S2 6.7 Junk Rig Conversion

  • 31 Oct 2018 17:45
    Reply # 6882763 on 6882723
    Scott wrote:

    How tall can I make a tabernacle? Would having the hinge point 6 feet above the deck be at all possible? I am imagining the top of the tabernacle above one or two of the battens. That might allow me to lay the mast down without having an unreasonable length hanging over the stern. I have not seen anyone with a rig like this -- I imagine there might be a good reason.

    Thank you,

    Scott.

    The top of the tabernacle should be just below the foot of the sail, so that the sail doesn't have to pass over the extra width as it is reefed and furled. Even with the type of tabernacle that puts a hinge into the mast itself, with a tube that drops over the hinge to carry the bending load, it's not really practicable to have the hinge higher than that.

    When the mast has been lowered, it's not usually left in the tabernacle, but is commonly taken out and laid along the boat, perhaps resting on top of the tabernacle. 

  • 31 Oct 2018 17:30
    Reply # 6882723 on 6882236
    Anonymous wrote:

    Scott, 

    is there any strict restriction with respect to mast position and interior?

    Arne


    Arne,

    The major constraint is due to the very large trunk for the lifting keel. Right now there is just enough room to duck and waddle around the trunk on all sides. There is currently a compression post directly under the mast. If the aft side of the mast (tabernacle) is placed any further aft than the current compression post I think it will make the meager accommodations even less comfortable. I attached a drawing showing the keel trunk and compression post locations.

    I do not know what structural factors come into play with putting the mast through the cabin top instead of the fore-deck.

    David,

    I would really prefer to minimize the length of the mast. I would like to make it practical to trailer the boat regularly and use a tabernacle.  I have no doubt that the Weaverbird sail has all the advantages that you describe. It seems the trade off is a tall mast. I saved a copy of the Weaverbird CAD files. I will consider following this design.  Unfortunately I think it might not be trailer sailer friendly.


    How tall can I make a tabernacle? Would having the hinge point 6 feet above the deck be at all possible? I am imagining the top of the tabernacle above one or two of the battens. That might allow me to lay the mast down without having an unreasonable length hanging over the stern. I have not seen anyone with a rig like this -- I imagine there might be a good reason.

    Thank you,

    Scott.


    Last modified: 31 Oct 2018 17:31 | Anonymous member
  • 31 Oct 2018 11:17
    Reply # 6882236 on 6872873
    Anonymous member (Administrator)

    Scott, 

    is there any strict restriction with respect to mast position and interior?

    Arne

  • 31 Oct 2018 10:13
    Reply # 6882159 on 6882147
    Arne wrote:
    Scott wrote:

     I think this fan sail design is the right way for me to go.

    Scott.

    Scott,

    The fantail sail has proven that it works. 

    Yes, it works, but it's not a universal rig, to be chosen without considering the alternatives. Annie decided she didn't like it very much. I sailed with it from NZ to Alaska, and decided I didn't like it as a big boat's rig. David Thatcher is still using it, satisfactorily.

    Just take care and terminate the battens flush with the leech. 

    Highly desirable on all junk sails.

    This will minimise the sheet tangle. Would it be an idea to lengthen the top sheeted batten? Annie should be the right one to answer on this.

    Or me. Or David Thatcher. We've all used this planform. The extended top sheeted batten is also highly desirable on all junk rigs (except the double port/starboard sheeted ones, of course).

    Arne


    Scott,
    Depending on accommodation constraints, you might like to consider my Weaverbird planform of sail, which is easier to use the the Fantail planform - fewer issues with helm balance off the wind, deep reefing, sailcloth stability and parrel loadings. The S2 6.7 has enough stability to carry a higher AR than the Fantail planform. On the other hand, the shorter mast of the Fantail planform may be of value, as you intend to trail the boat down to the slip to launch, meaning that a tabernacle is going to be useful.
    Last modified: 31 Oct 2018 13:09 | Anonymous member
  • 31 Oct 2018 08:56
    Reply # 6882147 on 6881688
    Anonymous member (Administrator)
    Scott wrote:


    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.

    Scott.

    Scott,

    The fantail sail has proven that it works. Just take care and terminate the battens flush with the leech. This will minimise the sheet tangle. Would it be an idea to lengthen the top sheeted batten? Annie should be the right one to answer on this.

    Arne


  • 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.

    Scott.

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

    Scott,

    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!
    Arne


    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.

    David.

    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,

    Scott.


    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.

       " ...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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