shows advantages when calibrated properly. When not calibrated, the impeller system will still show accurate, corresponding changes in speed.

* Information regarding GPS systems relates to commercially available instrumentation at comparable costs of impeller systems

NK Guest Blogger: Volker Nolte

About Volker Nolte

Volker Nolte is men’s head rowing coach and assistant professor at the University of Western Ontario, where he teaches coaching and biomechanics and coaches the highly successful men’s rowing team. He was the lightweight men’s national team coach with Rowing Canada from 1992 to 2000. His national team crews won an Olympic silver medal at the 1996 Atlanta Games, two World Championship titles in 1993 and 2000, and several medals at World Championships in recent years.

Nolte received a PhD in biomechanics from the German Sport University in Cologne, and is an internationally acknowledged expert in biomechanics. With his expertise in the coaching field, he presents frequently at scientific and coaching education conferences worldwide. This year, Volker published his latest book, "Rowing Faster".

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