Mashup Trimaran (continued)

Discussion in 'Multihulls' started by Bigfork, Feb 17, 2021.

  1. trip the light fandango
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    trip the light fandango Senior Member

    Shear, in terms of local catastrophic failure of a material arranged in a sheet form like a cut as opposed to say a delamination.
    I post to try and help the OP, it's a pity I'm getting up your nose.
    I just looked up sheer, curvature of a deck, I didn't know that, which I assume added a little pepper, sorry.

    I don't think discussing what I think bulkheads do is of much value other than what I've already explained . I don't think your interest[in what I know about bulkheads] has much to do with helping the OP
     
  2. trip the light fandango
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    trip the light fandango Senior Member

    Thanks Bluebell, I realise that a lack of naval architecture terminology could annoy someone well versed in it. As you know Big forks project has similarities to mine so I think I can add some useful clues or opinion,. for no other reason than it's fun to mess about with boats .... ha, regards.
     
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  3. gonzo
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    gonzo Senior Member

    You are describing a tear and not a shear failure. Shear failure is how a bolted connection fails. For example, two plates are bolted together, and the bolts fails in shear which looks like it was cut at the plane where both plate touch at.
     
  4. upchurchmr
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    upchurchmr Senior Member

    Feathering is a valuable technique to save weight.
    Putting down the shortest reinforcement ply first will increase the load transfer capability.

    Blue Bell,

    Can you explain about his "shear" statement? Since "trip" can't or won't.
     
  5. upchurchmr
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    upchurchmr Senior Member

    Trip,

    If you don't know what you are saying then volunteering an opinion to BigFork can be more hurt than help.

    From your definition of shear you don't know where to put the material to help avoid shear failures. If you don't know what a bulkhead is doing then you also don't know how to avoid failures and you run the risk of adding material which will do nothing to help.

    By the way, have you ever heard of interlaminar shear?
     
  6. redreuben
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    redreuben redreuben

    Shear, put your hands together like when you talk to your imaginary friend, now slide them in opposite directions. That is shear. Common cause of core failure.
     
  7. trip the light fandango
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    trip the light fandango Senior Member

    Right ,thank you.
    Thank you , yes I didn't quite understand the terminology
     
  8. trip the light fandango
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    trip the light fandango Senior Member

    There are 2 ways a bulkhead in this structure can be not ideal, the first one is ,as stated creating a hard point in a shape that can flex a little, a bending failure. The second way is with a grounding where the submerged item drags along the hull while the hull flexs a little in that area, until it meets the hard point , the bulkhead, it catches, either bumps over it or makes a hole.

    I knew shear as in a bolt shearing , I didn't realise it couldn't be used to describe where a weak point splits .
     
  9. trip the light fandango
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    trip the light fandango Senior Member

    Big fork, I was describing a bending failure, a potential split most likely across the main hull either side of the bulkhead. This could also create a shear as in a delamination between the inner and outer skin of the foam sandwich, but that wasn't thinking that initially. I'll check my hulls again to see how far the sandwich extends into the bilge but I think it is the whole hull. Your bulkhead secures to the inner skin which is deliberately designed to be lighter and weaker.
    I can't see how it is going to add much value, particularly the sizes you mention. For the umpteenth time, I'm a backyarder novice...who shares an interest in this.
     
  10. trip the light fandango
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    trip the light fandango Senior Member

    The way I see it ,using epoxy and wood then glassing over it is a misunderstanding of what is required, adds unnecessary complexity and creates problems.
    The timber could be used as forms if you're really happy with the shape, swapping them for foam may not even be required at those thicknesses, just an extra layer or 2 of glass. Epoxy is chemically pretty toxic and only needed because of the timber.
    As mentioned earlier I think a bulkhead creates more problems than it solves and isn't required. Some extra glassing and mechanical bonding[ 60 or 80 grit ] for strength will be better than the bulkhead. Just use polyester or if you want vinylester. I'm only going to use epoxy where strength is really important from now on.
     
  11. upchurchmr
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    upchurchmr Senior Member

    There is too much opinion and very few facts from "Trip over his feet".
    Nothing anyone can do to help with this mess of unsupported statements.

    Big fork. Good luck. PM me if you want to discuss.

    Marc
     
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  12. trip the light fandango
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    trip the light fandango Senior Member

    Bigfork ,you could use the sandwich lamination that you cut out of the deck as a bulkhead/ waterproof box, just make sure to lay up a decent sized pad first to avoid a stress point, and using epoxy, too late I suppose.
     
    Last edited: Mar 3, 2021
  13. peterAustralia
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    peterAustralia Senior Member

    Not sue where your up to, some light pointers. The weight of your parts, support the part three places. then use a bathroom scale, record weights and add together. As for calculating waterline, you can use graph paper, sketch out sections as best you can using an a best guess waterline, do calcs on that waterline. and from there you may need to lower or raise, should only take 25 minutes. You make sections every 18 inches or so, work out cross section area using graph paper, and then work out volume in each division, not as good as software but gives a quite accurate estimate of where your waterline will be. As to crossbeams, suggest timber at the top, say 1 inch x 5 inch and timber at bottom, grain running length of crossbeam, then plywood fore are aft, then glass, the calcs for crossbeams are not super complicated, I can give a rough go at it... might be rough but better than nothing
     
  14. peterAustralia
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    peterAustralia Senior Member

    aside, what does up church mr refer to?
     

  15. Bigfork
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    Bigfork Junior Member

    Thanks for all the thoughts folks!
    I see what people are saying about the 'hard' point or stiffened location where the bulkheads and storage locations have been fillitted/tabbed into sides of hulls. Before, the hull could flex and absorb vibrations and now, I've made a series of hard points that may induce cracking of laminate, de lamination of inner skin-foam-out skin relationship...As you all are likely aware, the skeg or bottom most surface of a beachcat is solid glass; the foam runs down the sides stopping 3-4" before the bilge. And yes, the inner glass layer is thinner than the gelcoat-ed thicker outer layer. Right now, my bulkheads are mostly tabbed in with one layer of 4" wide glass (2" on each side of the joint). I could feather wider strips of glass in to soften the "hard spot" (??). You can see pictures of the single layer tabbing. I also have generous bonding on the top surface (outerskin) of the hull. There's also a photo of the H18 main hull with the box beam "mount" which is braced back to the topside gunnel. I think that adds some (?) structural integrity.

    inked H18 braces.jpg


    The other 3 pictures are the rear box beam mount surface on the aft end of the Solcat floats. One is the inside view of the bulkhds, the other is the bulkhds poking up through the deck surface, and the lastly, finished up and ready for beam (with
    potential bracing drawn in)
    potential braces on rear float mount.jpg
    IMG_3842.JPG IMG_3871.JPG


    PeterAustrailia, I savvy what your saying about beam size but your suggestion doesn't mention height of box.
    I also don't quite get how to weigh the hull using a bathroom scale. I've seen physics drawings explaining how to do this, but looks beyond my comprehendo :) Well after some more reading, it seems really easy! Simply weigh both ends and add the number...?? seems too easy.
     
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