scarf strength question

Discussion in 'Boatbuilding' started by Charly, Nov 23, 2011.

  1. Charly
    Joined: Dec 2009
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    Charly Senior Member

    This is probably basic stuff, but I have been wondering about this one for a while:

    which orientation is stronger?

    Assume 3/4 x 2-1/2 stock, that is oriented so that the 2-1/2" face is vertical, with a vertical load.

    Now scarf another piece to that one, same dimensions. Say a 10:1 scarf.

    One way would be to cut the scarf plane on the narrow face of the stock. surface area of the scarf would be width 3/4 x run 25 = 18 3/4 sq in.

    The other way would be to cut the scarf plane on the wide face of the stock. surface area= width 2.5 x run 7.5= 18 3/4"

    The same.

    BUT, one is a tension/compression situation, and the other is a shear.

    Which is more suitable for a vertical load path?

    This is an oversimplification, of course, but I am about to build a crossbeam for a Catamaran that will be holding up a mast, so I figure it is worth asking.

    Thanks for all inputs!
  2. lewisboats
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    lewisboats Obsessed Member

    Is the load going to be directly on any part of the scarf or off to either side and by how much? I would say that tension/compression would be stronger just because I can visualize the shear situation being subject to both straight and twisting forces due to the lever arm of the hulls and the downward forces imposed by the mast. Others will be along to tell me I'm full of it :)
  3. Charly
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    Charly Senior Member

    Hi Steve, actually in this case the scarfs will be well off to the side. The beam is about 21 feet long and the stock I am working with is 20, so the scarfs will be about 1 ft from the ends, And each piece is actually two laminated 3/4 stock together... so it is really a theoretical question. But I am still curious...
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Scarfs are best if done in compression or tension. Shear loads invites creep. This assumes adhesives are employed. If strictly a fastened joint, then shear is more desirable, though still less so the compression or tension.
  5. tunnels

    tunnels Previous Member

    i like what you have written !! To learn about scarf joints we need to go back to times before the age of metal fastenings and the like !. We only need to step back a short time and do away with adhesives .
    So how did the ancients hold bits of anything together and against each other without adhesive or fastenings . wow thats a big ask !and toda we run to the computer for answers !!
    I live and walk around in asian countries and look at the old buildings and structures that were built and have stood for hundreds if years simply by using clever joints and wooden pegs and wedges . in Japan during earth quakes some of these buildings are still standing where new modern building have collapsed and crumbled in a million bits !!. where things can move and shift minutely there is give and take so absorbs movment but is done as a whole not just a part of .
    When i built the deck round my house it had no nails to hold any of the frame work . It has been up for 15 years and never moves , there is no creaking or noise , the timbers are straigh and have no twists or warping ,its as stable as a rock and i had lots a people come ,look shake there heads and not believe its all interlocking with rebates for the beams to sit into , all done with a jig clamped to the wood and a router ,chisle and hammer ! was all precut and shaped and paired ,coded, stacked , then errected by just myself and my 12 year old son ,all done in a matter of hours .
    I even had young moden day house builders come ,look ,shake there heads and walk away talking to themselves .
    Some of the face beams were 12 meters long and there were 3 joined together end on end tapped together and self supporting no fastening just well thought out fancy interlocking joints .:):D:p
  6. PAR
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    PAR Yacht Designer/Builder

    There are lots of joint types, in particular I can think of at least a dozen different scarf joints. One of the locking scarfs can work without fasteners, depending on load.

    What works in a house doesn't necessary work on a boat. The dynamic loads on a boat will tear apart most of the common land based variants of the same joint, which typically only experience static loads.
  7. buzzlymutt
    Joined: Jul 2008
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    buzzlymutt Junior Member

    For your beam scarves,if using epoxy glue,shear is best.Read Gougeon Brothers on boat construction,& J Norwood "high speed sailing" for beam strength & design.Good luck
  8. John Riddle
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    John Riddle Junior Member

    Eric Sponberg is good at answering a question like this - maybe he'll chime in but here's my take:

    If you're using a wood species that one would normally expect to find in a catamaran beam, and using appropriate epoxy or other glues, the strength of the bond will exceed the grain strength of the wood.

    I can't think of a situation where a 10:1 scarf joint near the end of a catamaran beam (or anywhere else in the beam for that matter) would see true tensile loads at all. Please correct me if I'm wrong. It seems to me, sheer and peel are the likely failure modes, and a well made, epoxy-joined scarf will not fail before the wood grain does. And the wood won't fail if the scantlings are appropriate.

    In the end, it doesn't make a difference whether you scarf the face, the edge or on a diagonal across both.
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  9. DCockey
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    DCockey Senior Member

    If a member has a scarf joint and the member is in pure tension, the stress across the scarf joint will be primarily shear due to the low angle of the scarf joint.
  10. Ad Hoc
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    Ad Hoc Naval Architect

    For shear, if you look at the location of the applied load, if the joint where a simple straight butt, the load must be resisted by the shear area of the adhesive alone. The assumption is that the shear strength of the adhesive is less than that of the material being used.

    fig.1 shear.jpg

    The same is also true when the applied load is tensile or compressive. The strength of the joint replies totally upon the tensile strength of the adhesive with the given available area, in a butt joint, is also usually insufficient.

    fig.2 tensile.jpg

    So does the method of the joint, i.e. its angle or orientation, become a factor? Well, if, in the case of tension, the joints strength of the joint relies totally upon the strength of the adhesive, not the material, and the same is true for the shear.

    So, the way to ‘design’ the joint, is to ensure that the amount of “area” available, of the adhesive is greater than or equal to the strength of the material being used. If you used an angle, for the scarf of say 30 degrees, the load on the joint, has an area of twice that of the material (sin 30 = 0.5). So is the area of that joint, with the properties of the adhesive equal or greater than the material? Probably not. So, you keep decreasing the angle until the amount of area and hence available strength of the joint is equal or greater than the material. Whether you’re looking at shear or tensile/compression strength.

    fig.3 load.jpg

    Now back to the scarfed joint. With an angle, or scarfed joint, when a shear load is applied it shall try to “slide”, owing to the composite nature of the joint. In which case, if you resolve the forces this shall give rise to tensile load (horizontally) as well as shear load (vertically). But, the joint relies upon the adhesive, therefore you must resolve the forces parallel to the plane, or angle of the joint, along the axis of the adhesive. Since if the joint was made with grease, the two faces would slide apart, which means the two surfaces are shearing apart.

    fig.4 forces.jpg

    So, you need to work out the minimum amount of area required, to transfer shear, at the same or greater strength than the parent material. This shall then dictate the minimum angle required.

    If you now apply the load as compression, the same shall occur, the joint shall attempt to slide apart, thus same result as for shear. If however you apply the load in tension, it is technically a pure tensile load, and which case the limiting factor is the tensile area. However, since the joint shall not be perfect and the applied load shall in some way be slightly eccentric owing to the geometry, thus shall give rise to both tensile and shear forces. If in doubt which has the lowest strength, if it is say shear, design the “tensile” load case using the shear strength.

    Also, if the scarf is a shallow angle, and if an applied load within the limits of the joint (point load), say shear, this shall give rise to bending, hence both compressive and tensile stresses in the joint. Thus you need to check the minimum amount of area against these two other properties, of the adhesive. That shall then provide you with a satisfactory joint.

    As a guide a nominal 5 degrees is considered acceptable.
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    Either way the scarf is cut the area is the same, but given the same angle the length is longer in one case; that will be the strongest for all load conditions except tension which is the same for both, but will waste more material.

  12. rxcomposite
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    rxcomposite Senior Member

    Here is a typical bolted scarf joint from Classic Boat Construction. It is also a direct copy from LR rules circa 1979 as it appears in the book. The l/m ratio is to be greater than 6. If you have the depth of the crossmember, I will look up the diameter of the bolt in the table.

    It is interesting to note that the author has some reservations on using epoxy glue due to HDT temperature which was discussed recently in this forum (black painted fiberglass. It applies to wood as well.). The author prefers Resorcinol glue which is an LR standard.

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