Birdsmouth spars revisited

[yokebutt]

I've been playing around with the birdsmouth technique a bit, what I've discovered is a nifty trick for putting it all together. No doubt somebody else has tried this before. First of all, as long as any two opposite pieces are the same width, it'll come out right, this allows you to make ovals, or, if you feel like it, really weird shapes. What I used was flat-braid straps, I just stapled it on to piece #1 and then held piece #2 in its proper position and stapled it to the strap, then I held #2 and #3 together and stapled #3 to the strap, then I held piece #3 and piece #4 together and stapled the strap to #4 and so on to #8. Once all the pieces were stapled on to the straps I just un-rolled the whole thing and spread on the adhesive, rolled it back up, and clamped it with hose-clamps. Pretty damn cool technique, if you ask me.

Yokebutt.

[yokebutt]

Come on kids, poke a hole in my balloon will you! Let's get a conversation started, so we can all benefit in the end! Bloody wankers!

Yokebutt.

[yokebutt]

Lately I've been amusing myself by cutting a shallow cove with the table saw on the inside of the sticks, and thus making the inside round too! Works for oval birdsmouth spars too, just takes a little bit more figgering....

Yokebutt.

[duluthboats]

I like the idea of using a strap; I have made a few small tests using tape in the same way.
Do you have a name other than the user ID; I get a little tick in my head when addressing “yokebutt”, I guess I can get use to it. ;)

Gary

[cyclops]

I have access to a lathe with 8' diameter and a 40' bed to "clean up" any rough edges. Or we could turn down a tree in 8 hours?

[phillip]

Sounds like a great method for assembly. I like the birdsmouth technique just wonder about swallow tail installation. Plugging the foot doesn't appear to be a problem, but rolling up all those staves with a birdsmouthed swallow tail???? There I said it!
Keep up the good work Yokebutt!!

Phil

[yokebutt]

Yes, I can certainly understand if duluthboats doesn't care to address my buttocks, just call me "yoke", my friends seem to think it applies well.

Now Phil, if you would just lay off the gay jokes, unless "birdsmouthed swallow tail" has a meaning in woodworking too! (but then again, I'm not a native speaker of english) actually, I don't know what a swallow tail "entails" in this particular instance, would you mind enlightening me?

Yokebutt.

[phillip]

Swallow tail is the name given to a plug installed in a hollow mast. The plug or swallow tail is installed at the point where the spreaders are to be installed. The ends of the insert are cut with v sections taken out. They are hollow and usually made of hardwood. The v sectioned fingers provide extra stength but cut down the weight of the insert.

Hope this helps

Phil

[PAR]

I've come up with several modifications to the bird's mouth technique to address the typical issues that come up during any built up spar construction.

I build the sticks as halves, first of all. I glue it up and assemble it as any other bird's mouth pole, but tape over the seams I don't want glue in. This allows me to open the spar and install hard points swallow tails and all. Putting a cove on the inside (yokebutt style) would make this job a lot more difficult and I don't like removing any more material then I need to. I usually design the mast and like to stick with the scantlings I've selected for the intended use, though I could factor in an additional 20% material to account for the cove cut on the inside, but I'm not sure of the need.

I've made oval masts with internal halyards and wiring, all the bells and whistles you'd expect to see on a well dressed spar.

As Phillip has pointed out, the swallow tail, refers to the ends of a tapered insert (hard point) You need hard points along the length of the mast. Areas like the gooseneck, gaff, spreader(s) stay and shroud lands, heel and head will all need some reinforcement from their attachments and the loading they exert. These areas may be to prevent collapsing the mast from compression of the gaff or gooseneck, support for spreader sockets, shroud tangs, anything that may require fasteners that could compromise the stave wall. Typically a hard point is inserted in this area, with gently tapering ends to spread the load. More importantly to not create a "hard spot" which will likely break the mast when under bending loads in use. When making a built up mast, these inserts will have to taper down the inside of each stave, which must have look like a swallow's tail to some really bored spar builder long since dead, but his hook still carries on. I make mine up individually and glue them into place inside each mast half. I find it's a lot easier to fit them and insure good gluing surfaces this way. Tapers are around 12:1 or better.

I also use an asymmetric layout for the staves, rather then the more usually seen symmetric notch. I take advantage of the thicker shoulder created and save a small amount of rounding effort. I do use the symmetric layout on internal halyard spars.

[cyclops]

A post in the steel boat section is from a guy who needs help on a mast specified in his plans . He can't nail down a few steps. His name is Phillip and I took the liberty to tell him to post on this section. Could you steer him straight? Rich

[yokebutt]

Phillip,
I think the roll-up technique would lend itself to leaving two of the joints dry and installing local reinfocements before closing it out, but I'll have to do some experimenting to confirm that.

PAR,
Cutting a cove on the inside was something did mostly because it seemed like a really cute idea, but the reality is that as you move materials away from the neutral axis their impact on stiffness increases by the cube of the distance, so I imagined the cove as a way of eliminating a little bit of material that doesn't contribute all that much to overall strength of a spar that gets forced to bend while sailing would be a good idea, I certainly appreciate your concerns about the difficulty of installing local stiffeners though.

The idea for bands to hold the pieces together arose from a teak and maple floor-board project we were doing at the same time that I was futsing around with the birdsmouth technique. What we did was to first glue together 2" thick teak boards with 1/4" thick maple, then we ripped battens that were 3/8" thick and 2 1/4" wide from those, giving us a maple strip already bonded to the teak. Bonding the two pieces together on a flat table first certainly helped, but they were still far from straight. To eliminate the last of the crookedyness, we laid out the teak/maple slats in the pattern/order as we were going to bond them to the plywood substrate, then we used bar-clamps to squeeze them all together tight, we then put down a strip of 3M fiberglass-reinforced packing-tape (very strong and very stiff stuff) across the wood-slats spaced every few inches. Once the tape was down and carefully rubbed on, we could lift the whole thing, and coat the bottom and sides of each individual slat as we progressively "rolled" them into place, once the whole "sheet" of slats was down, we assembled the processing-stack and the vacuum-bag. However, we had a few issues along the way, (or "opportunities to gain experience" as I like call them) for one, the packing tape didn't stick all that well to the teak, and on the first panel we almost had the slats fall off the tape before we could get them into place, we rectified this problem by alternating the packing tape strips with strips of really gooey tape that was very stretchy but kept the pieces together during glue-up. The right types of tape and liberal application of some thoroughly foul language seemed to do the trick. And that, at long last, is the moral of the story and the reason I used staples instead of tape for the birdsmouth, but I suppose you could use a band of fiberglass cloth and put a dab of epoxy on every slat if you want to avoid the staple-marks.

Yokebutt.

[yokebutt]

And of course, thank you both very much for elucidating the meaning of "swallowtail" to my rather dense and prejudiced mind.

Yokebutt.

[phillip]

Turned into quite an amazing thread yokebutt. Don't think for a minute that PAR didn't straighten me out also. I wouldn't have thought about a 12-1 taper on those fingers; But it sure makes sense. Unbeleiveable amount of information....I'll be saving this one.
I was never concerned about building a 25 ft. mast using these techniques but 36 ft. has me wondering. But! If everything does go to heck in a basket....... We do know a builder with a 40ft.bed that can do an 8" diameter right cyclops!!!!

Thanks Everyone
Phil

[cyclops]

AAAHHH, UUMMMM. No problem. Just remove the roots and wash the dirt off the rest. How long does it take to dry a whole tree without stress cracks? 4 score and 7 years ago our

[PAR]

Having just built a 26'er to replace the aluminum one that got bent (it's 19 oz. lighter too) I'd have to say a 36' stick, as a first attempt in mast making, is quite ambitious.

The method has a number of posts on this and other forums. You'd be well advised to do a search for them. With careful planning and lots of dry runs before the glue hits the staves, you can do it. Personally, I'd want a boom under my belt (hint) before I dug into a stick this length.

A 10" diameter pine tree cut down, not de-barked, heavily coated in oil, covered up and rolled 180 degrees every other month will be ready to use in a year, and shouldn't check to badly. Dry it faster in a solar kiln, maybe 3 - 6 months, depending on where you live.

[yokebutt]

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.

Yokebutt.

[galindo]

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

[RHough]

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.

[PAR]

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.

[BOATMIK]

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.

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

[Moss]

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!

[nordvindcrew]

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.

[Scott Carter]

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,
Scott

[PAR]

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.

[ancient kayaker]

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.