S2 6.7 Junk Rig Conversion

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  • 27 Oct 2018 09:34
    Reply # 6875822 on 6872873

    With this kind of boat, where the hull is very shallow and there's no skeg, it's the keel that's providing virtually all of the lateral resistance. Ignore the rudder and just consider the centre of area of the keel.

  • 27 Oct 2018 07:38
    Reply # 6875749 on 6872873

    Hi,

    generally the rudder is not included in CLR calculations. If included then the rudder will need to develop some force to balance the rig, that is to say it will create weather helm. 

    David.

    Last modified: 27 Oct 2018 07:39 | Anonymous member
  • 27 Oct 2018 04:13
    Reply # 6875650 on 6872873

    hi scott

    i checked your image with inkscape. (this free software can tell you the mass center of any area defined by a path,)

    your CLR calculation is correct as long as the area of the rudder is included. but i would at least like to know the CLR without it (the second yellow cross…) – and i might use some point between the two as a base for rig placement. (but i'm still an armchair junkie…)

    ueli

    Last modified: 27 Oct 2018 04:15 | Anonymous member
  • 27 Oct 2018 01:22
    Reply # 6875519 on 6872873

    Thank you Arne and David for the feedback.

    I am working on redrawing something closer to 20 or at most 22 sqm. My boat is significantly lighter than Weaverbird. I probably should not need any more sail than that. Point taken. 

    As some others have done recently I am planning to build a completely modular sail where the panels are never sewn together. I would like to have pockets or webbing loops or something similar to rig the sail to the battens, yard, and boom. I will consider adding some amount of shape to the panels when I get that far.

    I would like to have a vertical leach or maybe even one that has the upper battens very slightly further aft. From the little bit I have messed around with my Puddle Duck rig I understand why this may be important. I would like to have nice easy tacking without the sheets getting hung up.

    I am a little stuck on the sail as far as aspect ratio and location. I thought it would be better to go back to following the order in PJR and get an idea of the hull's CLR first.

    It seems like QCAD does not provide any way to mark the centroid of a polygon so I did the math myself following what Wikipedia told me. If anyone has a moment to check the image I am attaching I would appreciate it. It seems my 'eyeball' guess is consistently wrong regarding CLR locations on a hull. Does this drawing look something close to correct?

    Thanks again for the interest.

    Scott.

  • 25 Oct 2018 20:27
    Reply # 6873627 on 6872873
    Anonymous member (Administrator)

    Scott,

    although I like my boats to have some sail area, I, like David Tyler, think that 28sqm is a bit too much for a boat with a displacement just below 1000kg. As I wrote in ‘Junk Rig For Beginners’, my primary reason for converting to junk rig was that the Bermuda rig of my boat performed so badly downwind. If I suggest a very generous SA/disp = 24 , that would  result in a sail area of 23.6 sqm. Dropping to a more sensible SA/disp = 22, the SA comes out at 21.6sqm. On my present Ingeborg, I checked what the original area is with Genoa 1 set, and then drew up a JR with that area (result:SA=35.2sqm and SA/disp.=21.4). Ingeborg doesn’t feel under-rigged.

    As for camber, a daysailer, working on a lake, surely should have some, as you often sail from A to A with no help from any tide, that is, as much upwind as downwind. I have seen photos of Kurt Jon Ulmer’s boat, and the sails appear to have some camber, in particular in the top panels, which cover about half of the leading edge.

    One thing I take care to avoid, is to design a sail with forward-leaning leech. This is to avoid sheet tangle when gybing.

    Anyway, good luck!

    Arne


    Last modified: 26 Oct 2018 08:44 | Anonymous member (Administrator)
  • 25 Oct 2018 18:20
    Reply # 6873333 on 6872873

    The S2 6.7 is a little smaller than Weaverbird. I have a 22 sq m sail and that is quite enough. 28 sq m would be enormous and way OTT. Junk rigged boats sail downwind faster than bermudan boats because we deploy our area more efficiently - a spinnaker is really a very inefficient sail, area for area. I hope you'll rethink the area.

    "Windward performance is not really of much interest to me." It should be. I don't mean world-beating performance, there's no need for that, but I think you'd be disappointed with a flat sail, for lake sailing. As it's actually easier to make a sail panel-by-panel than the old flat sail method of sewing the whole sail together at once, you might just as well put in a little bit of shape while you're at it.

  • 25 Oct 2018 13:54
    Message # 6872873

    My boat is now stored for the winter with no access until sometime in May of 2019. I am planning to spend the winter drawing and planning the sail. Maybe the mast and tabernacle as well.

    I thought I would start with the 'post the wrong answer to get the right answer' method.

    Sailing down wind with Bermuda rigs has always irritated me. Why should the least technologically advanced point of sail be the most stressful and difficult? Following this line of thinking I got it in my head that I would like my junk sail to have the same sail area as a spinnaker that would normally be used on the boat. I want a big squared off sail for running down wind.

    I have also been persuaded by Kurt Jon Ulmer's 'long bold article' on the merits of flat-cut sails. I have decided that a flat sail with progressive batten angles (some fan) in the panels will be best for me. I am looking for simple, strong and proven. Windward performance is not really of much interest to me.

    I tried to draw some sails using the angles the Kurt describes in his article. 'I might use 10,10,12,14,17,22,60 now.' I did not have much success so I started looking at other sails that had some progressive fan. I was very happy to find the .dxf files for David Tyler's SibLib sailplan in the members area.

    I scaled the sail in his drawing down to 28 sqm (I will be trying very hard to do all work in metric units). I did my best to place it where I thought it looked OK on the line drawing for my boat.

    In summary my first try for an answer is a SibLib sail scaled to 28 sqm, cut totally flat and placed as shown in the attached drawings.

    I ended up with what looks like a BIG stick for a mast. This makes me think my idea is not practical.

    Any helpful advice would be greatly appreciated.

    Scott.

    Last modified: 26 Oct 2018 13:59 | Anonymous member
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