I need to span a 68 inch long by 54 inch wide

Discussion in 'Boat Design' started by Skua, Nov 17, 2013.

  1. Skua
    Joined: Apr 2013
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    Location: Hunt's Pier WW NJ

    Skua Senior Member

    deck area over a gas tank. I have 2 1/4 inch thickness to play with, as this is the deck thickness of the rest of the deck. I can span the area with 2x4's 2,8's etc, but this gets quite heavy, using sufficient cross beam support. Due to the gas tank there is not enough room to run 2x4's fore and aft. The rest of the deck is supported fore and aft as well cross beam, with 2x4's with 1/2 inch ply .

    It would need to support 2-3, 160--200lb adults,, not weigh more than 100 lbs approx, and not be crazy expensive. Any Ideas??
     
  2. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Make a perimeter frame of 1x2 rectangular tube (1/16" wall), with a few fairly closely spaced (12" centers) cross pieces, then glue and screw 1/2" marine plywood over the assembly. If in salt water, use 316 or 316L, if fresh water use 304. You could get by with well painted or powder coated mild steel for less money.

    To shave some weight off this, use angle stock for the cross pieces. The 1x2 perimeter tube will be about 20 linear feet and will weight about 26 pounds (304). If the cross pieces (4) are 1x2, they'll weight about 29 pounds and the plywood will weight about 35 pounds for a total of 90 pounds, not counting welds, fasteners or paint. If you substitute 1" angle instead of 1x2 tube for the cross pieces, you'll half the weight of the cross pieces for total weight of about 75 pounds.

    This is a stout assembly that will easily hold up some fat friends, Fidel the wonder dog and a cooler full of beer too. It can be easily fabricated by anyone with some welding skills.
     
  3. Skua
    Joined: Apr 2013
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    Location: Hunt's Pier WW NJ

    Skua Senior Member

    Hadn"t thought of that. Using any metal had completely slipped my mind.
     
  4. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    You can use wood if you want. Use a 2x2 perimeter frame of oak, with cross pieces of 1x2 SYP or Douglas fir. The weight will be about half that of the steel and though not as strong or dimensionally as stable, still capable of holding the load. I'd increase the cross pieces stiffness by installing "bridging" between them on 22" centers (2 bridges, per cross piece), also of 1x2 and since you'll be working with dimensional wood (.75" x 1.5") double plank the plywood top with two layers of 3/8". The 3/4" thick decking will feel like a concrete sidewalk, trust me. Naturally, for best results and longevity, encapsulate the whole shebang with epoxy and paint as desired, which will add 2 pounds to the job, but will stabilize the wood, sealing it from moisture (a good thing).

    There's several old sayings about fabricating these sort of things. The first is, " if you want it strong, put metal in it", then of course the obligatorily " if it needs to be really strong, through bolt it". Metal is the fast way to get strength and stiffness, if you have some welding skills. Even if you just bolt it together (not nearly as good for high vibration areas), it work. Even just a simple angle stock frame with some cross pieces of the same stuff will be very strong.
     
  5. Skua
    Joined: Apr 2013
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    Location: Hunt's Pier WW NJ

    Skua Senior Member

    I can weld, and have welding equipment. I had thought of going with 2x2 cross pieces, but in testing they didn't have the strength, over a shorter span than was required. Unsure of the species. Currently using 2x8's temporarily to walk on while I work. At 21lbs a board, that adds up fast.


    I have a number of 1x4 oak planks I could rip down to 1x2. Would a single sheet of 3/4 be better, or does the 3/8 x2 offer better stiffness?


    Strangely enough my first real answer, was a honeycomb sandwich laminate, which of course would have been crazy expensive.
     

  6. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Some random thoughts-

    What ever you do, treat the top of the tank if it isn't plastic. Also insulate to the extent possible. Condensation, either dripping on to the top of the tank, or forming inside the tank, is something you want to take action against.

    I'd go with the metal frame if it is an option and you have a poly tank. I'd probably go with wood if it was an aluminum tank. For wood, I'd resaw 4x4 oak to 2 1/8 x 3 or so and build the frame full depth and let the sealed ply panels into it. Add insulation to the ply to flush it with the frames underneath, then drop 1708 over all top and bottom. 54 inches is a fair span to pull off in wood. With 2 end frames and 4 cross frames, you get 5 ply panels 5/8" x 12" x 50" spanning 10" x 48" of space. 125# or so when done. It will hold a school bus, which is my usual safety factor.
     
    Last edited: Nov 18, 2013
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