NK Impeller: Small Investment, Big Payoff

For serious rowers, reliable and accurate data is essential. The NK Impeller is a game-changing tool that offers enhanced data collection and analysis at an affordable price, making it an invaluable asset for anyone looking to optimize their rowing performance. Discover how an impeller can help you finetune your technique, track your progress, and, ultimately, make your boat go faster.

Heavy rains in the watershed recently translated into high water levels and an elevated current in the river where I row. From what previously had been almost a lake in the laziness of the river's speed, there was now a significant difference between rowing upriver or with the current. Forty minutes to get to the turn-around point and 20 minutes back to the dock. The difference was clear on our SpeedCoach GPS 2 screen, with split times not comparable when a workout was done in one or the other direction. My doubles partner and I are not elite athletes; in fact, we rarely compete. While we love to train, learn, and improve wildly different data is more of a curiosity than a significant problem.

For serious rowers, reliable and accurate data is essential regardless of what kind of water you row on. Data and accompanying interpretation and insights give coaches and athletes feedback on training programs, equipment finetuning, technical skill, and fitness levels. Crew selection may be driven by results read on a screen in the coach launch. Incremental improvements could mean a win. Inaccurate data could lead to injury, burn-out, or worse.

Many variables influence the data collected in rowing and decisions linked to that data. Subtle equipment differences, how well nourished or slept an athlete is, and weather can all impact what is uploaded to the LiNK Logbook for later analysis. Boat speed is affected by water temperature, wind direction, and strength. Of all the possible factors, current is perhaps the most significant. It is not just upstream vs downstream. The impact of current on a boat is influenced by distance from the shore, water depth, and bends. The challenge of obtaining accurate and comparable on-water data is one reason that ergometer testing became so significant in athlete identification and crew selection. Even if you train on a lake, you might race on a river.

How The Impeller Works

GPS technology is generally understood. It is packed into the SpeedCoach GPS 2, essentially a tiny computer on your boat. Recent advances translate into rapid refreshes of data like boat speed. Strap it onto the rigger or a mount, push start, and begin rowing and data flows. The SpeedCoach GPS is generally accurate within 1%, translating to under 1.5sec / 500m in most conditions. Like all GPSs, it measures the movement relative to land. Fine if you are rowing on a dead calm lake. Add in a helping or hindering current, and the data is less accurate and potentially not as useful.

Sykes Racing explains the problem with an easy-to-understand analogy:

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 at the gate, you are moving 1 mph. (In fact, he thinks about calling security.)

It is the same on the water. It doesn't matter to an observer on the shore if you appear to row 500 m in 2 minutes or 4 minutes. What matters is the speed of the boat in the water, not its speed relative to the land. If you are putting in the same work applying the same pressure, the speed will be the same regardless of the time it took. Make a small adjustment to the rigging or adjust your stroke to catch just a little earlier and you want to know if it made a difference, regardless of whether you were rowing with or against the current.

The best way to get accurate data in moving water is to use an impeller. Small and slim, the NK Impeller is mounted on the hull of the boat and connected by a cable to the SpeedCoach GPS 2. It has a small spinning part. The speed that it spins at corresponds with the speed of the boat relative to the water. Sensors in the boat record the spinning and translate that into what you see on your SpeedCoach screen. That easy and all just for $30¹ plus wiring (which depends on your boat).

Tips for Using the Impeller Effectively

1. Placement is important

The recommended positioning is 5 m or 16'3" from the bow. This is typically under the foot stretchers of a single, or around two seat of a larger boat. Do not install it further back than 6 m from the bow.

Laminar vs. turbulent flow is the reason for these precise instructions. Laminar flow begins at the bow of the rowing shell and is important in making the boat go faster. You do not want to impede this flow with the Impeller or anything else. Further back in the boat, the flow becomes turbulent which slows the boat. Anything installed on the hull creates turbulent flow, whether a fin or an Impeller. If the Impeller is too far back the water will be too turbulent for the impeller to spin consistently. 5 m is the sweet spot.

NK Sports provides detailed installation instructions (Impeller Wiring Installation) for mounting the Impeller, installing the wiring and connecting the wiring to a SpeedCoach GPS 2. It is strongly recommended to read through all the instructions before beginning. There is information on how to make a more temporary installation, should you wish to remove the Impeller periodically

2. Calibrate and recalibrate

While small and seemingly simple, the NK Impeller gives the most accurate comparative data once it has been calibrated. This is especially relevant if you are comparing data amongst boats or to published data. An uncalibrated Impeller still gives very useful information for that one boat, for example eliminating the impact of current on workout results, testing modification in technique or equipment and monitoring changes in athlete fitness and strength.

Calibration of the SpeedCoach GPS 2 and Impeller is like how you calibrate your car's odometer – checking what the instrument shows you when rowed on a measured course. See the installation instructions for details and formula. Calibration is boat and impeller location specific. If the unit is moved to a new boat, or the location of the impeller is changed, recalibration is recommended.

3. Keep it clean

Small weeds, bits of dirt adhering to the spinning part of the Impeller will impact its movement and the data it provides.

4. When to take it off

Rowing forums offers ample discussion and debate about how much drag (or not) is caused by the Impeller. Crews may choose to remove the device for a race. Be sure to not leave any residue on the hull, which might cause turbulent flow.

5. When you transport the boat, tape or remove the Impeller

Whether on top of a car or on a trailer, transporting a boat with an Impeller will cause wild spinning which wears out the magnet. Remove or cover the Impeller. In an entertaining and useful discussion forum about calibrating an Impeller, Carl Douglas, the renowned British boat builder, offered the suggestion of taping it before travel. He also digresses into the perils of rowing in mercury. All good fun and further benefit for that small investment in a NK Impeller - in addition to getting that extra edge in your training.

¹ 2024 pricing in USD and not including shipping