Making a plywood box beam

Discussion in 'Multihulls' started by luff tension, Aug 20, 2010.

  1. luff tension
    Joined: Aug 2010
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    luff tension Junior Member

    Im about to start restoration of a 38ft cat and it currently has a 6inch diameter alloy back beam which I want to cut out and replace with a plywood box beam which incorporates the traveller, tillers, seating and central mainsheet winch mount.
    Currently the alloy beam doesn't carry the traveller load and only supports ythe hard deck, the beam between the hulls is 3.87m.
    Has anyone got any specs to make a beam like this, I was thinking 120mm wide and 600 high in 8mm ply, two layers of 8mm ply on the ends where it goes through the deck and attaches to the existing aft bulkhead and two layers of 8mm ply top and bottom with bulkheads about every 800mm.
    All Double bias taped and coved inside and skinned outside with a 6oz.
    Is this overbuilt??
    Appreciate some feedback
     
  2. jamez
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    jamez Senior Member

    Sorry, can't help with specs for a beam, but will be interested to see what is suggested. What design cat is it?
     
  3. catsketcher
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    catsketcher Senior Member

    Need more specs

    Gday

    Can you tell me what the wall thickness of the alloy beam is. Then we can reverse engineer the wooden one.
     
  4. luff tension
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    luff tension Junior Member

    Looks like it is 3mm wall
     
  5. stewart hyder
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    stewart hyder Junior Member

    8mm seems a bit light to me, I assume you are thinking of making an I beam as opposed to a box beam. maybe an idea would be to look at scantlings for a similar size ply design, say Richard Woods or James Wharram? What design is the boat anyway?
     
  6. luff tension
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    luff tension Junior Member

    Its a Tennant 37.
    I was thinking a box would be stronger and also look heaps better, besides on a previous boat I have had a rear beam failure because it was an I beam. - it didnt do too well in compression.
    just checked the ply Ive got is 9mm not 8.
     
  7. catsketcher
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    catsketcher Senior Member

    You have to calculate the second moment of area of the current beam. A tube has the formula

    Ix =pi (do4 - di4) / 64 – do means outside and di means inside

    Use metric units and calculate

    You will get a number in mm to the power of four

    As the beam is made from aluminium it has a stiffness times its second moment of

    E(stiffness) times I (second moment)

    So calculate this for aluminium. Aluminium has an E of 69 GPa

    So if we use an E for timber of 9 GPa (and forget about the glass for now) you need to increase the E of the beam by 10 times to get the same EI ratio

    Then calculate the I of the box beam

    The formula is

    I= width x depth (cubed)/12 – so making the beam deeper pays very quickly

    You need to calculate the I of the total section and then subtract the I of the void inside.

    I would expect that your beam would be way stiffer than the alloy one. If it is twice as much on the EI stakes I would be happy to put the traveller on it. I would not go I beam. A cat beam has to deal with torsion and a box beam does very well under torsion. If you are close and you want to bulk up the beam run some unis along the top - the compression face - or run some extra timber laminations along the top corners of the beam.

    BTW - My aft beam on my 4000kg 38ft cat is an 8mm box beam of pretty similar dimensions.

    cheers

    Phil
     
  8. catsketcher
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    catsketcher Senior Member

    Forgot one important thing about calculating I for plywood beams. Assume that only HALF of the ply is usable structurally in calculating I. You probably have 5 ply so you can go to 60%. This is because half of the ply goes across the load and doesn't take much load.

    Instead of having two layers of ply top and bottom consider a lamination of thicker pine or fir in the interior edges. Then all the timber is doing work

    cheers

    Phil
     
  9. luff tension
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    luff tension Junior Member

    Thanks for that Phil, seems we are on the right track. This one only weighs 3000kg. Just new here, can we post photos in these forums?
     

  10. ThomD
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    ThomD Senior Member

    I'd run Phil's numbers.

    There are 5 other strategies I would run that allow you to design your way out of situations you don't feel able to calculate your way out of (or do both):

    Design something other than a beam. You mention a bunch of functions this beam might have. Often if you build stuff like seating, cockpit bulkhead, etc... You can end up with a lightweight structure that is way out of the scale of a beam. The strength and stiffness will skyrocket to the 2nd and 3rd power. Just be sure that faces of this structure that will be carrying the main loads will be thick enough to deal with these loads.

    Which brings us to strategy two. Often if your beam will carry walking loads it is thick enough to carry the other loads. Not a scientific thing, but if you can't happily dance a jig on it...

    Third, run a weight budget on it. What can this beam weigh consistent with the previous beam and the carrying capacity of your boat. What would be a reasonable weight? Calculate a beam based on that weight, what is the max amount of material you could build within your budget. One way or another this can surprise you. With a reasonable budget the beam can be huge. No points for over designing, but cockpits and salons have big bits that can hide significant structure. Remember to include weight saved in these areas if your beam does more than one thing.

    Four, optimize materials. Plywood is a good conceptual material, but for beams it can be a little non-linear. You can pick up a lot of strength by substituting linear wood in appropriate glass skins. You can also pick up load carrying in the webs by orienting the ply in the best direction for the webs.

    Optimize your shape. Square shapes are better for beam loads which are pretty linear. While tubes are great for dealing with loads in every direction, think of fishing rods which get rolled in all directions (though they have a spine but nothing other that tubes or hexes has really worked). Compare to a bow where rounded shapes concentrate far too much load and leaf spring sections rule. I realize you are planing on a box, but it will make for a much more efficient beam. In rough terms while wall thickness in beams ought to go up significantly moving from alloy tubes to wood, it actually goes up relatively little most beams seam to increase about 4 fold, and I figure the shape difference is a large part of that. Both the square section, and the ability to assign functional areas in the beam.

    Five, You can also rig the beam if it is large enough. You can build in internal wires or bars into stringers etc... The Tiki 30 hides a sneaky little wire in the beam...

    All that said, the beam you describe is relatively light, and I wonder if it is a spreader loaded mostly in column?
     
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