Results 1 to 19 of 19

Thread: Scarph vs motive and tenon

  1. #1
    Join Date
    Oct 2017
    Location
    Charleston, SC USA
    Posts
    81

    Default Scarph vs motive and tenon

    I have a festool domino tool. The tool was designed for cabinet and furniture joinery. It drills rectangular holes into which one side is inserted a peg. Walaaa! Instant mortise and tenon joint with great precision in minutes.

    I occurred to me to wonder what the strength of this joint would be in a boat building application, butt joint with pegs (mortise and tenon ) with epoxy vs a standard 8 or 10:1 scarph joint.

    A major difference between these two uses of the tool is that furniture is not exposed to the vibration a boat experiences. The pegs may create high load points vs spreading the load across the joint since the joined wood would be stressed with a hard point where the pegs are located following fair curves and flexing a boat must endure vs static 90 deg angles on furniture.

    Has anybody tried this or have the materials/engineering knowledge to comment on whether this is a good or a bad idea?

  2. #2
    Join Date
    Oct 2008
    Location
    Walney, near Cumbria UK
    Posts
    41,466

    Default Re: Scarph vs motive and tenon

    A glued scarf transfers all of the stress through all of the thickness and width as a shear force with some tension.
    A mortice and tenon transfers the stress as shear through half of the width at about i/3 of the beams depth. the rest is end grain tension (glue failure). So no. The nearest equivalent joint is where the stem or sternpost is halved into the keel, but both are backed up by solid knees.
    It really is quite difficult to build an ugly wooden boat.

    The power of the web: Anyone can post anything on the web
    The weakness of the web: Anyone can post anything on the web.

  3. #3
    Join Date
    Nov 2004
    Location
    Port Townsend WA
    Posts
    11,823

    Default Re: Scarph vs motive and tenon

    I just bought a biscuit/plate joiner for doing some special work in the remodel of our kitchen. This method of joinery for a structure that does not go to sea or is subject to moisture invasion is in no way compared to using it for boat construction where rot and wood movement is a constant factor of concern. By the same token, the Fess Tool tenoning peg system is great both in the expense of the machine and in the engineering for making joints that are both fast and strong for land locked joinery work. Both systems are viable for rapid construction that is not destined to go down to the sea in ships.
    Jay
    Last edited by Jay Greer; 06-30-2018 at 10:35 AM.

  4. #4
    Join Date
    Dec 2003
    Location
    Portland, Oregon
    Posts
    50,430

    Default Re: Scarph vs motive and tenon

    I think your instinct is steering you well.

    As Jay mentions - biscuits and mortise+tenon are not particularly apt joinery for boatbuilding (with certain limited exceptions).

    Scarf and m+t are not interchangeable joints (or even comparable - again, with certain limited exceptions).
    David G
    Harbor Woodworks
    https://www.facebook.com/HarborWoodworks/

    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  5. #5
    Join Date
    Oct 2017
    Location
    Charleston, SC USA
    Posts
    81

    Default Re: Scarph vs motive and tenon

    Thank you all for shooting down that idea. That was my gut feeling. It’s good to have informed independent verification. Motive and tenon? Darned autocorrect! :-)

  6. #6
    Join Date
    Oct 2000
    Location
    Padanaram, MA USA
    Posts
    9,458

    Default Re: Scarph vs motive and tenon

    Still, as I have some furniture work in my future, I wish I had a domino tool.

  7. #7
    Join Date
    Apr 1999
    Location
    Now, there's a long story...
    Posts
    1,561

    Default Re: Scarph vs motive and tenon

    The Festool is more "floating tenon" than it is biscuit. I've always seen it as a way to attach things at right angles, where I see scarfs as more of an "in line" pull.

    I'd like a Domino... but so many other things I need more first.
    Heute ist so ein schöne Tag...

  8. #8
    Join Date
    Jan 2011
    Location
    Norwalk CT
    Posts
    987

    Default Re: Scarph vs motive and tenon

    The Domino takes the basic biscut joiner to a whole new level. Its a very handy tool and is worth every penny.

    BEWARE!! If you use the Domino without active dust/chip collection like the Festool standard vac the tool will fail and Festool will NOT replace or repair it. It is easy for any dealer to see if the tool has been used without dust collection.

  9. #9
    Join Date
    Sep 2011
    Location
    New jersey
    Posts
    2,929

    Default Re: Scarph vs motive and tenon

    I agree that biscuits are not structural but those Festool things look like tenons to me. I would use the dominos in place of
    mortise and tenon in most applications.....if I were rich.

  10. #10
    Join Date
    Jun 2010
    Location
    PNW, an island west of Seattle
    Posts
    1,771

    Default Re: Scarph vs motive and tenon

    Quote Originally Posted by Boater14 View Post
    I agree that biscuits are not structural but those Festool things look like tenons to me. I would use the dominos in place of
    mortise and tenon in most applications.....if I were rich.

    In the appropriate application I consider biscuits to be quite structural. I wouldn't bet my life on them, but have placed many books in danger by supporting their shelf with biscuits. Domino "things" are floating tennons in every sense. They are immensely useful in furniture and cabinetry work. If one considers them for a boat be aware of the wood species used. I'm not at all sure that the standard tennons are rot resistant. In that vein, I've gone to the trouble to make them myself out of durable wood. This endeavor is not cost effective. But neither is a lot of what I do.

    Jeff

  11. #11
    Join Date
    Jan 2011
    Location
    Norwalk CT
    Posts
    987

    Default Re: Scarph vs motive and tenon

    Festool sells the Domino tenons in at least five species.

  12. #12
    Join Date
    Nov 2004
    Location
    Port Townsend WA
    Posts
    11,823

    Default Re: Scarph vs motive and tenon

    Biscuits will be prone to rot if used on a boat.
    Jay

  13. #13
    Join Date
    Mar 2014
    Location
    Hasslö, Blekinge, Sweden
    Posts
    629

    Default Re: Scarph vs motive and tenon

    Loose tenons of the same material as the pieces joined together will be almost as strong as a real M/T joint; but all glue lines may fail eventually and you have twice as many of them.
    But consider a mast glued together from two pieces and a M/T joint. The mast will be about as strong as a mast with the diameter of the tenon!
    Remember that the end tree gluelines have practically no strength.

    /Mats
    My blog about my time as a boat building student, a rigger apprentice and Journeyman http://kaptenmohsart.blogspot.se/

  14. #14
    Join Date
    Mar 2014
    Location
    Hasslö, Blekinge, Sweden
    Posts
    629

    Default Re: Scarph vs motive and tenon

    I think that the M/T joints are used excusively for joining end grain into flat grain if you wish; never to join end grain onto endgrain, even in cabintry making. It's just not a good joint for that.

    /Mats
    My blog about my time as a boat building student, a rigger apprentice and Journeyman http://kaptenmohsart.blogspot.se/

  15. #15
    Join Date
    Mar 2014
    Location
    Hasslö, Blekinge, Sweden
    Posts
    629

    Default Re: Scarph vs motive and tenon

    And biscuits are a whole another animal.
    Biscuits are used to align two boards being glued together. The whole glue power is between the two boards, the biscuit has no holding force to talk of.
    This is for furniture making, a tabletop is glued up from three pieces; all gluelines will be straight grain to straight grain, no need for any enforcement.

    /Mats
    My blog about my time as a boat building student, a rigger apprentice and Journeyman http://kaptenmohsart.blogspot.se/

  16. #16
    Join Date
    Jan 2010
    Location
    Northern Europe
    Posts
    9,448

    Default Re: Scarph vs motive and tenon

    All this talk of biscuits.....do want a Rich Tea or Digestive with you mug of tea?

  17. #17
    Join Date
    Jan 2011
    Location
    Norwalk CT
    Posts
    987

    Default Re: Scarph vs motive and tenon

    Biscuits are used to align two boards being glued together. The whole glue power is between the two boards, the biscuit has no holding force to talk of.

    Really?? Have you ever tried to separate two pieces edge glued or end grain to edge with biscuits that are glued in?

  18. #18
    Join Date
    Dec 2003
    Location
    Portland, Oregon
    Posts
    50,430

    Default Re: Scarph vs motive and tenon

    Quote Originally Posted by mohsart View Post
    Loose tenons of the same material as the pieces joined together will be almost as strong as a real M/T joint; but all glue lines may fail eventually and you have twice as many of them.
    But consider a mast glued together from two pieces and a M/T joint. The mast will be about as strong as a mast with the diameter of the tenon!
    Remember that the end tree gluelines have practically no strength.

    Oops... not quite. Floating tenons are just as strong as fixed tenons. And... having restored and repaired a fair amount of antique furniture, I have so say that glue lines are not doomed to failure. Even with the easily reversible hide glues in use at the time, the majority of the joints are still intact. It's true, though, that a glue-line CAN fail...

    /Mats
    Quote Originally Posted by mohsart View Post
    And biscuits are a whole another animal.
    Biscuits are used to align two boards being glued together. The whole glue power is between the two boards, the biscuit has no holding force to talk of.
    This is for furniture making, a tabletop is glued up from three pieces; all gluelines will be straight grain to straight grain, no need for any enforcement.

    /Mats
    Simply not true. Biscuits are, indeed, handy for aligning joints during glue-up. But they also provide a significant increase in faying surface - making the joints stronger. At least in a non-marine environment.

    In a marine environment... one would have to do three things to make them viable. First - track down biscuits made from a durable species, in a durable fashion. I've never heard of such an option being available. Second - take very particular care to make sure that the biscuit slots were fully and completely stuffed with glue during the glue-up process - so as to leave no potential pathways for moisture ingress and rot-pocket development. Thus moving biscuits from a low-skill, very accessible process to one requiring some fussiness and judgement. Third - use waterproof glue. That one, at least, is easily managed.
    David G
    Harbor Woodworks
    https://www.facebook.com/HarborWoodworks/

    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  19. #19
    Join Date
    Mar 2014
    Location
    Hasslö, Blekinge, Sweden
    Posts
    629

    Default Re: Scarph vs motive and tenon

    Loose tenons provides strenght.
    Biscuits, in any way I've seen them, do not.

    What I'd typically do when joining two boards is cut a groove in the center of both boards where they are to be glued up.
    Then I cut out a spline, that has oposotite grain to the boards, and fits snuggly into the grooves.
    Then glue it all together with the spline in the middle providing strength to the two glued up boards.

    /Mats
    My blog about my time as a boat building student, a rigger apprentice and Journeyman http://kaptenmohsart.blogspot.se/

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •