Glass wrapping stainless angle irons

Discussion in 'Fiberglass and Composite Boat Building' started by fallguy, Jul 25, 2019.

  1. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    7BB9D1DD-C792-442B-A682-F20E6D069256.jpeg My cockpit 88x106" is supported by a center iron on the 88" side. Boat is demountable.

    The irons are two ss angles 1.25"x1.25" welded together to a t shape with an arch formed in them. The thickness is about 1/8" or jist under.

    The arch was formed at 3/4".

    We sat two of us (400#) on the teed angles and lost about 1/4" of the arch permanently.

    When we walk onto the cockpit; the arch is gone fully, so our tee is too weak.

    So, can I wrap the angle irons in fiberglass?

    Sort of a metal sandwich thing?

    My cost to get the angles and ironwork is about $3-400 for a beefier angle iron like say 2.5x2.5x3/16. I am just nervous it'll still be weak.

    Will the fiberglass metal sandwich crack off with epoxy or will it give less and enough to perform? Thanks for any replies.

    I can provide a picture of the metal as well, bit it os just two angle irons welded back to back on staggered welds about a foot apart on the bottom and staggered offsets about a foot apart on top offset 6" from the other side.
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    I'm not at all clear about what this item is, and its function, but I'd have little confidence adding glass/epoxy to it will decrease deflection much at all.
  3. fallguy
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    fallguy Senior Member

    Here is a cross section one minute sketch. The arrows are pointing to the welds in red. The part is 88" long with a 3/4" arc. The welds are about a foot apart and staggered so the top weld is 6" away from a bottom weld.

    The deck sits on top.

    Material is 304 ss, 1.25x1.25x1/8", iirc...not at location now.

  4. fallguy
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    fallguy Senior Member

    If you look at the picture of my son; he is standing on a greenish laminated deck section. The white section to his right or the starboard side is not laminated. The angle iron sits under the intersection of those two sections running for an aft to fixed mounts.

    Thanks for replying; sorry if I was unclear.
  5. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    Fiberglass-metal sandwich not a great idea.
    How about using a manufactured T shape with a longer web?
    Is the piece just a deck support, or does it involve other loads and forces?
    The supplier will usually furnish engineering specs, you need to calculate the load and deflection.
    Foam/glass “hat” section reinforcement underneath the deck will work, but that needs calculations as well.
  6. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I ran all the engineering numbers and we need to use a beefier angle...the stuff I have is weaker in deflection by a factor of like 500-1000%. I ordered 2"x2"x0.250" angles in 316 because they will also not get crevice corrosion when joined. The low oxygen idea of the metal sandwich is also a problem I think; especially with 304 stainless.
  7. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    Being an avid reader of Old Multis posts, that state that most beams are designed to be 5 x breaking strength[Crowther]., 5x overall weight of loaded boat I presume, The vertical height seems too low for SS , a Warram using mild steel RSJ. is mentioned. the answer directly above your sons head?, impressive looking build, great stuff.

  8. Barry
    Joined: Mar 2002
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    Barry Senior Member

    Is there a reason that you don't increase the vertical height of the beam?
    Two angles oriented as you have is an extremely poor usage of the amount of material that you are using to resist the load, ie minimize deflection in the beam

    The moment of inertia is the parameter that deflection depends upon when the load and the distance between the end points are constant. The max deflection is given by
    y=wlll divided by 48EI (lll is l cubed). So everything else a constant, E, 48, w ( load) l (length), then y varies as the inverse of I

    Note, I am using a simply supported beam as we don't know if your end conditions of the beam are sturdy enough to hold the resulting bending forces in the beam.

    The moment of inertia of your two 2x2x.250 angles will be .70, So the deflection is K/.7 , assume K is .3, probably close, so then the deflection is .42 inches
    If you were to make the two angles into a 4 inch channel with 2 inch flanges, The deflection would be .3/5.8 or .05 inches. This is a significant difference for the same weight
    of the two angles.
    Alternatively you could purchase even 2 x 2 x .25 channel, and the deflection would be about .36 which is lower than the back to back angles and would weigh much less.

    Alternatively, you could go to a 2 x 3 by .188 channel, deflection prox .16 inches or even 2 x 3 x .125, and get a deflection of prox .24in
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