Which composite material?...pics

Discussion in 'Boat Design' started by jahmes143, Dec 14, 2018.

  1. jahmes143
    Joined: Dec 2018
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    Location: Florida

    jahmes143 Junior Member

    Boat is a 1995 Chapperal Sunesta 250, 25' cat-style hull deck boat that runs 45mph.

    I want to re-do all the stringers (1 per cat), bulkheads (6 of them in the in each cat hull) and deck supports (in the center of the boat). Originally all material was 1/2" to 3/4" ply with what looks to be a single layer of glass (maybe 2 on the stringers).

    The stringers - about 6" high made of 3/4" now-rotten wood

    Bulkheads - join the outside of the cat to the inside of the cat, but do not make contact with the deck

    Deck supports - 3/4" ply with another 3/4" strip on the inside and outside at the top 2", thus creating about a 2.5" wide footprint for the deck. Deck was affixed to deck supports with a ~3/4" thick hard caulking (plaster?) type material which easily separated while lifting the top cap off the hull.

    Local FGCI store has 3/4" 5lb honeycomb for $70/sheet, but is the orientation of the honeycomb 90 degrees wrong for this application? Would a closed cell foam of a certain density work?

    I'd like to experiment with composite materials but need help selecting the proper types and densities for the various applications. Any help would be greatly appreciated.

    Likely using epoxy resin sourced from USComposites.

    01.JPG 1.JPG
     
  2. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    Perhaps the best composite with Resin is ....Wood.

    If the stringers and b'heads aren't rotten after being surrounded by presumably Poly Resin, why would you want to use other cores ?

    Encapsulated ply is hard to beat in this situation from what I can see.
     
  3. jahmes143
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    jahmes143 Junior Member

    Agreed wood is an excellent choice. However I’m looking to gain experience with other material types, regardless of weight or money savings.
     
  4. ondarvr
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    ondarvr Senior Member

    Just about any material can be used as the core of a stringer, you just adjust the laminate thickness according to the physical properties of the core you choose.

    I like hollow stringers with plenty of limber holes.

    Honeycomb doesn’t make a good core if your standing it on edge and using it for a stringer, but it can use as a rot resistant core for the stinger, it just won’t add as much strength.

    In a typical rebuild on an old glass boat I don’t normally recommend epoxy, it’s a better product, but its better properties aren’t really used/needed in these older boats.

    But if someone wants to experiment with different materials as part of the enjoyment of the project, then there’s no limits on what can be used in the rebuild.
     
    Last edited: Dec 15, 2018
  5. jahmes143
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    jahmes143 Junior Member

    Ahhh “hollow” stringers using the core simply as a form material. I like that idea. Absent math formulas (I don’t have the know-how there), does 2 layers of 1708 over a cheap stringer core of, say PU foam, sound suitable for this application? 3 layers for extra safety margin?

    What about bulkheads…could they be “hollow” as well, were a PU foam is simply used as a form? 2 layers of 1708? Would wrinkling, buckling, or other failures be a concern? I’m generally familiar with fiberglass used in a traditional sandwiched core such as decking, but standing it on end such as bulkheads, I’m not familiar with its performance.
     
  6. SamSam
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    Location: Coastal Georgia

    SamSam Senior Member

    No comment on materials, just a reminder to make sure you don't lose the shape of the hull or the cap.
     
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  7. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Pay attention to weights in and out.

    Avoid overbuilding.

    Coosa and corelite are pretty spendy.

    If you use epoxy and fully encapsulate it; plywood won't rot. Ingress at the top is the only real worry and you can cap it all.
     
  8. ondarvr
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    ondarvr Senior Member

    When I say hollow, I mean hollow. The stringer shapes are pre-made over a 2”x12” or something. This is just a thin skin, one or maybe two layers of glass. The size and shape of the stringer determines how thick it needs to be for handling.

    This thin skinned stringer shape is trimmed to fit in the correct place and then glassed over.

    With no core you can typically go with one more layer of glass.

    Most old boats were over built, the wood cores would turn to pulp and the owner wouldn’t notice it until they wanted to do a little work on the boat.
     
  9. ondarvr
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    ondarvr Senior Member

    For rot prevention in an application like this the type of resin doesn’t make much of a difference, the wood doesn’t care which resin is used if someone drills an unsealed hole or doesn't cover it correctly with glass. Attention to detail is what will prevent rot.

    People that use epoxy tend to care more about what they’re doing and research it better, so they use better techniques.

    New construction on these boats was about as sloppy and cheap as possible. Polyester resin was used and it took the blame for rotten wood when the real cause was a total lack of concern, training and knowledge on the part of the workers building the boat.

    These boats would have rotted away in the same amount of time even if they used the best epoxy available.
     
    Last edited: Dec 15, 2018
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  10. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    Amen!
    Hollow stringers with generous limber holes are forever.
    For a mold, I use a doubled 2x12 to get a 3/4” radius both sides and 1 1/2” flat on top, a very solid rest for the floor.
    Cover it carefully with packing wrap, and lay it up.
     
  11. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    I took apart a 1974 Starfire with solid mahogany stringers. Zero rot; despite atrocious end preservation and poly and

    the balsa core was rotted thru n thru

    Made me a bit sad when it landfilled.
     
  12. jahmes143
    Joined: Dec 2018
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    jahmes143 Junior Member

    Thanks for all the guidance, folks.

    Understood. Took plenty of measurements and made a few jigs to maintain shape of both.

    Help me out with bulkhead/deck support members. I've used 20# coosa for these applications before. But spendy and heavy. Would a light density non-structural closed-cell foam be effective in this rebuild application, with 2 layers of 1708?
     
  13. fallguy
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    fallguy Senior Member

    Non-structural?

    No.

    5# marine foam-yes
     
  14. jahmes143
    Joined: Dec 2018
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    jahmes143 Junior Member

    Thanks!

    So to be clear, polyurethane is a no-go? I think it has poor friability (surface crumbles/degrades when rubbed and causes a delam of sorts)?

    Does thickness matter in my application? 5# 1/4" Divinycell is much cheaper than 1/2".

    Or is the foam simply acting as a form? In other words, could I put release wax over a cut-to-size MDF bulkhead, layup a few layers of 1708, then pop the MDF off the fiberglass and have nothing but a solid 3 or 4 layer fiberglass bulkhead?
     

  15. fallguy
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    fallguy Senior Member

    See embedded replies.

    To the last question. Yes; the only question is how strong or how many layers. I lack the credential to advise the final number of layers. But three layers of 1708 makes a nice part that holds its shape pretty well for a foot or two of height. You might need to tab in something to keep the parts from collapsing on on the bottom for stringer.. someone here can probably advise.
     
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