Measuring junk sailing performance

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  • 11 Nov 2018 20:10
    Reply # 6899470 on 4913961

    FYI---I just noticed that 3 of the Poppy CSVs are empty.

    PS actually 6, s5,6,and 7  and p4,5,6.

    Last modified: 11 Nov 2018 20:24 | Anonymous member
  • 11 Nov 2018 19:14
    Reply # 6899435 on 4913961

    Poppy prelims. Even TWSs. Histograms, etc here. I also have a split junk rig so found these results particularly interesting.

    In Slieve's AYRS catalyst article he mentions that Poppy goes at about half the wind speed. These preliminary results supports that. I point to the TWS=6knt polar plot which, most likely, all 7 panels were up. N=1100 plus data points. Many of those point in the 30,40,50 and 150,160,170 and 180 degree bins, i.e. upwind and downwind TWAs. Median boat speed 3 knts or better in all bins in the 6 knt wind.

    I'm wondering about the "30 deg" TWA...is that realistic for a westerly longbow? Seems unusually close-winded for the design but I don't know the boat. There are 200 points in that bin having TWAs between 25 and 34.9 degrees. Are all/most due to tacking? It's fairly easy to look at the time series of d(twa)/dt and d(tws)/dt. Obvious tacks should show up as hills or valleys and "steady state" as flats.....maybe.

    rself

    Last modified: 11 Nov 2018 19:26 | Anonymous member
  • 11 Nov 2018 18:45
    Reply # 6899424 on 6899329
    Anonymous wrote:

    Hello Robert,

    The CV7 wind sensor is outputting eg

    $IIMWV,047.0,R,014.9,N,A*32

    where 047.0 is wind direction
    R is reference
    014.9 is wind speed
    N is knots
    A is 'correct measure'
    *32 is checksum

    Also $WIXDR wind temperature which we aren't interested in.

    The DX900+ log sensor is outputting eg

    $VMVBW,4.65,-1.00,A,,,V,,V,,V*64    dual speed
    $YXXDR,A,-3.4,D,PTCH,A,-10.6,D,ROLL*6D pitch and roll
    $VMNLA,25.6,A*06     leeway
    $VMVLW,38.68,N,28.07,N,,,,*5E    distance
    $VMVHW,,,,,4.96,N,9.19,K*5D     boat speed

    I think that NavMonPC is then adding date and time, because that sentence is $GPZDA which doesn't appear in the text files. We would need another source if we don't use NavMonPC.

    I've put a full list of the NMEA 0183 sentences here





    OK. Thanks David. Looks like only the strings that need to be pulled are $VMVHW and $IIMWV to generate the stw, awa and aws values in the  .csv's. I'll do a test run with the first weaverbird text file and see if get any sequences of values that resemble what's in the csv.
    Last modified: 11 Nov 2018 18:48 | Anonymous member
  • 11 Nov 2018 16:13
    Reply # 6899329 on 4913961

    Hello Robert,

    The CV7 wind sensor is outputting eg

    $IIMWV,047.0,R,014.9,N,A*32

    where 047.0 is wind direction
    R is reference
    014.9 is wind speed
    N is knots
    A is 'correct measure'
    *32 is checksum

    Also $WIXDR wind temperature which we aren't interested in.

    The DX900+ log sensor is outputting eg

    $VMVBW,4.65,-1.00,A,,,V,,V,,V*64    dual speed
    $YXXDR,A,-3.4,D,PTCH,A,-10.6,D,ROLL*6D pitch and roll
    $VMNLA,25.6,A*06     leeway
    $VMVLW,38.68,N,28.07,N,,,,*5E    distance
    $VMVHW,,,,,4.96,N,9.19,K*5D     boat speed

    I think that NavMonPC is then adding date and time, because that sentence is $GPZDA which doesn't appear in the text files. We would need another source if we don't use NavMonPC.

    I've put a full list of the NMEA 0183 sentences here





    Last modified: 11 Nov 2018 16:26 | Anonymous member
  • 11 Nov 2018 14:14
    Reply # 6899240 on 6886808
    Anonymous wrote:

    Ideally, we should write our own software for the recording programme as the prospects of getting the authors of NavMonPC to update theirs seem slim.

    Hi Alan--Question about the processing steps:

    1. All the data collected by the electronics is in the .txt files (??).

    2. .txt files read by NavMonPc --- user sets some parameters---then outputs a .csv file.

    So date, time, aws, awa and port/stbd information are all in the .txt file?

    Found a Matlab nmea decoder in the Mathworks file exchange but none of $G--strings in your files match the strings in the prog. Do you have a source for the sentence definitions?

    I'll post first half of Poppy results pretty soon.

    rself

    Last modified: 11 Nov 2018 14:21 | Anonymous member
  • 11 Nov 2018 12:04
    Reply # 6899181 on 6898120
    Alan wrote:

     I think it confirms my impression that the approach of taking the median of all the data leads to an unrealistically low result.

    It occurs to me that a better approach might be to use an idea similar to the concept of "significant wave height", which is the mean of the highest third of the data, the data being the measured wave height of thousands of waves. This gives a figure that corresponds to an experienced observer's assessment of the sea conditions, but effectively smooths out any extremely large waves, and discounts the lowest two thirds of the data. Maybe we could and should use a similar approach to extract a result from the data that an experienced observer would recognise as "reasonable".

    The Pareto Principle, or 80/20 rule, might also be applied. That is, it's easy to get 80% of available performance with 20% crew effort/concentration, but getting that last 20% of performance requires a much higher level of skill and concentration. I would guess that most of us sail at the 80% of available performance level (unless we're racing and concentrating 100%). Sea state, too, will have an enormous influence. In flat water, we might get 100% of available performance, but in wind against tide conditions we might struggle to get 50%. 

    Once Anthony's approach has discarded data that is not steady state, because the boat is turning, or the wind is changing, then perhaps a line drawn through the 80% points would be quite realistic, as a guide to what performance is readily achievable.

    Last modified: 11 Nov 2018 12:16 | Anonymous member
  • 10 Nov 2018 12:46
    Reply # 6898120 on 4913961
    Anonymous member (Administrator)

    That is very interesting because it shows the wide distribution of the data points about the median points and the smoothed curve through the median points.

     I think it confirms my impression that the approach of taking the median of all the data leads to an unrealistically low result.

    It occurs to me that a better approach might be to use an idea similar to the concept of "significant wave height", which is the mean of the highest third of the data, the data being the measured wave height of thousands of waves. This gives a figure that corresponds to an experienced observer's assessment of the sea conditions, but effectively smooths out any extremely large waves, and discounts the lowest two thirds of the data. Maybe we could and should use a similar approach to extract a result from the data that an experienced observer would recognise as "reasonable".

    I'm delighted that we are investigating several approaches to analysing the data.

    Anthony Cook is refining his software to eliminate points where the wind speed drops more than 0.7 knots per second, and where the wind direction changes by more than 5 degrees per second. The intention is to eliminate any data that may show a false over-performance because the boat speed drops off more slowly than the falling windspeed, or change of angle. He is also eliminating all the subsequent data points until the boat speed changes, to be sure that the effect of the change of wind speed or direction has been reflected in the boat speed.

    I hope, Robert, that you will be able to extend your analysis to the other boats. It would be very interesting to see the resulting curves for all the boats at one wind speed on the same graph.

  • 09 Nov 2018 14:14
    Reply # 6896406 on 4913961

    More weaverbird plots. Should have started with a picture of the raw data before reduction to medians then furthur reduction by a smoothing function but the coding is more difficult so saved it for last.

    I created polar plots showing the point scatter about the median STWs (red squares) separately for each TWS bin. If combined into one plot it'd be a hard-to-interpret mess of points. Only the "even" TWS bins (4,6,8,... knts) plots are shown below. The "odds" plots which use the other half of the data are similar and show the same trends and can be found here:

    The medians tend to be in the center of mass of the point scatter. If sample size is small then that median is suspect.

    If sample size per bin is large relative to the natural variability then outliers have very little effect on the median.

    If the intent is to use these data to detect "before" vs "after" rig performance on the same hull then:

    Might be tough....the variability is large and so the data overlap will also be large given you're trying to detect small percentage changes.

    One, perhaps convincing, result would be if all the medians for rig A were higher than rig B even though the points overlapped.

    Another possibility if comparing bermuda vs junk is a take-off from the perception that cambered junk is slower beating but faster off the wind. If true then there should be a STW crossover around 90 deg TWA? in the data.

    rself


    Last modified: 09 Nov 2018 17:47 | Anonymous member
  • 03 Nov 2018 13:42
    Reply # 6886808 on 6886773
    Anonymous member (Administrator)
    Anonymous wrote:

    The DX900 log unit is capable of outputting a figure for the leeway, as well as for forwards speed. If we are moving away from Polauto in favour of in-house processing of the data, could the vector diagrams and the code be updated to include leeway?


    The problem here is that NavMonPC does not at present give us the option of recording leeway, but it is recorded in the .txt file of NMEA sentences. Unfortunately, because of the problem with the Vyacht Mx it was only recorded intermittently, like the boat speed.

    Ideally, we should write our own software for the recording programme as the prospects of getting the authors of NavMonPC to update theirs seem slim.

    Any volunteers out there to write a suitable program?

  • 03 Nov 2018 13:38
    Reply # 6886804 on 6885376
    Anonymous member (Administrator)
    Anonymous wrote:

    Reading the manual for the LJC Capteurs CV7 wind sensor, I note that it updates the output at the rate of 2 per second. Wouldn't that be a reasonable rate for water speed as well? Is there any advantage in collecting water speed data at 5 per second?


    The new multiplexer should allow us to comtrol these things better than the old one. The old Vyacht Mx had no priority controls, so in practice the boat speed sentence was being crowded out by other sentences which were less important or irrelevant to us. This meant that regardless of how frequently the ST900 sent boatspeed data, it only changed every few seconds, as you can see from the .csv file.

    Personally I think recording the data every second is plenty. bear in mind that if you double the rate you double the amount of data to be processed.

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