S2 6.7 Junk Rig Conversion

  • 30 Oct 2018 23:51
    Reply # 6881688 on 6872873

    Thank you Arne and David for the feedback on the hull CLR 

    It seems the following options give me fairly close to the same result:

    1. 6% aft of the conventional rig for the CE

    2. 9% forward of the CLR without the rudder

    3. Arne's "CE placed at, or just a bit aft of the leading edge of the fin keel"

    I looked at a few photos of Arcadian. That is a very nice looking boat. I would say that compared to my S2, Arcadian is more of a Ship than a boat. For now I will assume that the 17% lead required does not compare to my very different hull shape. What I have is a 22 foot dinghy with some lead ballast.

    It seems I have been (poorly) re-inventing the fan sail the last few days. With a little push from Kurt Jon Ulmer I revisited the great work done by David Tyler that allows less talented people, like me, to scale his fan sail design to any sail area.

    I made up my mind that 21 sq m is enough for a 2200lbs boat and placed a scaled version of Mr. Tyler's fan sail on my QCAD drawing. I am really happy with this idea. I plan on moving forward with checking out this plan to see if a sheeting arrangement can be made to work.

    My apologies to Arne regarding the leach. I am sure that it is a lot easier to prevent tangled sheets if the aft part of the sail is vertical but all things considered I think this fan sail design is the right way for me to go.


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    Last modified: 31 Oct 2018 00:46 | Anonymous member
  • 29 Oct 2018 11:09
    Reply # 6877772 on 6872873
    Anonymous member (Administrator)


    Your newest rig looks better in my eyes. I notice that you have drawn in a very short halyard drift, as seen on some western lugs. I suggest you make the mast 40-60cm taller. This both reduces the wrenching forces as the sail is squared out, and also gives you freedom to shift the whole sail forward or aft, to get steering balance perfect. This special ability  of the JR saved me on my present boat, Ingeborg: I had underestimated the need for lead, but by later shifting the sail forward of the initially designed position, the steering became good.

    On a fin-keel boat with a big rudder on the stern, it is hard to get it wrong. The rudder, sitting far away from the turning axis, and in un-disturbed water, can handle most balance issues. I have had three boats with similar keel-rudder configuration. With the sail’s CE placed at, or just a bit aft of the leading edge of the fin keel/cb, the boat will most probably balance nicely when close-hauled. As one bears away onto a reach or run, or if the boat is heeled well over, weather helm will increase, and there will be more and more need for a rudder to stay on course.

    A long-keeled boat is much trickier to get the lead right on, for two reasons:

    ·         The actual centre of pressure, CP, sits quite far forward of the geometrical CLR, so the lead should be closer to 14-17% (not counting the rudder into the CLR).

    ·         The rudder itself is often sitting at the trailing edge of the long keel, where the water flow has been slowed down a lot, so the rudder is much less efficient than a freestanding rudder of the same area.

    One more thing: A boat with a free-standing rudder and no skeg in front of it, is likely to be directionally unstable. No big deal  -   my last boat, Frøken Sørensen was like that. I solved it by adding a simple tiller lock/brake on it. This freed both hands to let me handle the sheets or whatever, and still stay roughly on the course.

    Good luck!

    Last modified: 29 Oct 2018 11:20 | Anonymous member (Administrator)
  • 29 Oct 2018 01:23
    Reply # 6877318 on 6872873

    Hi Scott,

    the CLR is usually calculated from the underwater profile of the boat without the rudder. Where the boat is shallow draft and has a substantial fin keel this will generally not differ much from the CLR of the keel only. However where, as in my boat Gypsy Rose, the fore part of the hull is deeper than the aft part (or vice versa) then there can be some difference. To be safe I would calculate based on the complete underwater hull profile without the rudder.

    Regarding the lead percentage, I have found that a lead of 9% may not be enough; on Arcadian I allowed 11% and found that she carried quite heavy weather helm which was not neutralized until the lead was 17%. Having a sail plan that can be adjusted either by mast tilt or swinging the sail forward or aft can usually compensate for miscalculations of this sort. On Arcadian I added about six feet to the back edge of the keel to correct the weather helm.


    Last modified: 29 Oct 2018 01:29 | Anonymous member
  • 29 Oct 2018 00:24
    Reply # 6877262 on 6872873

    David Tyler and David Webb,

    Can you please confirm that I am following your advice properly?

    I have an extremely shallow hull (10" draft on the specs). Because of this I should ignore everything except the rectangular shape of the keel to determine the CLR and then set the CE of the single junk sail at 9% of the LWL forward of the CLR I determined.

    As you might imagine I get very different results using just the keel rectangle vs. using the entire underwater profile from the drawing.

    I attached a screenshot of my current 'maybe' idea in case anyone is interested.

    Thanks again,


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    Last modified: 29 Oct 2018 01:21 | Anonymous member
  • 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


    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. 


    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…)


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    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.


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


    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!


    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.

       " ...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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