Measuring junk sailing performance

<< First  < Prev   1   2   3   4   5   ...   Next >  Last >> 
  • 14 Oct 2019 18:32
    Reply # 7959583 on 4913961

    Well, before the output from the instrumentation is throughly trashed I'll publish my summary graphs and opinions.

    The new boats added this year were Weaverbird fitted with a new wing sail JR (same sail area as the 2018 single-ply JR) and Amiina with a split JR.

    Weaverbird comparisons;

    Upwind the wing JR and single-ply, hinged, cambered JR are nearly equal.

    On a beam reach I'd say a slight advantage goes to the single-ply although there is substantial overlap.

    Down wind the advantage goes to the wing JR.

    In the down wind plot note the small print in the individual panel titles. My attempt to uncover bias in the frequency distributions due to inadequate sample sizes for high or low wind speeds. The ideal is an equal number of measurements in all combinations of TWA and TWS. But there is no way to fully control the wind speed and course sailed on the day data is collected. The idea is to make the viewer aware. Up to them to disregard, in part or in whole, the graph. For example Amiina is missing data from 6-9 knt wind speeds. Only showing data for 10,11,12 knt wind speed. Higher windspeed, boat sail faster. So there is an inherit upward bias to the Amiina graph, out of the control of the data takers, when the viewer compares Amiina boat speeds to boats which are not misssing wind speeds in this category. Perhaps, if the 6-9 knt data had been collected the Amiina frequency distribution would extend down into the 2 knt boat speed region.

    Amiina comments;

    I found Amiina a surprising overall performer. Alan's scalar says it all (see spec spreasheet below). The higher the scalar the slower the expected boat speed. The scalar is an imperfect handicapper or rating designed to normalize boats of unequal length, weight, sail area, etc.

    That said, Amiina has the shortest waterline hence lowest maximum speed. Moderate D/L of 252 hence is not a light-displacement planer like a X-99. Low-ish ballast ratio of 32% not indicative of standing up to the wind when beating like a folkboat with 50% ratio. Lowest SA/D of 14. Under-powered relative to the other sailboats. By these numbers Amiina's "as measured" boat speeds should be measureably less than the other designs. But they are not. Amiina performs on level upwind and abeam. Down wind speeds are potentially biased high.

    Applying the scaling factors (red histograms), intended to achieve an apples-to-apples comparison, Amiina is a visible outperformer. Unless there has been some radical changes to Amiina compared to the Splinter 21 specs on the sailboatdata page. Perhaps something is going on here(?) i.e. the split JR.

    view and download link:

    https://drive.google.com/drive/folders/1b3zneYJrA8ATxeI8bsgFa3Q2025VAFEy

    robert self


    Last modified: 14 Oct 2019 22:06 | Anonymous member
  • 14 Oct 2019 16:39
    Reply # 7959405 on 4913961
    Anonymous member (Administrator)

    Robert, I have no problem with the orientation of the diagram. Whether the wind comes in from top or from right is just the same for me. It is the shown results, which I think is un-achievable for a Splinter (with any rig), I have problems with.

    I have eye-balled quite a number of polar diagrams to learn about the windward performance of different boats, and their VMGs to windward seem to culminate at between 40 and 50 degrees from the (true) wind. 

    Even a 6mR would struggle with 30 degrees, not to mention having best VMG at that angle.

    Arne

    Last modified: 16 Oct 2019 13:18 | Anonymous member (Administrator)
  • 14 Oct 2019 16:18
    Reply # 7959390 on 7958948
    Anonymous wrote:

    Robert,
    that polar diagram of Weaverbird makes sense. It shows that best VMG is reached at 40-50° from the wind, and is dropping at 30° from the wind. I even understand the green VMG-dots, although I don’t see a need for them as they are just the horizontal component of the STW curve (vertical component on the Contessa’s diagram).

    However, several of Amiina’s polar diagrams indicate that she is at her best when pointing only 30° from the wind.
    Amiina has shown before that she sails smartly to windward, but there is nothing supernatural about her close-windedness  -  which the polar diagram below appears to indicate.
    This is why I suspected that there was something wrong with the algorithm.

    Arne


    Arne--OK.

    I did not realize this orientation would be so confusing. Between now and next year when (hopefully) data from new boats becomes available I'll look for a matlab prog which defaults to the usual sailing orientation--0 degrees up and clockwise rotation.

    Last modified: 14 Oct 2019 16:22 | Anonymous member
  • 14 Oct 2019 14:12
    Reply # 7959186 on 7959022
    Anonymous member (Administrator)
    Alan wrote:

    The problem here is that. as we discovered last year, the wind sensor mounted on the transom is affected by downwash from the sail when sailing close hauled.

    Last year we developed the idea of correcting the TRUE wind values between 0 and 70 degrees so they are reduced by 10 degrees at 35 degrees off the wind, and the correction tapers off to zero at 0 degrees and at 70 degrees.

    This is a completely arbitrary correction to take care of the perceived affect of downwash on the results, and just adjusts the clearly incorrect upwind performance values to something more reasonable.



    During my life as an electronic engineer, regularly checking the integrity of critical navigational aids on (mostly) Stavanger Airport, I and my collegues learned to be aware of any error sources.
    We used to say -
    “Garbage in results in garbage out”

    This is why I became sceptical to those polar diagrams. To just add a suitable (..‘arbitrary’  -  now I learned a new word...) correction factor in the ‘think-of-a-number’ class, is no good. The upwash and downwash error will no doubt vary with the sailing angle, as the sensor’s vane will alter position with respect to the sail.

    It therefore seems to me that the found results have little value, so one is back to square one.

    My guess is that the wind sensor needs to be moved at least one sail chord’s length away from the sail (I suggest in the bow) to minimise the error caused by the upwash.
    If I remember correctly, the speed (STW) sensor was calibrated with a GPS in slack waters. In that case, we can at least trust that sensor.

    Good luck!

    Arne

    PS:
    With the wind sensor on a long stick, set forward of the bow, the sensor will of course be blocked when running straight before. Still, I would rather sacrifice measurements downwind to achieve good data from a broad reach and up to fully close-hauled.



    Last modified: 14 Oct 2019 16:34 | Anonymous member (Administrator)
  • 14 Oct 2019 12:05
    Reply # 7959065 on 7959041
    Anonymous member (Administrator)
    Anonymous wrote:

    I agree, Arne, there’s something wrong somewhere. My observations as a seat of the pants sailor are that Weaverbird points a little closer, and has more power through the seas, with the wing sail than with JR; and that Weaverbird points a little higher than Amiina but is no faster. I’m not seeing that reflected in the diagrams. 

    Alan, I found that there was a lot of upwash with the wind sensor on the bow, so that doesn’t help.   


    I guess it would need to be on a super long bowsprit!

    Unfortunately mounting the wind sensor at the top of the mast makes shifting it from boat to boat very difficult, and junk masts wobble about a lot which could also affect the results.

    So making a 10 degree correction for downwash is probably the best option at the moment, especially for the data we already have.

    Other crazy ideas could include mounting the sensor on a drone!

  • 14 Oct 2019 11:28
    Reply # 7959058 on 7957524
    Anonymous member (Administrator)
    Anonymous wrote:

    Robert.

    I took the liberty and downloaded the polar diagram of the Contessa 32. This is what I call a true polar diagram. The sailing angle of the boat is no doubt related to the true wind direction. This is how textbooks present them, and also yachting magazines.

    I have picked the polar diagram with the wind speed at 8kts as an example and have added two horizontal helping lines to indicate the best VMG upwind and downwind.

    My point is that it appears to me that the ‘polar diagrams’ for Amiina and Weaverbird have been drawn using apparent instead of true wind angles. To make these diagrams compatible with true polardiagrams, like that of the Contessa, I am afraid they have to go through another smart algorithm (..which it is too late for me to suggest… a serious exercise in trigonometry is called for...).

    I hope this makes sense.

    Arne

     

    (PS:That polar diagram, below also shows how useless the Contessa’s Bermuda rig is downwind, without a spinnaker  -  or a JR...)

     



    You need to be very careful about what you are comparing. This Contessa 32 polar looks to me as though it has been generated by a computer program, and not from measured performance. As such it will represent the best theoretical speed the boat can do.

    Robert's analysis takes the median performance value from the full range of data we have recorded. This includes lots of low speed data when the boat was not being sailed well, and so it is inevitably much lower than the maximum potential performance, so his polars cannot sensibly be compared with the Contessa 32 polar.

    The polars that Anthony & I produce are based on the maximum recorded performance values, with various corrections and filters to eliminate misleading data, as discussed in the articles I wrote last year. Therefore they are more comparable to the Contessa 32 polar, but still not truly comparable. 

    If you want to compare our data with a Bermudan rigged boat, then you should use the Calisto data, recorded using the same instruments, and subjected to the same corrections and filters as our junk rigged testing.

    When Anthony has finished running all the boats tested this year and last year through the new smoothing software, I will be placing the results on the website so everyone can see them.

    Last modified: 14 Oct 2019 11:30 | Anonymous member (Administrator)
  • 14 Oct 2019 11:17
    Reply # 7959041 on 4913961

    I agree, Arne, there’s something wrong somewhere. My observations as a seat of the pants sailor are that Weaverbird points a little closer, and has more power through the seas, with the wing sail than with JR; and that Weaverbird points a little higher than Amiina but is no faster. I’m not seeing that reflected in the diagrams. 

    Alan, I found that there was a lot of upwash with the wind sensor on the bow, so that doesn’t help.   

  • 14 Oct 2019 11:04
    Reply # 7959022 on 4913961
    Anonymous member (Administrator)

    The problem here is that. as we discovered last year, the wind sensor mounted on the transom is affected by downwash from the sail when sailing close hauled.

    Last year we developed the idea of correcting the TRUE wind values between 0 and 70 degrees so they are reduced by 10 degrees at 35 degrees off the wind, and the correction tapers off to zero at 0 degrees and at 70 degrees.

    This is a completely arbitrary correction to take care of the perceived affect of downwash on the results, and just adjusts the clearly incorrect upwind performance values to something more reasonable.

    David Tyler & I discussed the possibility of mounting the wind sensor in the bow just for sailing upwind, but we were unable to come up with a satisfactory scheme in time for this year's testing.

    So meanwhile we continue to apply the same arbitrary correction to all the boats so it is at least consistent.

    Anthony Cook has been developing smoothing software so we can get sensible looking polars from the jagged recorded data, and has today just sent me a smoothed version of Amiina's polar, which also allows you to see the smoothed and unsmoothed curves next to each other. These polars contain all the corrections which we discussed and published in the magazine last year.

    This is an Excel file. Look through the various tabs to see the comparison graphs.


    Last modified: 14 Oct 2019 11:05 | Anonymous member (Administrator)
  • 14 Oct 2019 08:09
    Reply # 7958948 on 4913961
    Anonymous member (Administrator)

    Robert,
    that polar diagram of Weaverbird makes sense. It shows that best VMG is reached at 40-50° from the wind, and is dropping at 30° from the wind. I even understand the green VMG-dots, although I don’t see a need for them as they are just the horizontal component of the STW curve (vertical component on the Contessa’s diagram).

    However, several of Amiina’s polar diagrams indicate that she is at her best when pointing only 30° from the wind.
    Amiina has shown before that she sails smartly to windward, but there is nothing supernatural about her close-windedness  -  which the polar diagram below appears to indicate.
    This is why I suspected that there was something wrong with the algorithm.

    Arne


    Last modified: 14 Oct 2019 08:19 | Anonymous member (Administrator)
  • 13 Oct 2019 22:34
    Reply # 7958430 on 7958420
    Anonymous wrote:

    These polars are based on true wind speed and true wind direction values. In my diagrams the wind is coming out of the west instead of the north and the boats are on stbd tack. The frame of reference is rotated to the right otherwise everything else is the same.

    The green dots are the magnitudes of the median vmg's calculated from the actual field data. No drawing of lines is necessary. If you wish you can drop vertical lines to the axes from the red squares but they won't necessarily line up with the green dots 'cause the dots come from the raw data. It's a given that the direction of vmg's are straight upwind or downwind so no need to show direction. The relevant information that I hope is transmitted is boat speed and direction relative to the wind direction (out of the west), the associated magnitude of the related vmg's, and (what you'll never see) the variability of actual field measurements. Less confusing? 


    Robert,

    in that case, both Amiina and Weaverbird must be very close-winded, indeed. It appears to me that their VMG keeps improving  until they are pointing 30 degrees from the true wind. I struggle with believing that.

    Arne

    Arne--Yes east.

    This is what I see in the plot below.

    vmg @ 30 deg course is around 2.1 knots in the straight upwind direction. Just read the magnitude off the radial scale....we already know vmg's are straight up or down wind.

    vmg @ 40 deg is higher about 2.3 knts because stw increased but not offset by 10 deg angle change.

    vmg @ 50 deg came down a bit to 2.25 knts so increase in stw is offset by 10 deg course change.

    vmg @ 60 deg decreases to 2 knts even though stw increase because the course change offsets the stw increase even more.

    same for 70 and 80 deg. even though stw increases the length of the vector on horizontal x-axis gets smaller and smaller and goes to zero at 90 degrees because on a beam reach (i.e. north) the boat is travelling perpendicular to the upwind course.

    So I see vmgs initially increasing then decreasing to zero as the course of the boat rotates from eastterly (30 deg off the wind stbd tack) to north on a beam reach.

    PS-

    See text file below. I'd need to understand then manipulate this code in order to change the default orientation. Any volunteers?

    Last modified: 13 Oct 2019 22:59 | Anonymous member
<< First  < Prev   1   2   3   4   5   ...   Next >  Last >> 
       " ...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