Fabricated aluminium alloy yards

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  • 28 Mar 2012 10:49
    Reply # 870482 on 868877
    Anonymous member (Administrator)

                                                                       Stavanger, Wed.

    It appears that the yard on Fantail’s sail is quite a bit shorter than the lower battens so may be well less than 5m(?). A wooden yard shouldn’t be too heavy then.

    When I moved up from Malena’s 32sqm and 4.6m yard to Johanna’s 48sqm and 5.8m+ yard, I just scaled up the PJR yard and then got a heavy monster, over twice the weight, so I had to find another way of doing it.

    The use of a shorter yard than the lower battens, as on Fantail’s sail seems to make a lot of sense. I used the same method when designing the rig for the schooner Samson, with HM-style sails. The idea with doing it was to reduce the size of the top panels to keep control of the twist, but a good side-effect was to save weight of the (wooden) yards. On Samson’s main sail the lower battens are 6.7m, the top batten 6.4m and the yard 6.2m. I guess I will make use of this method more often on big sails.

    Arne

    PS, a couple of hours later: I just found a Q-CAD drawing of Fantail’s sail which let me find the length of battens and yard. The yard, at only 3.75m, should come out pretty light, less than ¼ of Johanna’s wooden yard and just over half of the 4.6m yard on Malena’s sail. This kind of shortened yard may call for slightly stronger top battens due to raised compression, but overall one should still save quite some weight with a yard that is made so much shorter than the boom (4.85m). The more I look at it, the more I like it.

    Last modified: 28 Mar 2012 14:45 | Anonymous member (Administrator)
  • 28 Mar 2012 01:47
    Reply # 870103 on 868877
    I've been talking to Annie this morning, and if she goes for an alloy tube, it will be 86 x 3 from Ullrich, which is 11% heavier than a fabricated yard to Arne's scantlings, but a very great deal stiffer and stronger sideways. However, she might just go for a wooden yard to PJR scantlings, or 10% more. Weight is less of an issue than wanting to avoid any further unreliability.
  • 27 Mar 2012 20:03
    Reply # 869751 on 869533
    Gary King wrote:David or Arne,
    What tube size would you recommend for the yard on our 30sqm main? (3.6m length on the modified HM rig). This is without bracing tubes -  like Footprint's new yard (I think).
    The head of Fantail's sail is 3.75m, its area 29.5sq m. I recommend to Annie that if she goes for a tubular yard, she gets 80dia x 2 wall. I calculate that this is 1.16 times as stiff and 0.94 times as strong as 65dia x 3.5 wall, and its weight is 62% of the weight of a 65dia x 3.5wall plus a 25dia x 1.6wall bundled together, a close approximation to the weight of a fabricated yard. Of course, it is not as stiff in a vertical direction as a fabricated yard, so the head of the sail needs a round of 1 - 2% to compensate. If the halyard block is on a span, that will also help, and it's a lot easier to make than a fabrication.
  • 27 Mar 2012 16:23
    Reply # 869533 on 868877
    Deleted user
    David or Arne,
    What tube size would you recommend for the yard on our 30sqm main? (3.6m length on the modified HM rig). This is without bracing tubes -  like Footprint's new yard (I think).
  • 27 Mar 2012 09:41
    Reply # 869320 on 868877
    Anonymous member (Administrator)

                                                                   Stavanger, Tuesday

                                      Learning by doing

    I share David's concerns about making yards this way - after all I started using the method back in 2003. Still, I stick to this braced aluminium tube yard as one saves so much weight over the wooden PJR version. The challenge is to do it right.

    There are two traps one can fall into when making this braced aluminium yard.

    1.) One may be tempted to choose a too light main tube since the bracing tube "will take the load anyway". As David points out, it is easy to under-estimate the side forces. The solution is to use a thicker main tube or (more complicated) to beef up the centre third of it with a second close-fitting tube around it.

    2.) It is tempting to attach the bracing tube almost to the ends of the main tube. The problem then is that the angle between the main tube and the bracing tube gets very shallow and this leads to very high compression loads on the main tube. High compression loads combined with some sideways bending forces are not so good.

    As can be seen in the end of my Chapter 6 of "The cambered panel junk rig", the main tube of Johanna’s 6m long yard is 65mm diameter with 3.5mm walls. The bracing (or supporting) tube is attached 1m from the ends. This gives a reasonable angle between the two tubes, but at the same time leads to more bending of the end of the yard. This yard is on the light side, but has held for my easy coastal sailing. Fantail’s yard has only 2" main tube on her yard which is about half as strong.

    The solution to Fantail’s problem should be to replace the bent main tube with one of around 65mm diam. With Fantail being lighter than Johanna and her yard shorter, this should produce a safe offshore yard which still will be much lighter than a wooden yard.

    I am sorry I don’t have more skills as a stress engineer, so I can’t give hard and fast scantling rules. Learning by doing is the name of the game here.

    Arne

    Last modified: 27 Mar 2012 12:55 | Anonymous member (Administrator)
  • 26 Mar 2012 23:01
    Reply # 868921 on 868877
    Sorry to hear about Annie's Yard. Will she have to build a new one?
    I think I should recheck the ratio of my timber Yard as well.
  • 26 Mar 2012 22:07
    Message # 868877
    During a beat against a stiff breeze, at the NZ rally, Fantail's yard took on a dramatic bend in the centre.
    I have long had misgivings about this way of making a yard, and now that I have seen a failure at first hand, I feel emboldened to speak out against the method, of taking a small diameter straight tube and adding a reinforcing tube above it, spaced away in a shallow triangle by piece of sheet or plate. 
    A great deal of stiffness is gained - but only in the plane of the reinforcement. 

    A little anecdote:  a lifetime ago, I raced dinghies, and on one boat, I made a boom from a piece of wood about 4" deep and 1" thick. As soon as I applied tension to the kicking strap, the boom immediately collapsed sideways, choosing the easiest way to bend. There is an unstable equilibrium position where the load is in line with the strongest, stiffest axis, but if anything at all causes the load to be applied away from this axis, then, sooner or later, dramatic failure must result. This can easily be demonstrated by cutting a profile of such a yard out of stiff card or thin plywood, suspending it from the mid point and progressively loading it at the ends.

    PJR gives proportions for a rectangular-section wooden yard of 1.54:1 for the depth/width ratio, and this is clearly safe enough, because it's been used for so long. I would tentatively suggest that a ratio of 1.75:1 or even (perhaps) 2:1 might be tried, but under no circumstances would I want to make a yard with a greater depth/width ratio.

    I specified a round tube for Footprint's new yard. A round tube has equal stiffness and strength in all planes and is clearly not prone to instability. 
    A square tube has equal stiffness when a load is applied in line with its faces or across its diagonal - but, counter-intuitively, when a load is applied across its diagonal, it has only 71% of the strength that it has when a load is applied in line with its edges. So even here, there is a possibility of unexpected failure.
    The situation gets worse when the cross section is oval, elliptical or rectangular; but as I said above, a ratio of 1.54:1 has proved acceptable. At any greater ratio - well, I think the risk is unacceptable. 

    For Tystie's new yard, I will be using a round tubular section. I will be laminating it from carbon fibre braided sleeve, so I have the opportunity to increase the wall thickness in the centre, which I will do. The wall thickness will be the same all round the circumference, so there will be no "weak" axis. However, I will be attaching the halyard by means of a soft rope span to the one-third and two-thirds points along the yard, so that I need not make the yard excessively strong horizontally in order to get sufficient strength and stiffness vertically. This practice has been used for a great many years on the peak halyards of gaff-rigged boats, albeit with wire spans: Dyneema and Spectra are the modern equivalents.

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