rowsandall/rowers/templates/physics.html

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<div class="grid_6 alpha">
<h3>How we calculate things</h3>
<p>You are reading this because you want to understand how the wind/stream conversion and the conversion from OTW pace to OTE pace works.</p>

<p>The conversions are done using a one-dimensional mechanical model that is
introduced <a href="https://sanderroosendaal.wordpress.com/index/">here</a>.
The model takes into account, among others, the following parameters:
<ul>
	<li>Stroke rate</li>
	<li>Stroke length</li>
	<li>Rigging parameters</li>
	<li>Rower and boat weight</li>
</ul>
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. 
</p>
<p>
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. 
</p>
<p>
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. 
</p>

<p>
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.
</p>

<p>
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.
</p>

<p>Not taken into account are the following factors:
<ul>
	<li>Water Temperature</li>
	<li>Heavier/shorter/wider boats than the ones used by the elite</li>
	<li>Bungees, weed, or other artefacts slowing down the boat</li>
	<li>Boat stopping technique flaws</li>
	<li>Effect of wave height or cross-wind</li>
</ul>
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.
</p>

<p>
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 <a href="http://www.biorow.com/RBN_en_2007_files/App2007RowBiomNews08.pdf">here</a> 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.
</p>

<p>
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.
</p>


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<div class="grid_6 omega">
<h3>Manual</h3>

<p>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.
</p>

<p>Recipe for success:
<ol>
	<li>Click on the workout. This will bring you to the workout Edit view</li>
	<li>Click on the "Advanced" button</li>
        <li>Click on "Geeky Stuff"</li>
        <li>Look for three buttons labelled "Edit Wind Data", "Edit Stream
          Data" and "OTW Power"</li>
        <li>If you have wind or stream data, click on the appropriate
          button and enter your data.</li>
        <li>If you have both wind and stream, click the shortcut button
          on the respective page to take you to the other parameter</li>
	<li>Click on OTW Power</li>
	<li>Select the boat type and enter the average weight
          per crew member.
	  Do not use the crew total weight.</li>
	<li>Click "Update & Run"</li>
	<li>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.</li>
        <li>Progress can be monitored by clicking on "here" in the message
          at the top of the page advising that the calculation has
          begun.</li>
	<li>
          When the calculation is complete, go back to the "Geeky Stuff" page
          and click on "Corrected Pace Plot" to see the result. From here, you can re-run the calculation with different parameters.</li>
</ol>
</p>

<p>
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.
</p>

<h3>Why does the calculation take so much time?</h3>

<p>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.</p>

<p>
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.</p>

<p>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!
</p>

<img src="/static/img/validation.png" width="450">

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