Birdsmouth or Barrel Stave Mast

[Grey Sailor]

Is there any test/experience data out there that compares the strength of birds mouth vs. barrel stave mast construction on short masts (20’)?
Thinking that on a 3” spar, the benefit of the extra glue surface area of the bird mouth would be lost to the smaller “wings” of the cut.

[StevenBauer]

Birdsmouth is so much easier I'd use it even if it wasn't as strong, but I think it's probably stronger.


Steven

[Jlaup]

Not a comparison, but a good article on bird's mouth construction.
http://www.duckworksmagazine.com/08/howto/birdsmouth/index.htm
John

[David G]

The coopered mast (barrel stave) is perfectly adequate strength-wise. The birdsmouth, however, offers a bit more gluing area - and therefore will be a bit stronger. As noted, it's self-indexing during the glue-up - and therefore a bit easier to juggle. If for any reason, though, you're disinclined to do a birdsmouth, but comfortable with coopering, you should proceed with no qualms.

[kc8pql]

The coopered mast (barrel stave) is perfectly adequate strength-wise. The birdsmouth, however, offers a bit more gluing area - and therefore will be a bit stronger. As noted, it's self-indexing during the glue-up - and therefore a bit easier to juggle. If for any reason, though, you're disinclined to do a birdsmouth, but comfortable with coopering, you should proceed with no qualms.
I agree, and I also agree that the birdsmouth is much easier to assemble.

[James McMullen]

The physically interlocking joint of the birdsmouth has simply got to be stronger than the purely chemical bond of a simple glue joint.

There's also no question that the spar is easier to assemble with the birdsmouth technique. I can't think of a single reason to cooper instead.

[David G]

James,

There's no appreciable mechanical strength increase. The real upgrade comes from the increased gluing area. And, of course, the keyed structure makes the glueup much simpler.

[Yeadon]

I'm making a birdsmouth spar right now using some spruce. It'll be 15'3, with a major diameter of 3 1/8", or so. This spar will be the new main for my peapod, which is about to receive a balance lug. I'm using the birdsmouth spar directions from WoodenBoat magazine. (Not sure which issue.)

I don't know if it's less time intensive than working a solid spar, but I'm getting the feeling that it's easier to end up with a nice consistent taper. For me, there's a value to this. I do think there's quite an art to shaping a solid spar, one that I consistently screw up.

Overall, it's an interesting project. I'm enjoying it. But I always enjoy a new method or type of material. Keeps things interesting.

[Grey Sailor]

Thanks for the feedback guys!

[WX]

I've started cutting the birdsmouths in my staves and I'm beginning to think maybe I should have done them before I did the scarfs. When you don't have much of a run in table it's tricky getting the scarf end lined up with the table saw. Guess I need to modify a sawhorse to take the weight of the stave so I can concentrate on the business end.

[JimConlin]

As the strength of a glued joint is the lesser of the strength of the wood and the strength of the glue, added glue area has no effect. The only advantage to the birds-mouth method is that it's self-aligning.

[StevenBauer]

I think Clint has some good ideas on his birdsmouth page, including a neat Birdsmouth Calculator. Here's the link:

www.clintchaseboatbuilder.com/12.html

Gary, have you cut the scarfs but not glued them up yet? I'm not sure exactly where you are in the process.

Steven

[WX]

Hi Steve, I've just finished gluing up the first stave which is the worst one for number of scarfs...4. All the rest are mostly one scarf though there are a couple of 2s there. 9 metre lengths of Oregon are hard to come by. I'm gluing them because I want to taper the staves before I assemble it all. There are some excellent calculators on Duckworks. look for Bird's Mouth Spars revisited by Gaetan Jette.

[cap'nRod]

The physically interlocking joint of the birdsmouth has simply got to be stronger than the purely chemical bond of a simple glue joint.

While that may have been true in the past, it is no longer true with today's glues. Many, many wood glues today form a bond that is literally stronger than the wood(s) it is adhering together, and attempting to break the joint actually tears the adjoining wood and not the joint itself. There are also some theories that joints that utilize angles, (such as birdsmouth cuts) as opposed to relatively flat pieces of wood adhering to each other, actually forms a weaker joint, since the angle is a natural failure point that you've now just introduced into the wood grain.

[David G]

Rod,

Exactly so.

The phenomenon that you're describing is known in engineering terms as a stress riser.

And you're correct that - with a properly fabricated joints and any adhesive that's suitable for a spar in the first place - the wood will fail before the glue joint.

But all of that's just for us Wood Wonks. The simple answer to the OP is that either a coopered or a birdsmouth spar will function just fine, and there's not a lot to choose between them. I prefer the b'mouth only because of the simpler glueup due to the tendency to self-index.

[WX]

I reckon I'm going to like my new mast. :D

[Clinton B Chase]

There are some excellent calculators on Duckworks. look for Bird's Mouth Spars revisited by Gaetan Jette.

FYI: The calculator on my website is the really useful one and Gaetan was wonderful about working with me in getting it onto my website. I do some talks about Birdsmouth stuff to the boat schools, and this was so the students can have a go to place. It is a good calculator when you are given the plans for a solid spar and want to go birdsmouth. When in doubt err on the side of higher stave thickness.

[WX]

Hi Clinton, I ran my dimensions though your calculator just refresh my mind.
8 staves
OD 210mm
H 40mm
L 82
A 0.670
K 0.487
M 2.556

[WX]

Here's a youtube clip on a woman building one.

http://www.youtube.com/watch?v=jgWRCt0x9nA&feature=related

[Clinton B Chase]

Looks good to me, WX. I don't mind the ratios, just worry about making a good mast. The article link is fixed now and one can go there and see the background behind this calculator.

http://tinyurl.com/BirdsmouthSpars

[S B]

Leevalley tools has a router bit that allows up to 16 staves, 7/8" thick.

[WX]

I've posted a few new photos on my Redwing Update thread.