Measuring junk sailing performance

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  • 18 Nov 2018 09:02
    Reply # 6912441 on 4913961

    While Robert and Anthony continue to work on improving the analysis of data acquired, I am coming towards the end of rebuilding the kit used to acquire that data. It fits into a bag meant for kiteboards and associated equipment, so that will be much easier to transport from boat to boat.

  • 16 Nov 2018 15:48
    Reply # 6910584 on 4913961

    I've put the vmg's onto the polar plots. See examples, formulas and vector plots. The vector angles and magnitudes from CAD match the formula results. Point scatter of vmg's not shown...not visually different from stw's. 

    All completed plots here:https://drive.google.com/drive/folders/1d5lF1XT4DlbX9-ie7WZpWFcVDLl9FPbm

    or https://drive.google.com/drive/folders/12SnnPouRegorGlzFRwViCk-fRIQeMHgq

    Plot interpretation--the red square is the median boat speed (STW knts) in the direction shown (TWA deg). The aligned green circle is the boat speed (VMG knts) straight up- or down-wind. So if traveling 90 deg to the wind direction your vmg approaches zero. Plots make sense.

    rself

    Last modified: 16 Nov 2018 15:59 | Anonymous member
  • 16 Nov 2018 12:59
    Reply # 6910344 on 6905534
    Anonymous wrote:
    Anonymous wrote:
    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


    Hi Robert,

    I think David has answered some of these questions, but the sequence is not quite as you describe, in that the .txt and .csv files appear to be recorded simultaneously with NavMonPC adding the time stamp to the .csv file and recording only pre-selected data, but the selection does not seem to be under user control. We have to start and stop the recording of each file manually on the laptop at the start and finish of each run.

    I probably got the wrong impression that the csv's were derived from the txt files and that there was information overlap between them and perhaps unused info. But, maybe not...i.e no added info in the txt files.

    So your appeal above is then for a custom instrumentation controller application to replace NavMonPC, right? Any "double E" JRA members? Matlab does sell electronic instrumentation toolboxes. With time and the right tools it is possible. I was involved in a project many years ago where an applied physics graduate student built a controller (took months) to talk to his submerged instruments.

    Last modified: 16 Nov 2018 13:16 | Anonymous member
  • 15 Nov 2018 10:53
    Reply # 6905534 on 6899240
    Anonymous member (Administrator)
    Anonymous wrote:
    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


    Hi Robert,

    I think David has answered some of these questions, but the sequence is not quite as you describe, in that the .txt and .csv files appear to be recorded simultaneously with NavMonPC adding the time stamp to the .csv file and recording only pre-selected data, but the selection does not seem to be under user control. We have to start and stop the recording of each file manually on the laptop at the start and finish of each run.

    I did play a bit with trying to set up filters in NavMonPC (to eliminate the non-existent GPS data) but it seems they only work if we send the output elsewhere. They did not seem to affect the .csv file recorded by NavMonPC.

    I can't recheck that as I have sent all the electronics to David so he can try out and calibrate the water sensor in his new float.

    I had to search around on the web to find definitions of the NMEA sentences, and I have created a little summary of what I discovered which I will send to you, and I have put on this page on the JRA website. http://junkrigassociation.org/page-1858647 which you can also find under Junk Information/Sailing Performance/Interpreting NMEA Data.

    Last modified: 15 Nov 2018 12:14 | Anonymous member (Administrator)
  • 14 Nov 2018 13:18
    Reply # 6904112 on 6899435
    Anonymous wrote:

    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?

    rself

    Turns out only 3 of the points came from upwind tacking situations. The csv enclosed has been re-sequenced by time instead of the sequence in which the original data files were read. The first col is time in seconds since 15/09/2018 00:00:00. Stepping down through the file I only found one sequence around line 280 where the port-stbd switch showed large changes in twa. All the other p-s switchs were down-wind legs where twa hardly changed.

    So the "30 deg" TWA bin was "steady state" in the sense that the coxswain was pinching up then falling off probably following Poppy's instruments as described by Slieve.

    Last modified: 14 Nov 2018 13:35 | Anonymous member
  • 13 Nov 2018 17:24
    Reply # 6902791 on 4913961

    Robert, Poppy was fitted with racing instruments, giving water speed, true or apparent wind speed and I think direction as well as VMG to up wind or down wind with an expanded scale apparent wind direction from directly up wind or down wind. This latter instrument was very useful and if I remember correctly for beating I simply selected the apparent angle as appropriate to the sea state, trimmed the sails to best speed (all luff tell-tales flying) and stuck to that apparent wind angle. This got the respect of the Bermudan boys.

    Unfortunately I cannot remember the exact range for apparent wind angles I used as I used them as pips on the scale rather than numbers, but they could be remarkably small. I do remember that pointing above 45° true wind angle did not really increase VMG to windward, but it was often useful to point much higher if you sheeted in further trying to pass a moored boat or buoy and know that the speed would not drop of significantly and stall the keel. The 30° angle does not surprise me as I have a recollection of tacking through 60° one occasion.

    Poppy could point much higher than her Bermudan sister, if for no other reason than the headsail sheeting angle on her sister was right out on the toe rail and the split rig could be sheeted almost to the centre line so that the jibs only had their inbuilt sheeting angle, which was about 10° (I think). The split rig can be sailed very tightly sheeted, but not necessarily with a better VMG. The Longbow is very susceptible to sea state as the bow is quite blunt to give a good fore cabin berth, however in flat water (it does occur occasionally) it was fun to out point all the Bermudan boats. We got some very funny looks at such times.

    As for downwind speed, on the run we always got boat speed equal to relative wind speed which meant running at half true wind speed. This held up to 6 kts boat speed, but above that LWL length limitations slowed the increase down. Going onto a broad reach could increase the boat speed in winds under 12 kts, but the VMG down wind would only increase a little so we didn't really bother to tack downwind.

    I see little point in comparing the boat speeds recorded for the various boats, even with corrections (or fiddle factors) as a slight variation in sea state can make a mockery of any results, and that is why I have always tried to assess performance when sailing beside similar boats of known handicap (even though handicaps are not perfect). I think the only thing that might come out of this exercise would be to show the shape of the windward performance curve and close windness of a rig, but on Poppy I wasn't worried about that as we had the VMG meter to confirm our performance.

    I hope this is of some use.

    Cheers, Slieve.

  • 13 Nov 2018 15:08
    Reply # 6902464 on 4913961

    dd/mm/yyyy is right for Weaverbird: 06/08/2018 = 6th August.

  • 13 Nov 2018 13:55
    Reply # 6902357 on 4913961

    Now that I'm manipulating the date-time stamp just want to make sure about format. Poppy files are obvious dd/mm/yyyy (15/09/2018). Weaverbird format could go either way: 06/08/2018.

    Thanks

    rself

    Last modified: 13 Nov 2018 13:56 | Anonymous member
  • 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
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