best way to stiffen a bridgedeck?

Discussion in 'Multihulls' started by Charlyipad, Apr 22, 2021.

  1. Charlyipad
    Joined: May 2014
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    Charlyipad Senior Member

    Hello everybody! Looking for ideas here. I have a homebuilt 36ft cat. Bridge deck constructed as follows, using epoxy resin: 4mm oakume, 3/4inch balsa, 4mm oakume both sides encased in 10 oz cloth. It is a little too floppy to suit me. I think I would like to glass a hat section or an el bracket or something similar fore and aft under there to make the thing stiffer. I was thinking of laying up an el bracket from carbon cloth; maybe about four inches deep? Or would a half rounded section be better? Maybe layup a piece over a pic pipe as a mold? which shape is the most efficient? Or would it be easier to just through bolt an el section of aluminum? I have done about all I can do above decks to integrate benches, steps etc to the deck but I still have a lot of open "real estate" up there. Every time some sasquatch steps aboard and makes me spill my beer I swear I am going to do something about it. :) Thanks for any advice!
     
  2. fallguy
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    fallguy Senior Member

    The glass on the bottom should have been more than 10 oz woven. And a direction change of the fabric, like biax, would have helped.

    can you take the deck off or is it repair upside down only?

    Top hats really benefit from vertical sides. Rounds and angles are less effective.

    If you are repairing in place, use some plastic Downspouts as moulds and make the tophats on a table with plastic. Make a flange about an inch wide each side and bond the thing(s) to the deck bottom with thickened epoxy. If you have to screw it uo there, be real careful to repair the screw holes well.
     
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  3. fallguy
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    fallguy Senior Member

    My deck panels are inch plascore with 44 oz triax, span is 53". The plascore is far flimsier than you okume/balsa core, but the 10 oz is way too light to span much, imo.

    Rectangular tubing/shape is the way to go.
     
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  4. guzzis3
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    guzzis3 Senior Member

    A lot of the designs I am familiar with use L sections glued under the bridgedeck. You only need brace it in one direction. The hoirizontal flange is just a gluing surface some anything will do, 45/45 would be a good choice or alternate layers of 45/45 and 0/90. The vertical flange, the face that sticks down, is the one that does the work. This one needs uniaxial running along the length. I'd go for a thickness of 3mm (1/8") and made it 50mm (2"). Space them maybe 12" apart and stop/start around under deck protrusions (like nacell or whatever) as required. Pity you put so much money and work into your platform. You can get away with 12mm (1/2") foam with say 600 gsm 0/90 ot 45/45 both sides with teh bracing I describe above. Or you could have made glass skins either side if C sections made as above.

    I didn't know people were still using balse to build boats. Sigh...
     
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  5. fallguy
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    fallguy Senior Member

    I agree with much of what you said, but a square tube or something with two verticals and a flat is a bit better than L everytime.

    You can compare all of these and per pound easily.

    The issue with the L is the vertical twists easily.
     
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  6. waikikin
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    waikikin Senior Member

    Best off with a trapezoidal top hat section. Ls and Ts aren't self bracing under load and would need quite heavy tabbing each way to do so. You need section depth for stiffness, what that needs to be depends on the span... at least 90mm or more with some uniglass in the cap. If it's straight or you can loft the section and mold them off the boat then glue on.
    Jeff
     
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  7. Charlyipad
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    Charlyipad Senior Member

    I have been thinking along those lines. The span is around 16 feet fore and aft. Now to fu=rther complicate things... what if I wanted to taper the stiffeners (the top edge, that gets glued to the underside of the deck is straight. The bridge deck has camber but the fore and aft dimension of the bridge deck is straight) so, By tapering the stiffeners, I figured I could save weight by having the deepest part of the stiffener at the point of max deflection (ie, the middle) Now the question: Which form of taper is best to prevent deflection from a load above it? IOW, should the stiffener section be fatter, in the middle, tapering in depth towards the fore and aft ends? Or should the section itself be like an arch, with the narrowest point in the middle, and the deepest parts at the ends?
     
  8. waikikin
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    waikikin Senior Member

    Deeper in the middle..
    Lucky that it's straight. If I was doing this, I'd build a simple mold out of melamine sheet, like a small long trough, the taper at the forward end just sniped up at 2:1 or 3:1 with the end landing well past whatever transverse connective bulkhead is there, then the aft end more abrupt say 45 or 60/70 degrees & at the bulkhead connective, the draw angle on the sides might be 10 or 15 degrees. How much glass... who knows- the tech heads will:). You want at least 3mm in the walls and try and make sure there's some 45/45 in the walls, you need uni fibre at least in the cap and some wouldn't go astray in the "feet" that glue on. I'd go at least 5"/125mm deep & 2"/50mm on the cap... maybe a touch wider, maybe just put one down the centreline first and see how you & it feels, then if it needs more choose the centres of 400 to 600mm depending on the underwing width, this could possibly be a bit more at the edges depending how the underwing interfaces with the hulls.
    What does the designer reckon? 3/4 balsa isnt that thick for a big span like that, 40mm seems more aligned with work Ive done on cats and 16' a lot further.
    Jeff.
    PS:.. You wont end up saving very much weight with a continuous taper.. just complicate the construction for the sake of a can of beer or two each stringer, not worth it;)
     
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  9. fallguy
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    fallguy Senior Member

    No.

    I'd say you lose too much with the taper.

    Consider the extreme.

    If you only put a section of top hat across the four feet in the middle and have 6 feet unsupported each side, the top hat would stiffen, but the ends would sag and they would sag at the same amount as before.

    So, personally, I'd make them a uniform beam. Otherwise, at the very initial ends; your top hat is doing less than it can.
     
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  10. guzzis3
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    guzzis3 Senior Member

    Um no. If your bridgedeck is sagging enough to cause the L to buckle it is hopelessly underdesigned. You have built a trampoline. Given the origional description it should be reasonable stiff as is. With inverted L stiffeners as I described it should be rock solid.

    I'm about to start a catamaran. The bridgedeck is 4.5m long. The front center has a cabin to help stiffen it but the cockpit is 2m long. It's 12mm foam with 600gsm skins and stiffeners. Smaller than the one under discussion but lighter layup also.

    Anyway do as you please. Good luck.
     
  11. waikikin
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    waikikin Senior Member

    ... If you had something like these bench seat but continuous they would act like stringers but be very inconvenient, .. If there was some stringers underneath aligned with the longitudinal box sides it might not flex so much and be as stiff as might have been expected. The stringers underneath might just overlap by a good margin or taper towards the front..?
    Jeff.
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    [​IMG]
     

  12. Charlyipad
    Joined: May 2014
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    Charlyipad Senior Member

    Hi Jeff, Exactly. The original design has the seats extended the full length of the bridge deck. I cut them short as in the picture for that exact reason, and the boat is much more comfortable as a result. There is a full length rounded, deep hat section made of cedar strip running down the middle at the joint of the two panels. To make up for the shortening of the bench seats I glassed a small hat section fore and aft underneath in the middle. They are small, and a simple layup over a piece of 1 inch pic pipe. They helped, but are inadequate. The plan now, is to go full length with a 3-4inch deep hat section layup of uni and biax carbon, glassed midpoint between the centerline and the hulls. Probably will keep it simple (no taper) unless I can be convinced of any real savings. I haven't tried caulking the joints around the base of the seats, but am thinking about that. They are fastened to the deck with 3/8 nylon bolts. Maybe I will beef it up with ss bolts and a larger backing plate along with washers .The real weak spot is the base of the side steps (barely visible) where boarding passengers often don't have as ginger of a step as they should.
     
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