Front Cross Beam Tension

Discussion in 'Multihulls' started by Eliseviv, Aug 26, 2011.

  1. Eliseviv
    Joined: Jan 2011
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    Location: Darwin

    Eliseviv Junior Member

    Hello Fellas,

    Replaced the front beam in my cat, and it seems to be functioning as expected, and had it out in a few blows (35kts+ and 5 mtr seas) and the performance is as expected :). (10 knots is now normal speeds for me :))

    But its a bit unnerving the flex of the forebeam in moderate seas, as the seagull striker on the forebeam loads and unloads in each wave (with the flex in the front beam being tangible to feel through my feet, but can't see it).

    Obviously with the forestay attached there will be flex, but I was wondering from the consensus of this group, what is the "preload" I should put on the front beam via the seagull striker turn buckle. I have played with it and I can bend the front beam down pretty easily, but Im not sure what the science of this is?

    BTW Mai Tai is a 11mtr shockwave with 3 * 5.5 mtr cross beams, with rear hard deck but no "cabin" so to speak.

    Thanks in anticipation

    Regards

    Brett
     
  2. Bruce Woods
    Joined: Sep 2007
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    Location: perth

    Bruce Woods Senior Member

    Shockwaves were pretty lightly built, also with such low profile beam set up, one would expect the rig/forestay to be slopping around. Just as a recap the fore and aft beams hold the boat together and the mast beam sits (terminates) on the inside gunnels and merely takes the rig load. Is this correct? You have replaced the fore beam? Or the mast beam?

    Have you managed to have the boat weighed, maybe when it was lifted?. If its gained a lot of weight over the years its certainly going to load up those beams.

    Regards
     
  3. Eliseviv
    Joined: Jan 2011
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    Location: Darwin

    Eliseviv Junior Member

    G'Day Bruce,

    Yup you got it in 1. I replaced the forebeam and removed/inspected the others. The crane driver gave me a sub 2000kg weight which is pretty good when the design weight of the shockwave 37 with "blisters" (WAHOO) for internal cabin is 2200kg (mine is the "racer" without the blisters). So the weight is within a normal range.

    I expected a bit of rig movement on the lee side, but there must still be an optimal tension. Do you know how much tension is supposed to be on the front "seagull" striker. Should I load it up and put a 25mm bend down on the cross member and then "straighten" it with the forestay... Ive done a bit of hunting on the net/library (yep a place with books!!) and can't find a reference to the science for determining the tension of the front beam.

    Cheers

    Brett
     
  4. Bruce Woods
    Joined: Sep 2007
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    Location: perth

    Bruce Woods Senior Member

    Remember the wahoo variant was similar in name only, to the shockwave 29 and 36/37. From memory the displacement of the 36/37 was around 1400kg (just over 3000 lbs). So you are around 50% heavy in sailing trim than originally dsigned.

    Probably not much point in going for more striker tension than will give you a straight forebeam under normal rig tension.

    Regards.
     
  5. Samnz
    Joined: Nov 2008
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    Location: Auckland

    Samnz Senior Member

    On the racing GBEs (alloy front beam with wire stay) I have helped tune up over here we normally wind a little bit of pretension into them, so they are straight when sailing. The wire stretches and the beam compresses a little bit when sailing and I think it should be as straight as possible when sailing, so it only bends up slightly when flying a hull. the only way to achieve this is trial and error, have a look at it when sailing then adjust after it...
     
  6. Landlubber
    Joined: Jun 2007
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    Location: Brisbane

    Landlubber Senior Member

    Eliseviv,
    Basically wire rope tension is a matter of elongation measurement, 1/19 wire rope will stretch then reduce to almost the same length but needs to be stretched a few times before it becomes more stable.
    Like mast rigging tension in cap shrouds, you are looking at stable tensions of 15 to 20% of the breaking load calculated (by the manufacturer) for the wire rope size, is is very constant at 1mm of stretch for 2000mm of wire (regardless of the wire diameter) being 5% of the breaking load, so 10% would be 2mm, 15% would be 3mm and 20% would be 4mm stretch. Remember that the wire diameter is inconsequential to the above statement, as all diameters are constant.
    It is not hard to measure this stretch with a digital vernier and a yardstick (2M is better).
     
  7. Eliseviv
    Joined: Jan 2011
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    Location: Darwin

    Eliseviv Junior Member

    Hi Bruce and Samnz,

    Your coments are what i was after. Ill run up the front next time in in smooth water and look "along" the beam. and adjust from there...... Not sure Ill be up there while flying a hull though ;-). I think I probably have a straight beam now, but cant be sure till I sight along it.

    Bruce Ill look for my added half tonne... I guess if you add everything up it doesnt take long.....
     

  8. Eliseviv
    Joined: Jan 2011
    Posts: 46
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    Location: Darwin

    Eliseviv Junior Member

    Hi Landlubber, While not what I was after, a really good bit of information to store in the memory banks, its something Ive needed to know in the past, and for sure will again in the future.

    Cheers
     
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