Scarfing hints?

Discussion in 'Wooden Boat Building and Restoration' started by ted655, Apr 25, 2008.

  1. ted655
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    ted655 Senior Member

    I need to scarf some 2 X 4s, 8s & 12s. I'm asking for comments, links to jigs & general advice.
    I see quite a bit on scarfing plywood at 8:1 Same ratio for timbers? I'm thinking my router is the better tool to jig for. ??
    Thanks, Ted :) :confused:
     
  2. rwatson
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    rwatson Senior Member

    Hi ted

    I have a picture of a scarf attachment using a router. I have no idea where I got it from or how it works, but it might make sense to you.

    I have also attached a simple diagram that I think would be a better way to scarf stringers etc. One of the problems of scarfing is keeping the bloody ends stable while the glue dries. The diagram shows a method that only requires a little pressure at either end of the timber, and cant slide out of alignment.

    Hope they are usefull
     

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  3. alan white
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    alan white Senior Member

    Not difficult with a simple jig. 8:1 is adaquate for timber, but with timber a few fasteners are more necessary as solid dimensional wood works a bit during moisture changes. Through fasteners like bronze or stainless flat head bolts allow one side of the plank to be installed flush. Copper or bronze rivets would also work.
    You can screw a long (say 24") hardwood board or aluminum plate to the shoe of a circular saw so that it will ride on wedge-shaped ramps to each side of the cut. The ramps must be a ways out to prevent the saw blade from touching them. The saw cuts a series of slots which are knocked down and taken down flush with a 20-36 grit disc sander and then a sharp plane.
    I like a skilsaw better than a router because it's quicker, but the same jig can be made for use with a router simply by putting a long shoe under the router and using a 1/2" -3/4" dado bit. Aluminum make a sturdy base and it can be "machined" like wood on carbide-edged woodworking tools.

    Alan
     
  4. PAR
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    PAR Yacht Designer/Builder

    The easiest way to rig up a router is with inclined ramps bordering the scarf. The router rides on a plank shoe with the bit protruding below. The shoe glides along the ramps at the proper angle and you plow out wood as you descend the ramp.

    This is fine on thin stock, say up to 3/4", but tedious on thicker stock, like your 2 by's. I prefer a power plane and the inclined ramps. I can knock off the bulk very quickly with a power plane, then kiss it with a router or my usual choice a big angle grinder with 24 grit.

    If you're looking to epoxy 2 by stock, I'd like to suggest against it. 2 by stock is dimensionally large enough where any moisture gain can threaten the joint. This is why West System and the other manufactures recommend using only 1" or thinner laminations with epoxy.

    I also think you'll need a steeper scarf ratio on larger stock (like those 2 by's) at least 10:1 or better 12:1. With this steep a scarf ratio, you can use a standard structural glue like alphatic resin, plastic resin, urea formaldehyde, resorcinol or others, skipping the mess of epoxy and just screw it together, while the glue sets up. I'd just leave the fasteners in place and call it a done deal.

    Are you scaring together some stringers Ted? You could skip scarfs completely if you laminated two pieces of one by stock together (which equals the same as a single section of 2 by) with the joints well staggered and you could use epoxy on the thinner stuff.
     
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  5. ted655
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    ted655 Senior Member

    :) thanks.
    These are not stringers, (i chose to use butt blocks for that), these are keel timbers, (2 X12s), exterior chine boards & gunnel, (bumb), protectors on this damn barge I'm finishing from the previous owner. His design used too few & too small stringers on the ribs. He went ahead and sheeted the boat, so I'm forced to add longitudinal strength, on the exterior, (another subject, I'm sure).
    .
    Before I go whacking up good scraps for my jig, can someone go over the 12:1 measurement? This is dimensional lumber, so it's actually 1-1/2" thick.
    I once saw what I thought was a good design on a scarf joint. Instead of a sharp, thin end, this joint had a 3/8" end that fit a 3/8" "start" on the other board. I've no idea what type of jig was used. It was a 3" thick, curved gunwale around the whole boat. There was no splitting or curl at any joint! beautiful. I don't need that type of finish on this old barge, but it sure was something one remembers. :D
     
  6. rwatson
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    rwatson Senior Member

    You mean something like this ?

    Its not a hard join, and it just requires a good tenon saw. If you have the power tools, its even easier of course.

    If you have a crosscut power saw setup, you can run it across all the ends of the planks to a 3/8" depth, far enough back from the end to get a decent angle.

    I find that a band saw is the easiest to use to make the angle cut on timbers.
     

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  7. ted655
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    ted655 Senior Member

    Yes, that's the joint.
    I do have a nice power cross cut, (called a power miter here). I don't have a handsaw..
    I'm not sure I even need such a joint. I just think they show "class" :D
     
  8. alan white
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    alan white Senior Member

    The joint with the step is one of many used since long ago to join two short pieces to make a long one, but remember, the advent of waterproof glues has left us with only one simple scarf (which, if glued, is probably the strongest). In other words, the strongest is the longest.
    The older joints depended exclusively on fasteners in most cases (some fancy ones locked, requiring none, but they were always weaker than with fasteners).
    I would take advantage of modern glues (i.e., epoxy) and make pointy scarfs for maximum strength, using a few small ring nails to finish the thin ends.
    The step (shoulder) is better looking, yes, but save the finish work for the cabin interior or the cockpit, say I.

    A.
     
  9. PAR
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    PAR Yacht Designer/Builder

    I remember the photo of that frame and you're right, likely not enough longitudinal stiffeners, though the plywood sheathing will go a long way toward this.

    For that barge, skip the fancy stuff (a nibbled or stepped scarf is weaker then a plane one anyway). Cut your scarf, using the taper you want. This will be 12" long if 8:1, 15" long if 10:1, etc. Then pick up some asphalt roofing cement (tar), not the stuff with fibers in it, just plane old "black mammy" roofing goo. Butter up both sides of the scarf and scratch them with a 1/16" V notch trowel, just like if you were putting up new tiles around your bathtub. Then mash them together, using lots of screws to hold the pieces together. The tar will outlast the barge.

    A scarf joint maintains the stiffener's integrity across the joint.

    Again if you elect to use 1 by stock instead of 2 by, then you can stagger the joints and skip the scarfs all together. The pieces overlapping each other will act like continuous butt blocks. You could bed these in tar too. The longitudinal to frame fasteners will keep things aligned and secured well enough.

    To paint over tar you need to use the elasticomeric roof coating with the silver pigment in it, other wise it bleeds through eventually. You can over coat the elasticomeric paint with regular bottom paint.

    You could stil use better glues, but I suspect your barge is more dependant on it's fasteners then it's adheasives.
     
  10. ted655
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    ted655 Senior Member

    :) I won't do any mill work on it, that's for sure. I think I will spring for epoxy on the joints. The tar is going to complixate the seam taping I want to do, just befor I coat the underwater portion with Coal Tar Epoxy .. I definately don't want tar up on the gunwale either.
     
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    I wonder if finger joints would save work, time and material? Not sure if there's a router bit for that size stock though - I've lost my catalog - and you would need a router with a 1/2 inch collet.
     
  12. thudpucker
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    thudpucker Senior Member

    I gotta ask! I hope I dont screw things up with this...
    I built a house and put a beam from one end to the other.
    I made the beam out of 3/4" plywood all glued together.
    The beam was 10" tall, 8" wide and 52' long. Its holding up a roof.
    So thats 9 or 10 pieces of 3/4" Plywood, staggerd joints, all glued, using Screws (lots of screws) to hold the pieces together.

    Why couldnt you make your Keel timbers like that? Using Marine grade stuff of course!
     
  13. rwatson
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    rwatson Senior Member

    No reason at all - depending on boats size of course.
    They build whole boats like that - its called 'cold moulded', layers of plywood glued to each other. Sure , the layers are curved, but its the same principle.

    The beam you describe would be too big a section for a smaller boat, and for boats over say 30foot, the owner would probably pay a bit extra to get a number of much larger sections of a harder timber to join together. Unlike a house beam, there would be engine bearers, ribs and all the rest that needed to be 'joined' to the keel.

    Plywood is a great structural 'asset', but it is not as effective at supporting joins etc (the layers tend to splinter away from each other) Also, although its 'waterproof', more dense timbers perform better in bilge areas where oil, diesal, salt water, sand and all sort of other rubbish accumulate over the years.

    The 28ft trailer sailer I have only just decided to build in ply, doesnt need much of a keelson because of stitch and glue re-inforcing. A few layers of epoxy and glass will do. Maybe 1 3" x 1" 28 ft solid beam might be required.
     
  14. thudpucker
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    thudpucker Senior Member

    Yeah on the plywood coming apart at the seams. I've had the experiece.

    How about layers of treated Pine for your longitudinal beam?
    Something that's normally used to put right on the ground as a foundation wood for Barns and even Houses. My farmhouse sits on treated Pine thats a good 80 years old.

    Its heavy, will hold a screw very well and has tremendious strength. The 'treatment' should prevent any rot from developing between the laminate joints and between the butt joints in the laminated layers.

    How long does it need to be? I have a couple of Pines here that would go nearly 75' in a straight line. You could make a 15"X15" beam, 75' long! All Heartwood!
    Would that do? Assuming you could work out the Transportation details as well as a pressure cooker big enough to treat the wood.
     

  15. rwatson
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    rwatson Senior Member

    I remember helping my dad build a large church hall. It was about 20 foot longer than a basketball court, and the roof was held up by two laminated oregan beams that ran the length of the hall (not across).

    The beams were built on on the flat concrete floor, and were made of individual planks 6" wide, and no more than 20 foot long. The single planks were about 1" thick, and were glued layer on layer till the beam was nearly 3ft deep. The beam was built with a slight curve (high side up), so that when the beams sagged a bit under the roof load, they would end up straighter.
    The engineer told us that they were structurely stronger than steel of the same dimensions, and they sure didnt need such a heavy crane to get them twenty feet up in the air onto the walls.

    I think if you have some good, timber, then laminating a keel would be every bit as good as you would want. I was reading a magazine on the weekend where this guy is building a replica 50 ft square rigger out of pine that he has salvaged from farms all around his place, old trees that are too old and too big to act as effective windbreaks anymore.

    Depending on the timber suitability (some species dont hold up well in wet environments) lamination and rot proofing (painted on) would ensure a great keel. For major structural pieces.,some root timber or heavy limbs make great knees and stem posts. Just make sure they are well seasoned.

    If the pine is heavy, (after seasoning) and hold screws well, it sounds like thats a good sign. The denser light-woods perform much better.
     
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