Wooden Dowel Material????

Discussion in 'Materials' started by Phil Westendorf, Jan 8, 2014.

  1. Phil Westendorf
    Joined: Jun 2012
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    Phil Westendorf Junior Member

    I am currently building a 16' Mckenzie River Drift Boat (my first framed boat) and the hull is complete. I am starting to build the seats and other interior features. To date I have not used any screws or metal in the boat's hull, all epoxy, glass, etc.
    Frame members are 3/4" White Ash, Sides 1/4" WRC and bottom (two layers of 1/4" x 2 1/4" wide strips epoxied together), chine logs, sheer rails etc are all W Ash.
    Seat frames and supports are Blk Walnut and or W Ash. I am considering of using wood dowels rather than Sil-Brz Carriage Bolts to secure the seat frame supports to the Hull's Frame Members (4 places, Frame No.'s 3 & 8, Port & S'Bd.). The supports hold a pair of 3/4" Sch. 40 Galv. parallel mounted pipes that support the Fore-Aft. Adjustable seats (rower and guest seats)
    .
    I saw a video (online) where a large wooden boat's Garboard Strake was fastened to the keel using a series of wooden dowels with tapered wedges driven in the dowels from both sides, then trimmed flush.
    I'd like to try and eliminate the carriage bolts with something similar.

    My question is; The only non exotic wood, I found, that has a higher "modulus of rupture" (bending strength) than Ash (15,000 psi) is Hickory (20,200 psi). Is there any benefit in using Hickory as opposed to Ash for the pins ( approx. 1/2" dia)? I realize stronger is usually better but I can't see where a higher rating would add a heck of a lot more benefit.
    The framing and seat supports are 3/4" thick. I haven't done any calculations for the static or dynamic loading the pins would be exposed to. Static is easy but dynamic is something else. My weight is 120+ KG.

    Any assistance is appreciated.
    phil w.
     
  2. jehardiman
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    jehardiman Senior Member

    Normally trunnels (or "tre(e)nails") can be of any hardwood with long, straight, vertical grain. Elm and black locust were the woods of choice due to wet rot resistance.

    Or, because the smallest effective size of a trunnel is ~3/8 dia, you could use a copper "bolt" in the original sense of the word. This is a copper rod, drifted through a hole slightly smaller in diameter, then cut off just proud and peened over a tight fitting copper washer. Not as much damage to the plank and can always be brought up if needed.

    You may want to also go ask this on the WoodenBoat forum. I know it has come up before over there.

    http://forum.woodenboat.com/showthread.php?70709-Trunnel-Fastening
     
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  3. rasorinc
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    rasorinc Senior Member

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  4. Phil Westendorf
    Joined: Jun 2012
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    Phil Westendorf Junior Member

    Gents,
    Thanks for your replys. Going to start looking for Black Locust. I have several good wood sources here in Michigan.
    Happy New Year,
    phil w
     
  5. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    Work of fracture is the force required to break a material in two. In the case of bolt, it is the tensile strength because it is tightened but when the members that is being held together is thick, like wood, the dominant stress is shear. You should be looking at shear properties. Wood dowels are held by friction along the sides. There is no compressive or tensile strength as there are no means to tighten it.

    There were some discussions regarding the building of Tenacious, a wooden tall ship built with several layers of laminated wood and held together by glue and wooden dowels. It was to be certified by Lloyd's and the early discussion centered around the use of composite pins and wooden dowels. LR disapproved the use of stronger composite pins but approved the use of wood dowels. Maybe you can search the article on Tenacious.
     
  6. SamSam
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    SamSam Senior Member

    As an aside, if you do use 'wooden dowels with tapered wedges driven in', orient the wedges to apply force with the grain instead of across it, to minimize splitting of the wood being fastened. That might entail wedges at 90 degree angles to each other, as in fastening a plank to a frame.
     
  7. Petros
    Joined: Oct 2007
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    Petros Senior Member

    I have built a number of skin on frame hulls with wooden dowels as the fasteners. there are several things to consider: as pointed out, the wedge can actualy cause the plank you want to hold in place to split. They have little to no tensile capacity, and if the joint is designed correctly I do not think there will be very large loads on the pin itself.

    For example, if you join the two pieces of wood together with corresponding notches to take the primary loads, the dowel pin will only be used to hold the pieces together and not see significant loads.

    Even if designed to only take shear loads, the adjoining lumber has to be sized such that shear failure of the adjoining grain in plank you want to hold in place does not occur. So if you make the dowel larger that means there will be that much less wood in the plank to carry the loads.

    That is why if possible, just use the dowels to hold the parts together rather than take the loads in the load path. Much less risky. I have used both wedges and glue to hold them in place, the wedges left voids that could trap water and there was high tendency to want to split. I like using glue on them, faster, easier, stronger joint, less "redo" because of broken parts.
     

  8. Phil Westendorf
    Joined: Jun 2012
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    Phil Westendorf Junior Member

    Wooden Dowels

    Petros,

    I understand the concept of using the fasteners to maintain position of components and not be the load bearing member. It's a common principal in machine design.
    This had not come to mind as I am developing the features in the boat I am building. I'll have to do something to incorporate this into the support's design.

    I have used # 8 x 1 1/2 Lg. S.S. screws to secure the chines and sheer rails to the hull as the epoxy cures and then removed them.
    After removal of the screws I then drilled the screw holes, then with a tapered drill added a taper for the first 1/2" and then assembled wooden dowels with tapered wedges in the holes. I used mixed epoxy to hold them in place. The surfaces cleaned up nice.

    Thanks for your input, it's appreciated.

    phil w
     
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