Birdsmouth spars revisited

Discussion in 'Wooden Boat Building and Restoration' started by yokebutt, Apr 13, 2005.

  1. yokebutt
    Joined: Aug 2004
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    yokebutt Boatbuilder

    Ooooh, PAR, NOW I understand, I was a bit mystified for a while about what you ment by "assymetric joint", I'm not all that quick on the uptake all the time, I feel a little dense for not understanding what your idea was, but then, that's nothing new by any means. I've been doing something somewhat similar, albeit much cruder, by routing off the offending edge before assembly, but it's nowhere near as elegant of a solution.

  2. galindo
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    galindo New Member

    birdsmouth sparr

    its a little late but here's my response.:)
    sounds like a good idea- I building a 5m main spar for a fulmar using the birdsmouth technique and I reckon I'll give it a go. I've got 8pcs 35mm x 17mm and going to cut the 'V' using a tables saw. Wish me luck
  3. RHough
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    RHough Retro Dude

    This is probably a stupid question but I'll ask it anyway... :)

    Is it absolutely necessary to cut all the staves from the same board? Must each stave be full length?

    I can't imagine how long it would take to find a 20ft + bit of timber to start with. My guess is that Home Depot won't be the source.
  4. PAR
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    PAR Yacht Designer/Builder

    No, the staves don't have to be full length. Most of the things you look for in spar building stock aren't required in birdsmouth techniques. I still use tight and straight grained spruce, but it's not as important as the stock density. I scarf my short stave pieces, though I understand it isn't necessary from an engineering stand point. The adhesives do the work and the lumber should be uniformly dense. It would be nice if all our spar needs could come out of the same board, but it doesn't work out that way. This is that density thing. Each stave could be weighted, but that's a bit to anal for me. I have discarded pieces that seemed light or heavy from use in a stick before. I've also done the same for weird grain run out or other defects too, so maybe I'm more anal then I thought. Think of it as strip planking a spar. Some will edge nail, others will just slap on goo, folks will scarf or butt the strips (staves) depending on sexual orientation, etc. I don't use any edge fasteners. I use hose clamps, rubber bands, tape, clamps or whatever is handy to fix the staves in position as the glue sets. A well placed concrete block (about 35 pounds) can do wonders for talking a stubborn section into staying put.
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    BOATMIK Deeply flawed human being

    Scarf please.

    I think your instinct to scarf is a good one. The density issue is important too.

    Scarfs allow the stave to carry much the same tensile loads as a solid piece of timber. Butt joining (and end grain gluing) is weak by comparison. I suspect the statements that it is possible to use butts don't hold water.

    If a mast is stronger than it needs to be - ie heavier than it needs to be - then you may get away with butts. If the mast is about right for the job then butt joining staves may weaken it so it can't sustain the loads.

    If the mast is made up of the normal 8 staves then at the level of a particular butt join you have given away quite a good proportion of strength in tension. 12%. I know the mast is never purely in tension - but other loading situations are worse.

    For example hen a mast is loaded in bending one side will be in tension, the other in compression - roughly speaking you will have 4 staves with each type of load.

    So the mast may be losing 25% of it's capability to sustain bending loads with this very rough scenario.

    Starting to think in pukka engineering terms - the stave that does the most work is the one furthest from the neutral axis. So if the butted stave is in that position maybe you could be losing much more than 25 percent.

    I suspect the reasoning that you don't need to scarf would come from one of three areas and none is a sustainable argument.

    1/ that a stayed mast is never in tension and is more likely to fail in a compression mode. This is not true of unstayed masts or the top part of the mast in a fractional rig where bending is the dominant mode. It may also not be the case when the mast is knocked out of column by some mishap - big wave, broken rigging, misapplied runners, spinnaker pole loads etc etc etc

    2/ that the butted stave is supported by the staves on either side much like a buttstrap or buttblock works when joining ply or planking respectively. A properly applied buttstrap (length is 20 times the width of the ply or plank it is joining) does give much the same strength as the original piece. However you have two pieces of timber carrying the load of one. With birdsmouth there is no extra material added to carry the load across the butt - the staves to either side are being asked to carry their own loads as well as the load of the butted stave. So they are more highly loaded and may fail.

    3/ That the "glue will do the work". Do the experiment of getting a small piece of timber - a cedar strip is perfect. Cut it into two lengths. Cut a Scarf in one 6:1 scarf (some use 8 or 10 to 1 for spar work - but theres a few pages of argument there). Cut and butt joing the other. Let the glue cure and try to break them. Butted one breaks easily right at the butt. Scarfed one is hard to break and may not actually break in the scarf join.

    Butt joining staves in a mast doesn't sound like good engineering to me. So like you, I will be doing standard scarfs on my birdsmouth work unless someone can come up with a good argument as to why it is OK to do it another way.

    Best Regards
    Michael Storer
  6. Moss
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    Moss Junior Member

    birdsmouth alternative

    For smaller spars, I find heavy duty cable ties can be used in the place of hose clamps. Small cable ties can be useful, and quick, in the place of copper wire in stitch 'n glue applications too, as long as there's not too much torture involved!
  7. nordvindcrew
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    nordvindcrew Senior Member

    I'm working on glued birdsmouth oars. 10 shafts made and waiting for blades and handles ( this winter ) They feel very light, and I have suspended them on 4' centers and hung all my weight (170 lbs ) on them. strength seems good as well as light weight. Will update when we get some use on them. There will be two sets of 4 oars plus extras, each set progressivley 4" shorter than our current aluminum shaft oars. This will let us experiment to see the best length for wind and wave conditions to obtain the best speed our boat is capable of.
  8. Scott Carter
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    Scott Carter Senior Member

    - swallowtails- Need a picture drawn

    OK guys, I'm thick. I'm making the swallow tails for the 85' birdsmouth mast, and after re-reading this thread to refresh my memory, I think I didn't have a clear picture of what the swallowtails are suposed to look like. Fingers? I don't get it. I was picturing a solid plug, with straight sides in the middle (and therefore glued to inside walls of the staves in this mid area only) then all faces of the octagonal swallowtail tapering inward (as PAR indicated at about 12:1)away from the inside walls of the staves. Is this not right? Any chance someone could paint a clearer picture, direct me to another thread, or even better sketch it up and post a rough drawing of an example? I'm in the 11th hour. Much appreciated,
  9. PAR
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    PAR Yacht Designer/Builder

    If a solid area is installed inside a hollow mast, a hard point (stress riser) will be generated (under bending loads) at the starting and ending location of the solid section, within the spar. This is where the mast will not bend as uniformly as the rest of the stick, so it will break in this place, long before it needs to. This is what swallowtails are all about. The necessary hard point locations (spreaders, gooseneck, etc.) have to be there, but minimizing their impact on the spar needs to be addressed. If the "legs" of the pieces fitting inside the spar, which make up the internal reinforcement, are tapered back, into the spar walls, then the stress riser situation is greatly reduced. The spar is stronger in this location, but the hardness comes in gently (12:1 ratio) so as to not create a sudden increase in the stiffness of the section.

    Lets say your spreader hard point needs to be 6" long, inside the mast to receive through bolts, screws, whatever. You could have just a 6" long plug inside the mast, but this will weaken it in this location (amazingly enough). If the hard point was made longer, but had a taper (picture an inverted cone shape on each end of the hard point) which permitted the hard point to transition from standard wall thickness to completely solid, then you've have a swallow tail. They're called swallow tails, because this is what they sort of look like when you install the pieces, inside the stick.

  10. ancient kayaker
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    ancient kayaker aka Terry Haines

    Ybutt, thanks for a great idea, I've been trying to figure out how to assemble a birdsbeak spar without baptizing myself in glue for some time. Wish I'd checked out this thread before.
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