What is a doubler?

Discussion in 'Boatbuilding' started by CatBuilder, May 24, 2011.

  1. CatBuilder

    CatBuilder Previous Member

    All over my plans where I use some plywood, I see "doubler." I figured the meaning would eventually become clear, but it hasn't.

    What is a doubler?
  2. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Doubling means fixing side-by-side two identical frames instead of just one, in order to increase the frame stiffness in some critical areas. Or because a single ply sheet cannot arrive to a thickness necessary for a desired frame strength. Some call it sistering too, though the latter should refer to local reinforcement of a damaged frame (someone correct me if I'm wrong), where the sister serves just to transfer the stress around the gap or a fracture of the original frame.

    Doubling is typically used in double-sawn construction, when frames are made by joining multiple elements called futtocks. In that case frame pairs will have joints placed at some offset distance (along the frame) from each other, in order to avoid having weak points close to each other.

    Perhaps PAR might jump in to add a ton of more first-hand info on this subject, in which case I'm looking forward to a good reading. ;)
    Last edited: May 24, 2011
  3. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    Also part of a frame if I remember correctly, eg you may have another bit of plywood screwed to the top of the frame to help land a deck member.
  4. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer

    In modern construction doubler usually refers to skin plating rather than frames.

    Long ago wooden ships often would have the planking "doubled" when the original planks were old or worn.........Today classification rules call for extra skin thickness at point loads and stress risers, places in the structure like a hatch corner or the skeg root.......such loads on the skin can be handled with extra plating thickness or by use of "doublers".....essentially the addition of another layer of skin plating....... Which may be of the same thickness, "double", or more or less.......Sometimes the doubler is inside, like a backing block on a cleat or chain plate.....And sometimes they are outside, such as a doubler plate under a bollard on a steel deck......

    View attachment DesignGuidelinesforDoublerPlateRepairsofShipStructures.pdf

  5. CatBuilder

    CatBuilder Previous Member

    Thanks, guys.

    It's making sense now. Here and there, where an arrow points to something and says, "doubler", it is always some flat surface (gusset, bulkhead, etc...) that I can double up.

    Great input.
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