Free Data and Analysis. For Rowers. By Rowers.

 

 

 

 

 

 

How we calculate things

You are reading this because you want to understand how the wind/stream conversion and the conversion from OTW pace to OTE pace works.

The conversions are done using a one-dimensional mechanical model that is introduced here. The model takes into account, among others, the following parameters:

  • Stroke rate
  • Stroke length
  • Rigging parameters
  • Rower and boat weight
For this site, we use "standard" rigging parameters, blade shapes, and FISA minimum boat weights. For the eight, I add the weight of a cox (at the FISA minimum weight). The stroke length is also set at a fixed value, but in the future I will allow you to adjust to your own stroke length.

Knowing boat type (rigging), pace and stroke rate, and taking into account the influence of wind and stream (if provided), I am able to find the mechanical power that you provide to the rowing system by a reverse calculation. That is, I vary the input force until I find the one that corresponds to your actual pace at that stroke rate.

Knowing the power, I can calculate how fast you would have gone without external wind and stream influences by running the calculation in a forward way, using the power and force profile found. This is the wind/stream corrected pace, which I think is useful to know and be able to compare from training to training and between different rowing venues.

Using another algorithm to calculate total mechanical power on an erg, I can calculate what the erg display would show you if you rowed on the erg with the same average power, at the same stroke rate. The calculations are done for a statical Concept2 erg with a fairly standard drag factor. I cannot take into account the fact that you may use different technique or would row at a different stroke rate on the erg.

It is important to understand that the Power display on the erg is not showing you the complete picture. In my calculations, I use my proprietary algorithms to calculate the additional power that goes into moving your body weight up and down the slide on a static erg. To get the most accurate results, it is important to be honest about your weight and set it independently for each workout.

Not taken into account are the following factors:

  • Water Temperature
  • Heavier/shorter/wider boats than the ones used by the elite
  • Bungees, weed, or other artefacts slowing down the boat
  • Boat stopping technique flaws
  • Effect of wave height or cross-wind
The water temperature has a small but measurable effect on the water density (and thus on the drag). I am using the value at 20 degrees C, which is a good average over the OTW season for a lake in a temperate climate. All the other elements result in an equivalent erg pace that is probably slower than what you can achieve on the erg. So look at it as an incentive to improve your technique (big effect) and/or buy a faster boat (minor effect). If your OTW to OTE pace conversion results in numbers close to what you normally achieve on the erg, you are rowing like an elite rower (but possibly at a lower power)! When I get around it, I will try to model the effect of cross-wind.

I have checked the model both from a Physics perspective (I have a degree in Physics, if you are interested) and compared with the data available. An important data set has been published here by Dr Kleshnev. For sculling, my algorithms are extremely close in reproducing that data set. For sweep rowing, I am still fine tuning some parameters, but I am close for a pair and a four. I had to make assumptions about Kleshnev's data, especially about the stroke rate, but as I got realistic stroke rates (35 and higher) for world record performance, I am quite confident.

On top of that I am constantly comparing the model's results to my own sculling and rowing, and I will be the first to admit flaws and correct them. So please contact me if there are any inconsistencies, suspicions, questions or simply if you want to chat about Rowing Physics.

Manual

Here's the best way - in my mind - to use the Rowing Physics functionality. I am assuming you have successfully uploaded or imported a rowing workout. You must have position data (lat/long) with your row. A TCX from CrewNerd or RiM or a workout imported from SportTracks or Strava (where you see a map of your workout on those sites) should have those data. I am working on adding the FIT file format that is used by SpeedCoach GPS. For now, export the data to Strava and then import them here.

Recipe for success:

  1. Click on the workout. This will bring you to the workout Edit view
  2. Click on the "Advanced" button
  3. Click on Edit Wind Data (optional) to edit wind data. Go back to Advanced when you're done.
  4. Click on Edit Stream Data (optional) to edit stream data. Go back to Advanced when you're done.
  5. Click on OTW Power
  6. Select the boat type and enter the average weight per crew member. Do not use the crew total weight.
  7. Click "Update & Run"
  8. Go do something else. You will receive an email when the calculations are finished. The calculation itself will take about 10 minutes for an hour long row, but there may be other people's calculations in the queue, so it may take longer.
  9. From the "Advanced" view, click on "Corrected Pace Plot" to see the result. From here, you can re-run the calculation with different parameters.

Once you have run the calculation, the boat type, average crew weight, Power and corrected pace data are stored permanently on the site. If you would export the data to Strava or SportTracks now, those sites will have the Power data.

Why does the calculation take so much time?

I am running the calculations from a first principles base, so for each data point that I am calculating, I am finding the stroke average force, then calculating corrected pace (wind/stream) and finding the corresponding erg power. I am not taking any shortcuts. The advantage of this approach is that I can give you numbers irrespective of your weight, speed, stroke rate, sex, etc. The model can deal with circumstances it has not encountered before. The downside is that it takes time.

A much faster approach would be to simply take pre-calculated data from a table and interpolate. The advantage of this approach is is speed. The disadvantage is that extrapolation outside the limits of the available data is dangerous and will lead to erroneous results.

Future versions of this site will use a hybrid approach but only for pace/wind/stream/stroke rate/weight combinations that I consider well validated. For that, I need to collect data, so keep the workouts coming!

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