Measuring junk sailing performance

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

    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


    9 files
    Last modified: 09 Nov 2018 17:47 | Deleted user
  • 03 Nov 2018 13:42
    Reply # 6886808 on 6886773
    Anonymous
    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
    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.

  • 03 Nov 2018 12:55
    Reply # 6886773 on 4913961

    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?

  • 02 Nov 2018 12:03
    Reply # 6885376 on 4913961

    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?

  • 02 Nov 2018 10:04
    Reply # 6885272 on 6884231
    Anonymous
    Anonymous wrote:
    Anonymous wrote:

    That is very interesting, Robert, and thank you for doing this work, but I think the fact you are taking the median values from a group of data points will give a lower performance than it is realistically possible to obtain.

    I don't disagree. By taking high values you will be more likely to approach the boat's theoretical VPP values. It's easy enough to do max values and see if the plots change significantly.

    So I assume the 5Hz data is not available?

    I'm plugging bugs in my code. Turns out the TWS bins (4, 6, 8, etc) only use half the data. I'll have to do TWS = 3, 5, 7, etc bins to see the other half. I really don't think polars will change much.

    I'll put my Matlab code in google drive for anyone who happens to use the app. Alternatively there are free download Matlab-mimics like Octave that may work with minimal alterations.

    rself


    The 5Hz data is available in the .txt record of the NMEA sentences. NavMonPC does not allow the .csv file to collect more than once a second, so far as I can tell.
  • 02 Nov 2018 10:00
    Reply # 6885271 on 6884161
    Anonymous
    Anonymous wrote:
    Anonymous wrote:

    That is very interesting, Robert, and thank you for doing this work, but I think the fact you are taking the median values from a group of data points will give a lower performance than it is realistically possible to obtain.

    It would be better, IMHO, to take the maximum figure from each "box" of data. This is the approach taken by Polauto, and by Anthony Cook.


    It appear so me that median values will give a better picture of the performance than just picking the maximum figures:
    High peak values can easily be obtained by picking up speed on a lower course and then shoot up a little, which would give a very good VMG reading if caught in the right moment.
    Note: I don't accuse anyone for cheating, I just know from my own experience that it is difficult to keep a correct course all the time. Sometimes I aim a bit low (and pick up speed) and sometimes I aim a bit high. I guess I am not alone on this.

    Arne



    That is a risk, and when doing the trials we developed a technique of setting the sails for each point of sail and then hunting up and down a bit (quite slowly) through the pointing angles to get the best data we could with the wind that was available at that moment. It is clear that much of the data we gather is at less than top performance, which is why I think it is right to use the best performance we can measure.

    With Anthony Cook we are just now trying out a filtering process to take out data where the wind is dropping rapidly, which can also lead to falsely high performance, as the boat speed drops more slowly than the wind speed. We have looked at taking out data where the wind speed drops in one second by 0.7 knots or more, and also 0.6, and 0.5. The number of data points rises rapidly as the filter is reduced.

    The problem with using median values is that we record a lot of data at low boat speed, when we are starting and finishing a run, for example, or retrimming the sails, and these data points will pull the median values down in a fairly arbitrary manner.

    I tried to avoid any fast luffing manoeuvres when doing the trials, as they would create misleading data. I guess we can try editing out data where the wind angle is changing more than, say, 2 degrees per second, which would help to protect against any such cunning manoeuvres.

    Fortunately, once we have the data, we can choose how to analyse it, and discuss the merits of each approach. With no data, we have nothing.

  • 01 Nov 2018 16:15
    Reply # 6884231 on 6883717
    Deleted user
    Anonymous wrote:

    That is very interesting, Robert, and thank you for doing this work, but I think the fact you are taking the median values from a group of data points will give a lower performance than it is realistically possible to obtain.

    I don't disagree. By taking high values you will be more likely to approach the boat's theoretical VPP values. It's easy enough to do max values and see if the plots change significantly.

    So I assume the 5Hz data is not available?

    I'm plugging bugs in my code. Turns out the TWS bins (4, 6, 8, etc) only use half the   data. I'll have to do TWS = 3, 5, 7, etc bins to see the other half. I really don't think   polars will change much.

    I'll put my Matlab code in google drive for anyone who happens to use the app. Alternatively there are free download Matlab-mimics like Octave that may work with minimal alterations.

    The current version of the Matlab code is here:https://drive.google.com/drive/folders/1SKESwN1N6MENOjFelsj7vOPK9kmQUL7u

    rself

    Last modified: 02 Nov 2018 13:52 | Deleted user
  • 01 Nov 2018 15:37
    Reply # 6884161 on 6883717
    Anonymous member (Administrator)
    Anonymous wrote:

    That is very interesting, Robert, and thank you for doing this work, but I think the fact you are taking the median values from a group of data points will give a lower performance than it is realistically possible to obtain.

    It would be better, IMHO, to take the maximum figure from each "box" of data. This is the approach taken by Polauto, and by Anthony Cook.


    It appear so me that median values will give a better picture of the performance than just picking the maximum figures:
    High peak values can easily be obtained by picking up speed on a lower course and then shoot up a little, which would give a very good VMG reading if caught in the right moment.
    Note: I don't accuse anyone for cheating, I just know from my own experience that it is difficult to keep a correct course all the time. Sometimes I aim a bit low (and pick up speed) and sometimes I aim a bit high. I guess I am not alone on this.

    Arne


  • 01 Nov 2018 10:54
    Reply # 6883717 on 4913961
    Anonymous

    That is very interesting, Robert, and thank you for doing this work, but I think the fact you are taking the median values from a group of data points will give a lower performance than it is realistically possible to obtain.

    It would be better, IMHO, to take the maximum figure from each "box" of data. This is the approach taken by Polauto, and by Anthony Cook.

    The reason for this is that in your "box" of data for each wind speed and direction, you will have a bunch of data (in fact all but one of them) obtained when the boat is being sailed less than perfectly.

    By taking the maximum boat speed point for each "box" we are at least getting close to the maximum performance of the boat. It is clear from the results we have that in fact we still have a number of "boxes" where the speed is less than it could be, and we end up with rather spiky polars. 

    We could smooth them by putting a fitted curve through them, but again this will reduce the apparent performance of the boat, whereas by editing them manually we can fill in obvious low points, remove obviously excessively high points, and make everything smoother.

    I dare say that your results are more representative of the speeds that can be obtained on passage, but I think that to tease out the small differences of performance between different boat/rig combinations we need to look at the maximum performance data.

    I am really pleased that you and others are looking carefully at the data we have obtained and subjecting it to different methods of analysis. Anyone interested can then make up their own mind about which approach they prefer. I don't think there is a right or wrong approach as different analyses reveal different things.

       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

                                                              Site contents © the Junk Rig Association and/or individual authors

Powered by Wild Apricot Membership Software