Built up bow question

Discussion in 'Fiberglass and Composite Boat Building' started by Scuff, Nov 19, 2021.

  1. Scuff
    Joined: Nov 2016
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    Scuff Senior Member

    I'm building the main hull of a trimaran using foam over a male mold. The instructions call for the bow to be finished after the hull is laminated. Initially there's just a vertical layer of hull foam which is laminated with the rest of the outside. The bow is then built up with layers of foam and laminated. Does this two step lamination approach add additional stiffness to carry the forestay loads into the rest of the hull? I'm thinking it does add to the stiffness of the stem as you have two laminates sandwiched as opposed to just one if the bow was completed in one go. There is a keel doubler that is added between the front bulkhead that ties into the stem. Any insight appreciated.
    Thanks.
     
  2. bajansailor
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    bajansailor Marine Surveyor

    How will the forestay chainplate fitting be attached? It should ideally be secured through an area of single skin fibreglass, not foam sandwich.

    Can you post a photo of the bow, and / or a copy of the relevant section of the construction plan please, to help illustrate what you intend to do?
     
  3. Rumars
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    Rumars Senior Member

    The build up bow is a common procedure. I don't really understand your description, usually the hull terminates with a tiny bulkhead, and after lamination a block of foam is glued to its face, and the shape carved into it.
    The forestay chainplate is attached to said bulkhead. Where does your chainplate attach?
    A picture of the area in question will clarify things.
     
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  4. Scuff
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    Scuff Senior Member

    Rumars,

    It's as you describe it a small bulkhead .. the bulkhead area for the chain plate is a 3/8" solid laminate with a 3/8 high density layer on top of that. When the actual bow is formed he calls for a 3/8" solid laminate formed around a high density insert in the chain plate area. I duplicated this near the waterline for a bow eye. What I'm wondering though is does the 2 step process aid in transferring the forestay loads into the rest of the boat? It would be easier to do it in one step I think. I'll get some pictures.
     
  5. Scuff
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    Scuff Senior Member

    PXL_20211119_185340241.jpg PXL_20211119_185817540.jpg Pictures .. from the construction manual and then one of the insert for the chain plate.
     
    Last edited: Nov 19, 2021
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  6. Scuff
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    Scuff Senior Member

    One of the stem fitting PXL_20211119_201112489.jpg
     
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  7. Rumars
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    Rumars Senior Member

    Ok, it's very old school with an external chainplate. He wants you to laminate a ply block, glue it 90° to the small bulkhead to act as a spacer for the bolts.

    Today we have better options. What resin are you using, polyester or epoxy?
    If epoxy, get rid of the piece of wood and replace it with foam. Take a piece of G10 plate, cut the stem profile in it stem plate included and heavily tab it to the glassed bulkhead (you may need to slot the bulkhead, it's not clear to me how far back it extends). Over the G10 build up a composite chainplate with the appropiate angle using uni and biax fiberglass. Nothing to leak and bombproof.

    With poly it's a little bit more complicated, you have to first finish the entire bow, then tab the chainplate in with epoxy.
    Even if you you wish to stay with the metal chainplate and bolts I would replace the wood and ply with solid fiberglass and coosa.
     
  8. fallguy
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    fallguy Senior Member

    Get rid of the wood!!!

    Listen to Rumars.

    I have one place I used plywood as a locker top in my boat when I ran short of coosa. The locker filled with water and the bottom was not sealed well enough. It puffed and broke some of my laminate.

    You must have some coosa planned..
     
  9. Scuff
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    Scuff Senior Member

    I'm using pro set epoxy. The bulkhead is right against the framing so I wouldn't be able to slot it until I get it off the mold. To use the g10 it would need to be the width of the chain plate which is 1 1/4"? To also use it for the stem plate wouldn't it need to be narrower for the tack attachment?
    I would like to do the composite chain plate .. where would I find information on doing that?
    Fallguy, sadly I am having to wait for coosa .. evidently there's a shortage. I'm going to order some but have to wait until early December to order expected delivery sometime in January. The wood for the bulkhead (in the picture) is black locust it's very dense, rot resistant and takes glue very well.
     
  10. fallguy
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    fallguy Senior Member

    As long as you overbore and refill all the wood penetrations with thixo; it'll be okay. The rot resistance is not really the issue. I used marine ply. The issue is any water hits the wood or migrates through sealant and the wood will puff up. So, using wood makes hole cutting a two day affair is all.

    Spot the device and cut say 1/4" holes, then overbore to say 1/2" or 5/8" and make a batch of epoxy and cabosil and then fill it and tape or make sure it doesn't sag or shrink too much or it becomes a three day job. As of this moment, I have a few of these waiting for a lil more epoxy..but it got too cold here this last week.
     
  11. fallguy
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    fallguy Senior Member

    Also, black locust and coosa are not really comparable here. Black locust crush strength is 73MPa and coosa is say under 6 MPa. So the locust is 12 times less likely to crush. Just seemed important to point out in case you get some coosa in the future...

    and avoid breathing or touching the locust as you can develop a sensitivity to it
     
  12. Scuff
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    Scuff Senior Member

    Is there any reason that I can't integrate the forestay attaching hole into the g10 plate? He has another approach to the forestay when building with wood that actually may be a better way using the g10 plate I'll find that and post it.
     
  13. Scuff
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    Scuff Senior Member

    See below. It looks like I could easily use this approach with just the g10 plate? If I still cut the slit in the bow it would allow tabbing the plate to both sides of the bulkhead. Extend aft a bit and the deck would also retain it.
    PXL_20211121_145924305.jpg PXL_20211121_145932698.jpg
     
  14. fallguy
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    fallguy Senior Member

    You mean to replace 'solid' with G10?

    Sure.

    This is 413 MPa versus locust st 73 versus coosa at 6 being nice. Just pay attention to load paths. I can't help much since I am no sailor. I have a solid understanding of materials. Using G10 for say a cleat backing plate might be overkill versus a chunk of aluminum and thickened epoxy fill..but there might be high stress areas where it is smart...

    See what the sailors say. The G10 won't soak water n puff and looks like 5x less likely to crush than locust.

    Jamestown Distributors https://www.jamestowndistributors.com/product/product-detail/8758
     

  15. Scuff
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    Scuff Senior Member

    I'm talking about using g10 for the entire stem fitting .. the forestay attachment included. So in the last 2 pictures the g10 would replace the stainless stem fitting as well as the plywood built up on the inside of the bow that the chain plate bolts to.
     
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