April 14, 2016

Wind predictions (from windfinder.com) for the Brno lake for today:

Morning: 5 m/s (max 11) – 3 Beaufort

Afternoon: 8 m/s (max 10) – 5 Beaufort

So the decision was easy, given that our lake can be choppy already at 2 m/s, depending on the wind direction. I will row in the morning. Today’s session was a 24×45″/R60″ at 29-30spm.

Indeed, when I arrived there was a fierce wind blowing down the lake. Chop was not too bad though.

Workout Summary - 2016-04-14-0718.CSV
--|Total|-Total-|--Avg--|Avg-|-Avg-|-Max-|-Avg
--|Dist-|-Time--|-Pace--|SPM-|-HR--|-HR--|-DPS
--|07703|37:02.0|02:14.2|28.3|167.8|174.7|08.7
Workout Details
#-|SDist|-Split-|-SPace-|SPM-|AvgHR|MaxHR|DPS-
01|02015| 11:01 |02:44.0|18.3|145.0|156.0|10.0 | Warming Up
02|00200| 00:45 |01:52.5|29.4|153.0|170.0|09.1 | Tailwind
03|00198| 00:45 |01:54.0|28.0|168.0|174.0|09.4
04|00200| 00:45 |01:52.4|29.4|170.0|176.0|09.1
05|00196| 00:45 |01:55.1|30.7|168.0|175.0|08.5
06|00199| 00:45 |01:52.8|29.4|169.0|176.0|09.0
07|00200| 00:45 |01:52.4|30.6|171.0|178.0|08.7
08|00173| 00:45 |02:10.5|29.3|168.0|176.0|07.9 | Headwind
09|00177| 00:45 |02:06.8|29.4|171.0|178.0|08.0
10|00174| 00:45 |02:09.5|28.0|174.0|179.0|08.3
11|00175| 00:45 |02:08.6|28.0|174.0|178.0|08.3
12|00178| 00:45 |02:06.5|29.3|174.0|179.0|08.1
13|00181| 00:45 |02:03.9|30.7|174.0|180.0|07.9
14|00197| 00:45 |01:54.2|29.4|169.0|177.0|09.0 | Tailwind
15|00000| 00:45 |0000:00|30.6|172.0|176.0|00.0 | GPS error
16|00000| 00:45 |0000:00|24.0|170.0|174.0|00.0 | GPS error
17|00193| 00:45 |01:56.5|29.4|164.0|173.0|08.8
18|00194| 00:45 |01:56.0|29.3|169.0|177.0|08.8
19|00186| 00:45 |02:00.6|28.0|169.0|174.0|08.9
20|00169| 00:45 |02:12.9|28.0|165.0|174.0|08.0 | Headwind
21|00177| 00:45 |02:06.9|29.3|172.0|176.0|08.0
22|00176| 00:45 |02:07.9|28.1|171.0|176.0|08.4
23|00173| 00:45 |02:10.1|28.0|170.0|177.0|08.2
24|00174| 00:45 |02:09.1|29.3|170.0|176.0|07.9
25|00179| 00:45 |02:05.2|30.7|173.0|179.0|07.8
26|01619| 08:01 |02:28.8|20.1|150.0|158.0|10.1 | Cooling Down


It was a nice session. In interval 15, the XGPS160 decided to stop. This external GPS is nice. It saves phone battery power, is more accurate and has a higher data rate than the in-phone GPS, but it has it’s glitches. In interval 16, I got massively waked by a bunch of water quality scientists in a fast motor boat. I saw them launching on the bank when I turned after interval #7. When I rowed down the lake again, they were speeding up the lake. They take water quality and other scientific measurement at various buoys on the lake, but they clearly don’t understand how to be a good skipper.

Here are the graphs. They are slightly different today.

What’s going on? I have added some rowing physics to the python plots to create a Power plot. The power plot takes into account the wind direction and strength, and that also enabled me to calculate what pace I would have had in windstill conditions. The whole thing is massively experimental, but so far it works. To get the wind effect, I had to estimate the drag factor of a rowing shell. I used these data from Kleshnev:

They are fits to a very old data set collected by Klaus Filter. I couldn’t find the original data. Some more data (or perhaps the same data in a different representation) were found on the Australian Nielsen Kellerman website. I used that to tune the drag constant to achieve realistic results in my calculations. Here is the reproduction of the top left picture of Kleshnev:

I had to create a shielding equation for the bigger boats. My data will be more reliable for the single. Anyway, you can see very clearly what every rower knows. Headwind slows you down more than tailwind speeds you up.

I did my calculation using an average wind direction and speed for the entire outing. If I mounted a wind meter on the boat, I could even have processed instantaneous wind.

For now I had to rely on data from nearby weather stations. It was interesting to see that those that are closest to the lake reported windstill (less than 1 m/s) weather for the time of my row. I definitely know that this is not the case. Here is the SportTracks report:

I suspect those people have their weather station in their nice garden surrounded by trees. Definitely not representative for what happens on a large body of water.

At the same time Brno airport (LKTB) reported about 6 m/s, but that was on the other side of the town. But it is in line with the wind prediction by windfinder.com.

In the end I did my calculations for a 2.5 m/s wind and a wind direction of 290 degrees. Produced credible results, given that I estimate that the headwind cost me about 10 seconds of pace.

Anyway, all this is just theoretical rambling. I would have to get a wind meter, mount it on my boat, and start measuring. Not sure if I will do that … they already consider me a nutcase right now.

I just find it interesting … and I like Physics.