Comparisons - Split Junk and Cambered Panel Sails

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  • 20 Sep 2014 21:05
    Reply # 3103700 on 3103628
    Deleted user

    Ash, this sounds a great experiment, and I look forward to seeing the results.

    I don't feel knowledgeable enough to comment on the technical details, but I was wondering about the photography you mentioned.  Are you thinking to set up a screen, say a white tarpaulin, to avoid the background confusing your photos of the experimental set-up?  Also, if the three/four panels are in line on a common batten, does this allow unimpeded photography of an individual panel and its tell-tales?  I presume you are trying for results as closely positioned in time and space as possible - might the panels be mounted on separate battens each staggered so that the side view of each panel is not overlapping the others?

  • 20 Sep 2014 18:06
    Reply # 3103652 on 3103628

    Ash,

    First thought: this is a very interesting and worthwhile thing to do.

    Second thought: when this kind of 2D test is done in a wind tunnel, the test piece spans from wall to wall. Might you consider separating the panels with 8 x 4 sheets of inepensive material?

    Third thought: your measuring method does not measure the force that the panel exerts on the mast, just the sheet load. The CE of each panel may well be in a different place, and this will tend to influence your results. 

    Fourth thought: you will need to measure both the horizontal and vertical forces at both mast and sheet, to get a true picture of the lift and drag of your panels.

    Fifth thought: I wish I were in the UK and could make a panel with battens with a central hinge and camber only in the forward part of the panel. And I wish that I could make a wingsail test panel, too.

    Last modified: 20 Sep 2014 18:23 | Anonymous member
  • 20 Sep 2014 17:02
    Message # 3103628
    Deleted user
    I discovered the JRA about one year ago, since I have been engaged for many happy hours in studying the
    information of recent developments described by so many enthusiasts of Junk Rig within this Web Site, in the book "Practical Junk Rig", and in the books and videos by Roger Taylor .

    It's crunch time for me as next year I will be converting my old Bermudan Hunter 701 to a Junk Rig, I will enjoy doing all work myself but the first big question is "which sail?”

    My choice is between the 'Cambered Panel Sail' as designed by Arne, or the 'Split Junk' as designed by Slieve. I have discussed my plans (as described below) briefly with Arne and Slieve who have both responded positively giving loads of advice. I am hoping eventually to reach a decision, which is within my practical abilities and suitable for my
    boat's physical characteristics.

    Before I describe my 'test equipment' and 'how I will rig it', I believe that many of you will have your own good ideas, and may like to be involved or make suggestions - so better that I gather your comments and thoughts first, before embarking on the actual tests; to be able to incorporate changes or additions to the test rig from the outset. So
    please make suggestions if you wish.

    On conclusion I intend to post my findings and conclusions.
    I just hope that I am not being so smart that my sail ends up looking like a camel designed by committee!

    TEST EQUIPMENT:
    So far I have made 3 test panels from polytarp, each measuring 0.5 metres x 2.0 metres attached to two 25mm (1") diameter wooden battens about 2.4 m long. I have posted a few photos in "Technical Illustrations" to assist.

    I have found it is difficult or impossible to make the two cambered sails identical - (apart from their deliberate differences) - so from the outset errors have crept in - but I hope they are good enough to enable fair, simultaneous comparisons to be made, leading to evidence based conclusions, not merely subjective impressions.

    The 3 panels made so far are:
    "A." Flat with no camber built in. (I see this as a Control)
    "B" 10% Camber - constructed using the 'Arne' method.
    "C" 10% Camber - a 'Split Junk' constructed using the 'Slieve' method of construction.

    "D" May be required in due course to clear up inconsistencies, to vary camber or positions of draft, to establish the most suitable mast / sail balance position or maybe to carry out comparisons suggested by you.

    I would also like to make two cambered panels joined with the MingmingII hinged system and gaps as devised by Roger Taylor when converting his Achilles 24 MingmingII, to try to establish if the gaps in the "HHH” (Roger's 'Piano Hinges') assist in re-attaching laminar flow on the leeward side or in preventing separation as per the work carried out several years ago by Bunny Smith.


    HOW I WILL RIG IT?

    I plan to mount the 3 panels A, B & C, horizontally side by side (with a suitable space between the panels) on a horizontal 'mast' about 2m above ground, positioned 'head to wind' - so more like an 'upside down' aircraft wing than a sail.
    The battens of each panel will be roped to the mast, allowing vertical up / down movement, and permitting the wind to spill. I will initially attach using the front end of the battens, which project about 7" ahead of the actual sail luff - so there will be no  'sail / mast' balance - all of the panel will trail behind the mast.

    "Sheeting In" will involve lifting the aft ends of each pair of battens, in stages, up to a maximum of about 30 degrees from the horizontal 'spilling' position. Lifting, rather than lowering the leech of the sail in relation to the wind flow, will make use of gravity, which is already pulling the sail cloth down into its aerodynamic curve. 'Sheets' will go over a
    horizontal pole (through blocks) to a suspended bucket. Each bucket will initially be 'trimmed to neutral' by adding enough water to the bucket, so that it's sail is 'luffing' head to wind, i.e. just overcoming the gravity acting on sail and battens.


    Tools:-

    Scales.
    So that I can quickly weigh each bucket and water content (my strain gauges) - during and after each test.
    Large protractor.
    Attached to a spirit level - to measure angle of incidence.
    'Tell-tales'.
    Attached to a fishing rod or a series of tell-tales spaced out on a line that I can fix say just ahead of the luffs - to ascertain Upwash, laminar flow, stalled air, or leech vortex without getting my non-aerodynamic body in way of the air flow! I will also attach numerous stick on tell-tales to the upper and  lower sides of each panel.

    Bubbles - for the same reason.

    Wind
    Our autumn winds should do. I hope to be able to perform trials in a variety of conditions.

    Anemometer
    I could hire one for £ 31 / week but my calculator, stopwatch and tape measure coupled with bubbles or bits of straw seen to travel over a marked distance, will more readily comply with JRA Basic Principles, and will also serve to keep the assembling crowd of sceptical local onlookers; no doubt comprised of one or
    two 'BR' sailors agitating and tut tutting behind the scenes; amused for just a bit longer. I will compile a few ready answers for the inevitable question: "What are you doing"?



    WHAT AM I HOPING TO SEE AND RECORD?

    UPWASH (Well it will actually be Downwash in my case!)
    I hope that my small models can create the rapid flow and low pressure on one side of the panel, and slower flow and higher pressure on the other. On the two camber panels I hope that I will be able to 'see' and measure the upwash sufficiently well to measure and photograph the phenomena.

    I don't have pressure or speed gauges, but instead I hope to rely on the lower pressure being sensed by the wind approaching the luff, which instead of dividing equally to pass evenly down both sides of the sail, the wind will Upwash (Swerve) to get round the luff - effectively being sucked into the lower pressure area.
    When sailing close-hauled, Upwash is just like a permanent 'freeing wind shift' helping us point closer, we carry it with us until we tack, then 'lo and behold' it changes also, to free us on the new tack as well. So camber can provide smaller tacking angles and more power.

    I will be able to simultaneously observe the luffs of the 3 sails and I am just hoping that my rig is large and sensitive enough for the effects of upwash to be visible and measurable, and to compare the effect of each sail's shape and power produced.

    I know Upwash must exist because there are several jetliners over my house as I write.

    If the effects of Upwash are sufficiently created; I will be able to sheet in all panels gradually, adding water to hold the sheet in that position and weigh it, providing data &  evidence that a particular panel is still luffing / creating power /stalling / as the case may be; and distinguish differences in performance between the test panels.

    My trials are only intended to compare the actual sail panels, the presence of camber and the effect of split in the SJ - not other differences like lower yard angle or greater balance.

    A point that I hadn't anticipated before I started to make the panels was: 10% camber of Arne's Panel B dimension (2000mm) is 200mm, whereas on Slieve’s SJ main with a B of 1450mm, 10% requires only 145mm camber. This means the Arne panel has a lot more camber than Slieve's SJ Panel, let me try to explain.

    It is my intention that apart from their designed differences, the panels should be very similar in all other respects, so I automatically aimed for 10 % camber on both, expecting to be making 'like for like' comparisons, but in fact the SJ had a lot less OVERALL camber / draft than Arne's when viewed next to each other.

    Given that what I am trying to establish is "does the Jiblet and slot earn its keep, does it act like a Jib and Main on a Bermudan rig, and does it combine with the main to point and power better than a cambered main alone"?

    So I made a second SJ Main to have a similar OVERALL camber shape to Arne's panel but with a jiblet and slot being the only practical difference, to hopefully obtain comparable evidence.

    I am sure that Slieve and Arne will have thoughts on all of the above, which I should consider, and do welcome?
    NB. I sent them both copies of the above, they have both made positive and valid comments - I will post their responses in due course, or they may wish to do so themselves.


    A FEW THOUGHTS...

    I have noticed that camber appearing in photographs often looks greater than the real item.

    Two examples are:
    On my great day aboard "Amiina" (with the Split Junk sail) together her owner Edward Hooper, I felt that increasing the camber would be a good thing, but later at home and examining photos taken during the day, I noticed that the amount of built in camber seemed just right.
    Secondly:
    The camber visible in 5 photos of the 3 panels (A B C above) which I put in "Technical Illustrations" looks much more than it appears in reality.

    Just wondering if others have noticed this illusion? I often wondered why Arne commented that he may use more camber in future sails, when to me the photos of his existing sails looked just right.
    Obviously shadow and light can pronounce shape in different light. The only reason I mention it is that it may be misleading and give a false impression during these tests: Or could it be I need to visit 'Specsavers' again?

    In due course I will be attaching an Excel document with designed dimensions (in Red ink ) for the sail's intended construction ie. the amount of round "R" added, its position fore / aft in terms of % from the luff, the amount of camber intended; then a few columns of Results Achieved (In Black ink) after construction. I have also added details of the second SJ Main with increased camber.

    WHERE DO WE GO FROM HERE?

    Well, if any of this is of interest to you then I will be happy to receive your ideas.
    In the mean time I will press on as described above, in the hope of assembling all the bits, sorting out any practical difficulties and running a Dress Rehearsal before trials can happen probably during November.



    Last modified: 20 Sep 2014 21:34 | Deleted user
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