Measuring junk sailing performance

  • 28 Nov 2018 12:46
    Reply # 6936201 on 6929393
    Anonymous
    Anonymous wrote:
    Arne wrote:

    Robert,

    that was very impressive numbers. I have never seen polar diagrams where the boats sail 30 deg. from the true wind direction before. 

    Could this be apparent wind angle? In that case, the  VMG to windward will not make sense.

    Arne

    I've thought of something that may have a bearing on this:

    We rig the anemometer on a short mast way out on the quarter, as this is where we can get it furthest way from the rig. Yet it's not a perfect position. If you race, you know that if a competitor gets onto your lee bow, he deflects the wind that you feel, heading you so that you fall away into the windshadow behind his rig if you don't tack away. Our anemometer is reading a wind that is a few degrees more "on the nose" than the ambient wind, I think.

    An anemometer on a short mast at the bow is in a worse position, in the upwash as the wind approaches the rig.

    An anemometer at the masthead is also not in the true wind, and is probably going to be somewhat affected by the tip vortex at the top of the rig (and, dynamically, much affected by pitching and rolling). How much affected, I don't know, but that's the source of all the data that's been collected to date on sailboats' performance, and is the source of our feeling that 30˚ apparent, 45˚ true is what we should be aiming for as an achievable target.



    I think David may well be right that our anemometer position has meant that the upwind angles are being affected by the effect of the rig on the airflow. So what can we do about it?

    We could apply an arbitrary correction to the data we have, so that, for example, we increase the wind angle by 10 degrees at 35 degrees true and taper it off towards zero degrees and towards 70 degrees using a sine curve. That would bring the polars we have back into "sensible" territory.

    I suggest something along the lines of

    If TWA<70 then

    TWA corrected = (180/PI()) x (TWA +(10 x Sin((PI()/180) x (90/35) x TWA))

    applied during the true wind calculation would achieve the desired result. NB; The PI()/180 term converts from degrees to radians and the 180/PI() converts back from radians to degrees.

    Longer term, we can, and I think we should, retest some or all of the boats tested so far, with the anemometer mounted on a long pole attached to the bow to get it as far forward as we can, and at the same height as in the aft position. The cable on the anemometer is long enough to do this. We can than compare the upwind results we get with the results we already have to see what the effect really is of mounting the anemometer aft. Then we can adjust the correction factor above if needed based on observed differences.

    Obviously we should ignore any results with the wind aft when the anemometer is forward as they will be seriously affected by the sail!

    Then, for testing any further boats, we should put the anemometer forward for upwind sailing, and aft for downwind. It would not be hard to do that.

    Last modified: 28 Nov 2018 15:27 | Anonymous
  • 23 Nov 2018 19:29
    Reply # 6929949 on 4913961
    Anonymous member (Administrator)

    Alan,

    another idea:
    When I see the clouds of data on one of the diagrams in Reply# 6929495, I wonder; could it be that it is the sensors, or rather the sampling rate of them which is not optimal. I only own a simple Garmin 72 GPS, myself. However, I mean I remember that I could set up the damping of the speed. What was presented was an average over 2, 5 or 10seconds, something like that. With the shortest setting, the speed would fly up and down with over a knot, all the time. With the 10s damping, the speed only jumps with 0.2 – 0.4kts, at most, at each updating (once a second?). If the sampling of your sensors could be dampened in a similar way, there may be less of a job for later algorithms to struggle with.

    Just a thought.


  • 23 Nov 2018 17:37
    Reply # 6929719 on 4913961
    Anonymous

    Looking through the polars Robert has produced I can't help thinking the results are being skewed by taking the median value for each group of points. Taking Poppy at 12 kts wind speed for example, the values at 40, 50 and 60 degrees are pulled in by the number of low speed data points, while at 30 degrees the median appears near the top of the range of values.

    The display of the data points makes me think that taking the highest point at each angle would produce a better curve. Possibly taking, for example, the mean of the top 5 values would produce an even better curve, smoother and with a better defined optimum VMG.

    Just a thought!

    1 file
  • 23 Nov 2018 17:11
    Reply # 6929667 on 4913961
    Anonymous

    Robert, the smoothed curve for Miranda at 12 knots extends across the centreline, which can't be right and doesn't tie in with the data plot for that speed.

    2 files
  • 23 Nov 2018 12:19
    Reply # 6929495 on 6928814
    Anonymous
    Anonymous wrote:

    Summry PDF's of polar plots for all 4 boats at various true wind speeds and angles.

    Ancillary graphs, jpg's of all the polars and a cleaned-up version of the original data are here: Calisto, Miranda, Poppy, Weaverbird.



    Bravo Robert for a fascinating set of results. I particularly like the analysis of the raw data which shows how different the conditions were for the different boats. (See attached pics) Unfortunately I had to add the boat name to all the files to make these comparisons as when taken out of their own folder they all have the same file name. Maybe that's something to change for the future?

    It would be interesting to see the results for each tack on each boat, as the indications I have are that the results are significantly different. Again I like the histograms that show the data variation between tacks.

    Anthony Cook and I have been working on filtering the data to try and remove the suspect or dodgy data points which can be generated by rounding up, as discussed, or by the wind dropping suddenly, as the boat speed decays quite slowly compared to the drop in wind speed. I hope we will be able to share the results soon. At the moment we are eliminating any data points where the wind speed drops by more than 0.7 knots per second, and any where the wind direction changes by more than 5 degrees per second. We also eliminate all subsequent data points until the boat speed changes. These filter values eliminate about 10% of the data points.

    It is early days, and we are definitely still on the learning curve!


    4 files
  • 23 Nov 2018 08:34
    Reply # 6929393 on 6923085
    Arne wrote:

    Robert,

    that was very impressive numbers. I have never seen polar diagrams where the boats sail 30 deg. from the true wind direction before. 

    Could this be apparent wind angle? In that case, the  VMG to windward will not make sense.

    Arne

    I've thought of something that may have a bearing on this:

    We rig the anemometer on a short mast way out on the quarter, as this is where we can get it furthest way from the rig. Yet it's not a perfect position. If you race, you know that if a competitor gets onto your lee bow, he deflects the wind that you feel, heading you so that you fall away into the windshadow behind his rig if you don't tack away. Our anemometer is reading a wind that is a few degrees more "on the nose" than the ambient wind, I think.

    An anemometer on a short mast at the bow is in a worse position, in the upwash as the wind approaches the rig.

    An anemometer at the masthead is also not in the true wind, and is probably going to be somewhat affected by the tip vortex at the top of the rig (and, dynamically, much affected by pitching and rolling). How much affected, I don't know, but that's the source of all the data that's been collected to date on sailboats' performance, and is the source of our feeling that 30˚ apparent, 45˚ true is what we should be aiming for as an achievable target.


  • 22 Nov 2018 23:40
    Reply # 6929172 on 4913961

    i really like the polar plots for vmg (even if some of them miss the zero point at 90 deg…)

    …but something tells me that the interpretation of the data misses an essential point. i’ll try to put my finger on it:

    as a helmsman i never steer a really straight course. while meandering some 5 to 10 degrees to either side of the main course, the hull speed will drop a little bit, but the data collected on the windward side might show far better results than you could sail in this direction.

    a solution could be to sail a strait course for a couple of minutes and then interpret 30 second or one minute blocks of data to interpolate the main direction, main boat speed and main wind speed.

    imho this would give more accurate (but lower) performance results.

    ueli

  • 22 Nov 2018 18:47
    Reply # 6928947 on 6928855
    Deleted user
    Anonymous wrote:

    Another thing:
    The way the VMG is shown is confusing. Unless I am totally wrong, the VMG is the horizontal component (in these diagrams) of the STW, either straight upwind or straight downwind, so it makes no sense to draw it as a polar diagram, or in fact draw it at all. If we have the STW at a true angle to the wind, then the VMG will show itself automatically (if one knows what to look for).

    Conclusion:
    In case the numbers are right, and I am wrong, these boats (in particular Calisto) must be real world beaters.

    Arne

    Hi Arne: See my interpretation for red squares and green circles in 6910584.

    "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."

    The green circle (median vmg's) locations indicate derived from the STW data (blue dots) surrounding the red square (median STW). To get a vmg, off a polar, given only the location of the STW you have to mentally drop a normal to the zero degree axis. I thought it easier to read it off the radial scale...green circle location.

    I'm also skeptical of the Calisto results based on the actual data. The upwind boat speed through the water hardly changes as the wind changes by a factor of 2.

    "Smoothings" by definition are a mega-data-massage. It has to be to reduce ten thousand points by a factor of 10 into 105 that shows some kind of pattern.

    rself

    Last modified: 13 Oct 2019 20:51 | Deleted user
  • 22 Nov 2018 17:19
    Reply # 6928862 on 4913961

    I think it's worth saying that these were the very first trials, and we're learning how to gather data at the same time as learning how to record and analyse it. The numbers we recorded give Robert something to work with, but let's not read too much into the results just yet.

    I agree with Arne and Ketil, a cruiser is doing pretty well if it points at 45˚ true and 30˚ apparent. Weaverbird, Calisto, Miranda and Poppy ought to manage it, in a reasonable seastate. Not closer, though, at best VMG.

  • 22 Nov 2018 16:54
    Reply # 6928859 on 4913961

    Marie G can manage to sail at 29 degrees of aparent Wind, but achieves best VMG at about 32 to 34 degrees. It is also easier to maintain the speed at a wider angle, not having to keep her on the edge. Sailing agaist the Wind is more to keep up With windchanges to minimize the way to the eye of the Wind. I have written before about the performance of the Junkrig vs the Bermudarig. After 4 Seasons of racing, I think I can summarize: Beating to Windward 4-7% slower than pointy, 40-60 degrees offwind: 5-7% better than pointy. Running 7-10% better than Bermuda. Ufortunately racers use spinnakers, and leaves me behind by 3-6%. To quote a fellow racer: We are 5 sweathy persons onboard, you sit and drink coffee. The more Wind the better Marie G goes. She is hard to beat in 14 sek/m, (28knots) of Wind, With 3 panels Down and spray flying, luckily not as bad as Route de Rhum sailors.

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

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