re-building bow pulpit

Discussion in 'Fiberglass and Composite Boat Building' started by Egg, May 6, 2018.

  1. Egg
    Joined: May 2018
    Posts: 3
    Likes: 0, Points: 1
    Location: Canada

    Egg New Member

    I have a delaminated, fiberglass bow pulpit. The rotten wood core is now fully removed, the putty sanded off. My composite supply sold me some 1208 bi-directional (0-90) stitched cloth and suggested 20 layers of this stuff should suffice, with no more than 5 layers at a time to prevent overheating and shrinkage. No core will be used and the end bolted to the bow will be built up in solid glass. (I am considering some 5/8 marine plywood on the bolted end to save weight and time.

    First five layers are on, it went well (except for the fifth) but I feel this pulpit will be overbuilt. I searched for basic design guidelines but quickly found out this is a can of worms and that I don't have the time to learn about this stuff/and complete the work before next weekend's deadline.

    From the many experienced folk on this forum, do I really need 20 layers? See the pictures for context, the overall measurements are 15X48". The design has a strong structure with 1.5-2" deep sides and center ridge with equal depth. Only the first 10 inches are attached to the bow and the pulpit's main structural purpose is for the anchor and the load of a 200 pound man heaving a stuck anchor from time to time. The anchor line is rated to break at 5500 lbs. but a design load of 1500 lbs at the 30 inch mark should be more than enough.

    What do you think? Thanks for your input. pulpit top.jpg pulpit so far.jpg
    cloth.jpg
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    If you won't use a core, the thickness will have to be built with a lot of solid fiberglass. Otherwise, it will deflect too much.
     
  3. Egg
    Joined: May 2018
    Posts: 3
    Likes: 0, Points: 1
    Location: Canada

    Egg New Member

    Ok. If I used one layer of 5/8 mahogany marine ply I could reduce the fiberglass by how much?

    The proper method for bedding the plywood would be chop strand and then the wood clamped down into the wet bed? Or would a thickened polyester resin (I've got west system silica I could use) be the better way?
     
  4. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    If you want a solid laminate, yeah, 20 sounds about right, but you can decrease this amount with a core, which is why the wood was there initially. I'd use an inert material, such as Cossa board. Yep, it costs a fair bit more than plywood or even solid lumber, but it's not going to rot and bonds extremely well.
     

  5. Egg
    Joined: May 2018
    Posts: 3
    Likes: 0, Points: 1
    Location: Canada

    Egg New Member

    Thanks for your input! I have the marine ply and frankly if I get another 30 years from this job, it will outlast the rest of the boat. My initial plan was to use a light core material such as honeycomb or foam. It was not clear to me how much a full laminate job would weigh. It is heavy! If I were to do this again, or for my next project (likely a custom icebox) I would like to learn about the tensile and compressive properties of FRP because I am sure there is an opportunity to make these parts just as strong and much lighter. Probably not bullet proof like the full laminate though. If anyone has suggestions on a good book to start with, I would be grateful.
     
    Last edited: May 10, 2018
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