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Carbon wrapped birdsmouth masts


bill1111

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Hmmm . . . Carbon is very strong. Not sure what size mast you want, but for a small sailboat you could shape blue foam to the size, wrap that sucker in carbon, maybe use the Chinese Finger Trap style carbon tube socks, let `er cure, and in a safer no smoking zone,  dribble gasoline inside which will dissolve the foam.

If you do birdsmouth the wood weight remains. But I guess you could do an ultra thin-wall birdsmouth core.

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Hmmm . . . Carbon is very strong. Not sure what size mast you want, but for a small sailboat you could shape blue foam to the size, wrap that sucker in carbon, maybe use the Chinese Finger Trap style carbon tube socks, let `er cure, and in a safer no smoking zone,  dribble gasoline inside which will dissolve the foam.

If you do birdsmouth the wood weight remains. But I guess you could do an ultra thin-wall birdsmouth core.

Sure, you can do that.  What you will wind up with is a nice tube with no useful utility that I can think of.  The carbon Chinese sock offers very little stiffness in such a form.

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somewhere in surfing the core sound info I saw a picture of a thin wall birdsmouth that GB was doing for maybe the ec22

so must be of some use the wood then is only for a form?

Bill

The carbon sock on the top mast sections of the EC22 only offers hoop strength and the mast stiffness is from the birdsmouth fir  core. 

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All I know is that I unintendedly added weight to my mast top sections by glassing them.  I didn't use carbon, but rather 9 oz tape and epoxy.  I was shocked at how much weight it added to the tops of the masts.  I sought to add some strength up there, but am now concerned that the extra weight that the epoxy added will ultimately hurt me.  I'm not sure that I did the right thing.

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Hmmm . . . Carbon is very strong. Not sure what size mast you want, but for a small sailboat you could shape blue foam to the size, wrap that sucker in carbon, maybe use the Chinese Finger Trap style carbon tube socks, let `er cure, and in a safer no smoking zone,  dribble gasoline inside which will dissolve the foam.

If you do birdsmouth the wood weight remains. But I guess you could do an ultra thin-wall birdsmouth core.

Sure, you can do that.  What you will wind up with is a nice tube with no useful utility that I can think of.  The carbon Chinese sock offers very little stiffness in such a form.

You are most likely right, Tom. I've never used carbon sock tube, just strips. And then there would be the problem, either material, of a BLACK mast, and a toasty sun heating that bugger up and possibly degrading the epoxy. Reckon one could paint it white, or . . .

Back in the day of airplanes made of sticks and fabric, the prime coat on the fabric to prevent sun damage was aluminum paint. Yes, even colored paint is translucent, but the aluminum paint did a great job of keeping the underlying material in the dark.  Aluminum paint base, and then the color of your choice.

Sounds like work.  I'll stick to a wood only mast. I have two birdsmouth spars built for the Cat - a 22' mast, and a 7'+ boom. finish: 2 coats of 105/207 WEST and several coats of spar varnish.  If the Carolina sun starts to eat the finish I'll paint it. 

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  • 2 months later...

WB214 (May/June 2010, page 46ish) has a small piece on a catboat mast with carbon tapes & rods laminated to the inner face of the staves.

Seemed to be successful in their comparison test with uncarbon'ed birdsmouth masts.

If properly designed and constructed, this would be an ideal combination of the two materials.  Uni directional carbon fiber has lower elongation than wood fiber and should be used toward the center of the bending axis to balance the characteristics of the wood.  How far from the surface to place the carbon?  You'd really have to know the tensile strength of each material to do it accurately and that would be hard to come by (impossible) for the particular section of wood.  I'd think they just winged it in the WB example.

I proposed this exact layout to Graham last year as a possible better use of carbon in birds mouth masts.  I might make such a mast for Lapwing if I have confidence that it can be done at the same diameter of the aluminum so it would fit the boat as built.  Haven't really studied the problem enough to determine that yet.

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On a related theoretical note, I've wondered about making the following modification to a birdsmouth mast:

birdsmouth1.jpg

In this case, you would put a "cross" of some light wood.....say 1/8" baltic birch plywood.....inside the mast. This would be shaped to fit loosely, but close enough that there would be enough epoxy to hold it firmly in place inside the mast.

The thought being this is similar to how cardboard is built. The triangle providing substantial stiffness to the layup. Perhaps enough to really lighten the thickness of each stave?

Yes, I have wondered.....but not enough to build the thing to try it.  :lol:

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Howard, such a structure would add very little to the stiffness of a mast since it is located near the center of the bending axis where the stress is minimal.  It would not pay its way in weight vs added stiffness.  At the center of a mast, there is zero stress.  Stress varies exponentially with distance from the central axis.  That is why a hollow mast is the most effective in weight vs stiffness.  Its also why a thin wall aluminum tube is so difficult to beat for a mast.

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Thus another idea by a non engineer goes down in flames.  :lol:

My thought was each of those crosses would act as miniture I-beams or a truss to resist bending. Like taking a thin yardstick and bending it on edge vs. on it's side. The other cross would stabilize it in the column. Oriented at 45 degrees to the centerline of the boat, each I-beam would be in line with the sail hard on the wind, which is probably the only place it matters much. And it would probably only need to be in the upper 1/3 rd or so of the mast where most of the bending occurs. Aside from this, to a certain degree, the birdsmouth masts already have the I-beams in them. For nearly any direction of bend, you have two staves that are shaped as rectangles opposing the bend, and each acting as an I-beam.

Along these lines, for those with access to scantling details for these masts, which is stronger: a relatively small diameter mast, with thick staves, or a larger diameter mast with thinner walls? Somewhere in one of my resources I seem to recall someone saying that all things being equal, more staves (say a 12 stave mast) is stronger than less. Perhaps that is because less of the wood is planed off to make it round?

If you offset your birdsmouth joint closer to the middle to move the corner joint inwards, doesn't that require less wood to be planed off to make it round, thus adding strength?

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A large-diameter thin-wall tube is much stiffer per unit weight than a small-diameter heavy-wall tube because there's more material working in the area of highest stress. The downside is that a thin-wall tube is more vulnerable to buckling.

http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=213&pageid=2144417026

My favorite book on structures for non-engineers is J.E. Gordon's "Structures: Or why things don't fall down".  It's a bit dated, but it reads as if you're sitting in a pub with an enthusiastic retired professor.

http://www.amazon.com/Structures-Things-Dont-Fall-Down/dp/0306812835

Among other things, it lead me to Kipling's engineering short story "The ship who found herself"

http://www.readbookonline.net/readOnLine/2414/

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My favorite book on structures for non-engineers is J.E. Gordon's "Structures: Or why things don't fall down".  It's a bit dated, but it reads as if you're sitting in a pub with an enthusiastic retired professor.

http://www.amazon.com/Structures-Things-Dont-Fall-Down/dp/0306812835

Among other things, it lead me to Kipling's engineering short story "The ship who found herself"

http://www.readbookonline.net/readOnLine/2414/ 

I have Gordon's book and you are right.  It reads like a novel and offers a lot of knowledge in an interesting and simple format.

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Thus another idea by a non engineer goes down in flames.  :lol:

If you offset your birdsmouth joint closer to the middle to move the corner joint inwards, doesn't that require less wood to be planed off to make it round, thus adding strength?

I did that.  By adjusting the height of the router bit I made staves that had no overhang. The inside and outside corners were perfectly mated when glued up. I did not round the mast, just knocked the sharp corner edge off with a plane.

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