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yard design and construction

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  • 19 Mar 2013 19:56
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    Reply # 1246725 on 1054878
    Anonymous member
    My experience has been that the strength of a yard is the important thing, and if it is strong enough, it is also likely to be stiff enough. A yard does not need to be ultra-stiff in the vertical direction. A good approach to take is that adopted in the Finn, the OK dinghy, and similar racing classes. The mast is not ultra-stiff, but is of known flexibility, and the sail is cut to take into account that flexibility, and the weight of the helmsman, with the camber of the sail being more in light air, less in heavy air. Translating that into JR terms, a round, tubular yard of sufficient strength, without any bracing, has sufficient stiffness sideways, which is important to avoid reverse camber in heavy air; and has sufficient stiffness in the plane of the sail, provided that the sail is cut with some rounding to avoid "starvation" of the upper panels.
  • 19 Mar 2013 04:12
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    Reply # 1246155 on 1244817
    Deleted user
    David Tyler wrote:A correction, if I may, Barry:
    The stiffness depends on the moment of inertia, which includes the terms height cubed*width, but the strength depends on the section modulus, which includes the terms height squared*width.

    Mea culpa. I've got an electrical engineering background, so my mechanical engineering knowledge is based on one college statics class way too long ago, and a few evenings googling stuff.

    Now I'm wondering whether strength or stiffness is the limiting factor for an aluminum yard? I decided that stiffness was the major concern with battens.

    I guess I better spend some time reading up on strength and section modulus in addition to what I've figured out on stiffness and moment of inertia.

    Barry
  • 18 Mar 2013 18:54
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    Reply # 1245709 on 1054878
    Deleted user
    The links are fine, Arne. Well worth doing, thanks.
  • 18 Mar 2013 12:49
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    Reply # 1245340 on 1054878
    Anonymous member (Administrator)

                                                                      Stavanger, Monday

    The sideways strength relative to the vertical strength simply varies with the width-to-height (W/H) ratio of the solid yard and this goes for both rectangular and oval section.

    On the other hand the relative stiffness (inertia) sideways varies with the square of the W/H ratio.

    In other words, the standard HM wooden yard with B= 0.65H will be 65% as strong sideways as it is in the vertical plane, but the same yard will be only 42% as stiff sideways as in the vertical plane.

    It is easy to get lured into problems and forget that we are talking about relative values here: Square goes linear and cube goes square...

    Cheers, Arne

    PS: I have fixed the links to my posting here of 25. Aug 2012 so should be easier to read now.

    Last modified: 18 Mar 2013 13:56 | Anonymous member (Administrator)
  • 17 Mar 2013 19:53
    Quote
    Reply # 1244817 on 1054878
    Anonymous member
    A correction, if I may, Barry:
    The stiffness depends on the moment of inertia, which includes the terms height cubed*width, but the strength depends on the section modulus, which includes the terms height squared*width.

    Thus the HM yard proportions are a  little better for sideways strength than you've indicated.

    Still, the reasoning holds good - the best section for a yard is a very fat rectangle/oval/ellipse, or a round tube.
  • 17 Mar 2013 19:38
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    Reply # 1244812 on 1054878
    Anonymous member
    Arne and Barry,

    I agree.

    Unpredictable forces suggest beefing up in all directions, leading to a design more uniform - more circular - than any kind of I-beam or plank-on-edge.

    Cheers,
    Kurt
  • 17 Mar 2013 17:12
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    Reply # 1244738 on 1057113
    Deleted user
    Arne Kverneland wrote:

    I just looked up PJR Fig. 10.1 to find how the rectangular section of Hasler and Mcleod’s wooden yard is. The width is 65% of the height which, if my maths don’t fail me, means that the sideways breaking strength of that yard is 65% of its breaking strength in the vertical plane.


    I've been doing lots of calculations related to this--a solid beam's strength (and stiffness) is height cubed * width. So it works out to being 43% as strong or stiff in the horizontal direction compared to the vertical direction with only a 65% height to width ratio.

    I think this is why oval yards should be fat ovals, not skinny ones--the horizontal strength/stiffness falls much faster as they get away from circular.
  • 29 Nov 2012 06:10
    Quote
    Reply # 1148361 on 1054878
    Anonymous member
    Hi Graham,

    I'm not an engineer either. 

    Your pictured halyard span looks like a great sling point locator, but since the halyard shackle is at the yard, the beam-bending lifting load isn't being distributed. The 'weight' is taken at the point where that centre sling wraps the yard. 

    The other two lashings don't have any component in the 'up' direction, to lift with. Say you took the centre wrap out, they'd lift by compressing the yard between their locating eyes, hard, until the lifting component fraction became great enough. Spectra's pretty good stuff. Your eyes or frictiony wrappings would have to be strong as well, to transfer the compression to the yard.

    The forward-going spectra as pictured will take luff tension, if it goes really taut, but the yard could handle that. The aft-going one is slack in use, I predict. Compare a gaff halyard span, that has an 'A' of space above the gaff. The distance from the yard's top, to the shackle of the halyard block, determines the extent to which you can spread the force-at-right-angles, the 'weight' of the sail, along the yard.

    If you put your drift into the yard halyard span, you won't need it in the halyard purchase. The purchase blocks can go nearly cheek-to-cheek, giving you room underneath to spread the lift to a couple of points on the yard. But making them load evenly is another story. The gunter halyard bridle I inherited, has a sliding eye for the halyard. There goes your locating function...

    Cheers,
    Kurt
    Last modified: 29 Nov 2012 06:14 | Anonymous member
  • 31 Aug 2012 17:51
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    Reply # 1061677 on 1054878
    Anonymous member
    Having made a carbon fibre yard, I just moulded in some equivalents to the added saddles, but the principle is the same. I used 6mm bare braided Dyneema (4 tonnes breaking load, very chafe resistant and knots hold well), and tied it around a thimble with an Alpine Butterfly knot ( one of the mountaineer's knots that the middleman on a rope uses to tie in - will take a load in either direction). The halyard block is shackled to the thimble. The tails of the Dyneema are tied around the yard through the moulded in holes.
  • 31 Aug 2012 12:29
    Quote
    Reply # 1061486 on 1054878
    Anonymous member
    Peter, I will take a close up photo of my span tomorrow and post it on my member page.  I thought the existing photo might not show enough detail.  I'll also try to describe it more.  I like David's idea of using a span, both to spread the load and to allow variation of the sling attachment point.  I did not want to give up any of my meagre drift between halyard blocks however, so added the middle lashing as well.  I riveted on 2 heavy duty saddles with 6mm monel rivets to the bottom of the yard (my sail is laced to the yard),  at the 1/3 and 2/3 positions.  I attached a large shackle to the eye of the lower halyard block to which three point span is attached. The span is made up of three separate lashings of 4mm Spectra line.  One goes from the shackle through the aft saddle then back to the shackle on the other side of the yard.  This is repeated through the forward saddle.  The middle lashing goes from the shackle around the yard and back to the shackle.  It is not restrained by any saddles, to allow me to vary the halyard attachment position, which I can do by shortening or lengthening the fore and aft lashings and also to maximise articulation.  I have another saddle on the side of the yard for the yard hauling parrel and fixed yard parrels.  Not being an engineer, I don't really know how well this spreads the load but it sits very well and the lower halyard block remains nicely aligned with the upper block even when the yard is squared off.
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