rig tuning bow beam shrouds

Discussion in 'Multihulls' started by Charly, Dec 31, 2014.

  1. Charly
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    Charly Senior Member

    Hey guys, in building my 36 day charter Kurt Hughes cat my next exercise is getting the rig tuned.

    The mast is up. It is supported by the forestay, and two shrouds. Also there is one set of diamond shrouds swept back about 20 degrees, and the shrouds at the bow beam. The forward jumper was eliminated because in consult with the designer and mast maker the mast moments were high enough that they say it isn't needed. I am having the sails built now, but I cant sail anyway until I have the bow beam shrouds in place and secure.

    The bow beam is plywood/epoxy and also designed by Kurt Hughes. Basically it is a tapered box beam that is fastened to a composite bracket on the hulls with an aluminum chain plate that also has a hole to accept the jaws and turnbuckle for the wires that attach at the other end to the plate atop the gull striker.

    Now my main question is what is the proper tension for these wires at the bow beam, and how to tell without an instrument to measure it? Can it be done properly? A larger question that I have is what exactly are these wires for? I think I know the answer to the second question, but would like to hear from others that are more experienced.

    Thanks!
     
  2. upchurchmr
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    upchurchmr Senior Member

    How about a picture?
     
  3. RHough
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    RHough Retro Dude

    Basic starting point to tension a rigging wire is 10% of rating. In general this can be done with a wrench. The wire goes from "thud" to "tone" at about 10% tension.

    assuming the beam and tangs are equal and square, the pin to pin distances should also be equal. Go to a hand tight thud on both wires, then double check the pin to pin distance.

    From there tighten both wires equally one or two turns at a time until you have a tone.

    The exact loading on the wire can be measured with a tape and a ruler if the wire size, length, construction and alloy are known. For SS 1x19 wire the stretch will be 1mm in 2 metres = 5% If the wires are less than 2 metres long. Use a 1 metre rule and each .5mm (0.020") = 5% tension.

    Randy
     
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  4. groper
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    groper Senior Member

    Wow, a few interesting things there charly...

    Firstly, no forward jumper... I can't see how the mast can be sufficiently held In column when there is only cap shrouds without the jumper? Sure, if you have lower shrouds also then I could see it being fine but otherwise it sounds as tho your mast must be quite a large section? How much does it weigh?

    We've been doing all the same stuff as you lately with our cat. To tension the rig we basically just did it by feel until the forestay felt about right. Then to tighten the forward beam striker, simply give it enough tension so that the front beam itself, is straight. The forestay tension will deflect the front beam upwards in the center. The striker wire will push the beam down in the center. So tension both the forestay and the striker wire simultaneously ( a couple turns on one then do the other etc) until you have correct forestay tension, and check your front beam has zero deflection up or down.

    The broad question- the striker wire resists the upward bending moment in the front beam generated by the forestay. This could be a considerable tension when sailing in heavy air, so the front beam would deflect excessively and the forestay would go slack accordingly. The striker wire keeps it tight, and keeps your front beam from bending or breaking.
     
  5. Charlyipad
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    Charlyipad Senior Member

    http://www.boatdesign.net/forums/at...d1418318867-kurt-hughes-daycharter-36-007.jpg

    This is the best I can do at the moment. From the build thread in the boatbuilding section "Kurt Hughes daycharter36"

    This was taken when we stepped the mast. You can see the gull striker up fwd there. Disregard the halyard and bridle To the bows. That is just for temp stability. I should note that the diamonds are already tuned by the rigging crew. We didn't have the right fittings for the gull striker though, ( the jaws we had wouldnt fit over the 3/4 inch plate.... murphy stiles again) and I had to bring the boat back home to Georgia. So now I am on my own. When I get the right wires to put on I need to tighten them up.

    hey Randy
    When you say "tone" do you mean to pluck the thing like a Bass string??

    The head stay and shrouds are also loose enough so that in about 15kts of wind you can feel mast wobble a bit. I assume that they will need tightening a few times after putting the sails up.How do you know how much is enough? the designer recommends putting on a snubber on each shroud. is that normal on multis? I don't remember seeing that anywhere else.

    All comments appreciated
     
  6. Charlyipad
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    Charlyipad Senior Member

    Hey Groper, I don't remember the moments or the weight of the mst but Kurtwas satisfied with the numbers. Then the rigger tightened the diamonds to within a certain percentage of the wires breaking strength and measured the bend in the mast. This confirmed to Kurt that it was a good section. Because the mast didn't bend all that much. I am just a babe in the woods here. I have to trust the designer.

    that makes sense about the forestry and bow beam. I will have to figure. Way to accurately measure the deflection on site down at the dock. I may be able to pull a string on it. I still am paranoid about putting too much tension on anything though.
     
  7. groper
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    groper Senior Member

    yeah im sure they got the numbers right, i figured you must have a pretty heavy mast section to get that much stiffness from it. Our mast measures 210mm x 135mm eliptical section and it weighs 7.1kg per linear meter. We have a 14m mast and it does use a forward jumper, along with inner and outer diamonds of a single spreader. i can imagine cranking up the mainsail sheet and watching the mast bend backwards without it... weve ordered a square top main in technora fiber load path sail construction, somewhere between 45-50square meters. At rest, the forward jumper has almost no tension in it, but when you crank on the main sheet, it limits the mast bending which increases the draft of the sail. so you can control the sail draft and twist with a bit more detail by adjusting the jumper tension depending on how the boat and sail responds to main sheet tension.

    Weve only had it sailing with some freeby sails that were given to us, really small sails but they got us moving along ok:)

    heres our latest pics, heading off for a few days after xmas ;
    [​IMG]

    [​IMG]
     
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  8. RHough
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    RHough Retro Dude

    Yup ... when you thump the wire with your wrench or a hard rubber mallet it goes "thud" when it is loose. At about 10% it is tight enough to resonate and you have a tone. Just exactly like a guitar string.

    The working range for the wire is 10 - 40% of its rated strength. 10% is tight enough to not destroy fittings from shock loading a too loose wire. A decent starting point.
     
  9. waikikin
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    waikikin Senior Member

    Similar on my old Beach marine 10m/33' cat, just the diamonds although it probably did mean a "heavy" section, pretty typical, some had the forward jumper as does my Seawind, once I was in up to 70+ running under bare pole & there was some significant occilation, seeing where I was going was No 1 priority at the time....
    Next boat I might go simpler & just stay the rig with no spreaders, I'll leave it to riggers recommendation though

    Jeff.
     
  10. Charlyipad
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    Charlyipad Senior Member

    I have another issue with my home built rig. The gull striker and Chainplates on the ends of the beam are all fabricated from 3/4 inch plate, with holes for ss inserts that are 1/2 inch inside dia. So I need something with 1/2 inch pins to attach to them. problem is the jaw widths are not big enough to fit over the 3/4 aluminum plate when using something with 1/2 inch pins.

    I am trying to avoid replacing the ss insert bushings with.larger 5/8 if possible.

    ideas?
     
  11. upchurchmr
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    upchurchmr Senior Member

    Just remember to keep a 2D edge distance from the center of the drilled hole to the edge of the plate.
    I.E., if the drilled holes are 3/4" then you need 1.5" from center of the hole to the edge of the plate. Note: you can go less if you have enough engineering capability and know the loads applied. :D
     
  12. Charlyipad
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    Charlyipad Senior Member

    Thanks, that is good to know.

    Illhave to re measure, butI think the hole I already have was drilled3/4'" with the 1/2inch ID insert made from 3/4 SS rod. if that is the case, and I don't re drill the holeinthe aluminum any bigger, and cut a new bushing and drillit out to 5/8 ID. That would leave a bushing with a wall thickness of 1/16 of an inch. I wonder if that would be OK?
     
  13. groper
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    groper Senior Member

    There is a couple ways you could approach this without modifying anything. How about a pic for better consideration?
     
  14. RHough
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    RHough Retro Dude

    I just love when people fabricate fittings that will not accept standard hardware. :-(

    19mm (3/4") jaw marine forks usually have a 19mm pin.

    The wall thickness of the bushing will not be a concern. A close fit between the bushing and the aluminum!?!?! chainplate and between the pin and the bushing are the critical things to get right. We have to assume that *someone* figured a 1/2 pin and the matching wire sizes were correct for the loads.

    A SS bushing in an aluminum chainplate is not a great choice to begin with. If you have to make new bushings anyway, use bronze. SS in Aluminum will be a white chalky wad of corrosion in short order.

    Here is an idea if you can do it right. Consider casting the bushing from epoxy thickened with a structural filler. You get corrosion protection and a perfect fit compared to a machined bushing.

    Would be nice to see a drawing to see how the angles etc line up. I like to see toggles of some sort to help avoid point loading the pins is all the holes are not perfectly square to the loads.
     

  15. groper
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    groper Senior Member

    Look, simply placing aappropriately sized stainless shackle would do the trick. If you don't already have stainless wire, simply splice some Dynex dux to a thimble and the shackle connects the Dynex to the chainplate... but hey, we are using synthetic fiber for everything these days. I would suggest a soft shackle but the edges of your chainplate probably are not rounded off...
     
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