Re: [bolger] Re: urban myth -"epoxy join is stronger than the wood"

I don't disagree with that view Porky. The cheapest
source of good wood, unless you mill your own, is
plywood (glulam type plywood might be better, but it
isn't available localy, a regional thing). And my
only other point was that scarphs can be made to match
the quality of wood you get. The various Gougeon
boats have been made of pretty good materials, they
aren't sloppy. They recomend 8-1 splices, so I think
that is reasonable as a standards. They usualy don't
do stuff without serious testing. If you hang around
the wood aircraft people long enough, you get exposed
to the idea of longer slopes. The wood used in
official wooden aircraft is tested, and graded
carefuly, in ways that rarely apply to wooden boats.
My assumption is that with that additional control you
can get to the point where 8-1 splices may test out
lower than the surrounding wood.
--- porcupinefysh <porcupine@...> wrote:

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<tt>
Comments below... 
 
--- In bolger@y..., thomas dalzell
<proaconstrictor@y...> wrote:

> You don't need 12-1 scarphs in junk wood, scarph

angle

> is proportional to the quality of wood in

question.

> Aircraft spar grade, try 20-1. massively bad

junk

> wood, with grain that runs out on a 2-1 angle,

can be

> surpased with 3-1 scarphs. Something in the

6-1 or

> 8-1 range would probably do with home depot

2xs. 
 
Which begs the question, how good should the wood be
in a mast? Masts 
are, arguably, the most highly stressed wooden
structure on the 
average sailboat. They are also intended to be of
minimum practical 

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Comments below...

--- In bolger@y..., thomas dalzell <proaconstrictor@y...> wrote:
> You don't need 12-1 scarphs in junk wood, scarph angle
> is proportional to the quality of wood in question.
> Aircraft spar grade, try 20-1. massively bad junk
> wood, with grain that runs out on a 2-1 angle, can be
> surpased with 3-1 scarphs. Something in the 6-1 or
> 8-1 range would probably do with home depot 2xs.

Which begs the question, how good should the wood be in a mast? Masts
are, arguably, the most highly stressed wooden structure on the
average sailboat. They are also intended to be of minimum practical
dimensions and weight, mainly to reduce their windage and effect on
the boat's stability. The breakage of a eight or sixteen foot stick,
two or three inches in diameter, on a small boat is an inconvenience.
Having a big, heavy mast come down unexpectedly on a larger vessel
can be a disaster. IMHO, the wood used in a big mast should be of
such a quality that 12:1 scarfs are appropriate.

I'm not arguing that we should spend lots of money or buy from
a "marine" lumber yard. There are often excellent pieces of lumber at
your neighborhood home despot, perfectly acceptable for a big mast.
You just have to be able to recognize such pieces. More important,
you have to recognize pieces that are no good. Or, you can find out
later, when you hear the loud crash....

;o)

porky

--- In bolger@y..., "rlspell2000" <richard@s...> wrote:

> Hummm, where else would it fail? Five ft away from where you are
> putting the stress on it?

Exactly my point. It will not fail 5ft or 5 inches from the glue
join, but usually right next to the join, which does not necessarily
have to be in the mid-span.

> Actualy, I think the glued join IS stronger than the wood without a
> join.

Sorry, I cannot agree. Never have done tests as you describe, but my
observations are from days of experimenting with Finn masts back in
the times when they were timber(spruce).
DonB

>
> Someone want to perform a series of simple tests?
>
> 1) Joint your boards.
> 2) Say your scarph is 12" long, put two saw horse about three feet
> appart.
> 3) Put joint in middle of saw horses.
> 4) With a helper, push down on the protruding ends until failure.
> 5) Record results.
> 6) Report to group conditions of test, and results. Take PICTURES!
>
> Try it with different orientations, and different scarph ratios.
>
> Any would be scientistst out there?
>
>
> --- In bolger@y..., "dbaldnz" <oink@p...> wrote:
> > For a long time I have been a skeptic when someone says-"An epoxy
> > glued joint is stronger than the wood". An urban myth in my view.
> > True on the face of it, but have you ever noticed that the wood
> fails
> > right next to the glue?
> > Saying a glued scarph join is stronger than the wood, is not the
> same
> > as saying the glued join is stronger than wood without a join.
> > Of course if you cannot procure long stock, you have no choice.
> > DonB

You said it much better than I!
A previous post suggested that because the glue joint was stronger
than the wood, short scarphs could perhaps be used.
The only merit in scarphs is that full length timber is often hard to
find. Certainly not the first choice for strength.
DonB

--- In bolger@y..., "porcupinefysh" <porcupine@d...> wrote:
> The real urban myth is that that strength is a quality unique to
> epoxy. Actually, most glue lines are stronger than the wood.
> Resorcinol, urea-formaldehyde, even Elmer's Wood Glue generally
> results in breakage through the wood grain rather than the glue
line.
> This is because to stronger and/or harder glue line causes stresses
> to be concentrated in the adjacent wood, causing the failure to
occur
> there. This is also why scarf joints (especially in masts) should
be
> long and staggered. Having them short, or having them located
around
> just one part of the spar, will cause failure there from stress
> concentration. Scattering them and making them long spreads the
> stresses out so that more of the spar's structure can absorb it
> smoothly.
>
> People have been gluing up hollow wooden masts for a long time--
> around a hundred years now, I think. The techniques and rules of
> thumb governing their design and construction were determined
> empirically by practical men. If something didn't work, if the mast
> fell down, you didn't do it that way any more. By now all of these
> ideas--tapered plugs, 12:1 scarfs, wall thickness ratios, quality
> quarter-sawn lumber--are established and proven where it counts: at
> sea. Any good textbook on yacht design will cover the subject.
Bird's
> mouth masts and epoxy glues are just modern variations of an old
> theme, and compatable with it,
>
> porky
>
>
> --- In bolger@y..., "dbaldnz" <oink@p...> wrote:
> > For a long time I have been a skeptic when someone says-"An epoxy
> > glued joint is stronger than the wood". An urban myth in my view.
> > True on the face of it, but have you ever noticed that the wood
> fails
> > right next to the glue?
> > Saying a glued scarph join is stronger than the wood, is not the
> same
> > as saying the glued join is stronger than wood without a join.
> > Of course if you cannot procure long stock, you have no choice.
> > DonB

Well the limiting factor isn't the glue joint, it is
the surrounding wood, so the unglued piece isn't going
to be any better, unless you just designed it
perversely to be a stress raiser. Lamination proves
the concept, laminated wood is stronger, and not
merely because it evens out grain problems.

Piver had a neat method where he cut long scarphs in
2x6, these were scarphs beyond what was required
merely to join short sections, then he glued them
together, and got a much stronger mast in the process.

You don't need 12-1 scarphs in junk wood, scarph angle
is proportional to the quality of wood in question.
Aircraft spar grade, try 20-1. massively bad junk
wood, with grain that runs out on a 2-1 angle, can be
surpased with 3-1 scarphs. Something in the 6-1 or
8-1 range would probably do with home depot 2xs.

--- dbaldnz <oink@...> wrote:

<HR>
<html><body>

<tt>
For a long time I have been a skeptic when someone
says-"An epoxy 
glued joint is stronger than the wood". An urban
myth in my view. 
True on the face of it, but have you ever noticed that
the wood fails 
right next to the glue? 
Saying a glued scarph join is stronger than the wood,
is not the same 
as saying the glued join is stronger than wood without
a join. 
Of course if you cannot procure long stock, you have
no choice. 
DonB 
 
</tt>

 



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The real urban myth is that that strength is a quality unique to
epoxy. Actually, most glue lines are stronger than the wood.
Resorcinol, urea-formaldehyde, even Elmer's Wood Glue generally
results in breakage through the wood grain rather than the glue line.
This is because to stronger and/or harder glue line causes stresses
to be concentrated in the adjacent wood, causing the failure to occur
there. This is also why scarf joints (especially in masts) should be
long and staggered. Having them short, or having them located around
just one part of the spar, will cause failure there from stress
concentration. Scattering them and making them long spreads the
stresses out so that more of the spar's structure can absorb it
smoothly.

People have been gluing up hollow wooden masts for a long time--
around a hundred years now, I think. The techniques and rules of
thumb governing their design and construction were determined
empirically by practical men. If something didn't work, if the mast
fell down, you didn't do it that way any more. By now all of these
ideas--tapered plugs, 12:1 scarfs, wall thickness ratios, quality
quarter-sawn lumber--are established and proven where it counts: at
sea. Any good textbook on yacht design will cover the subject. Bird's
mouth masts and epoxy glues are just modern variations of an old
theme, and compatable with it,

porky

--- In bolger@y..., "dbaldnz" <oink@p...> wrote:
> For a long time I have been a skeptic when someone says-"An epoxy
> glued joint is stronger than the wood". An urban myth in my view.
> True on the face of it, but have you ever noticed that the wood
fails
> right next to the glue?
> Saying a glued scarph join is stronger than the wood, is not the
same
> as saying the glued join is stronger than wood without a join.
> Of course if you cannot procure long stock, you have no choice.
> DonB

Hummm, where else would it fail? Five ft away from where you are
putting the stress on it?

Actualy, I think the glued join IS stronger than the wood without a
join.

Someone want to perform a series of simple tests?

1) Joint your boards.
2) Say your scarph is 12" long, put two saw horse about three feet
appart.
3) Put joint in middle of saw horses.
4) With a helper, push down on the protruding ends until failure.
5) Record results.
6) Report to group conditions of test, and results. Take PICTURES!

Try it with different orientations, and different scarph ratios.

Any would be scientistst out there?

--- In bolger@y..., "dbaldnz" <oink@p...> wrote:
> For a long time I have been a skeptic when someone says-"An epoxy
> glued joint is stronger than the wood". An urban myth in my view.
> True on the face of it, but have you ever noticed that the wood
fails
> right next to the glue?
> Saying a glued scarph join is stronger than the wood, is not the
same
> as saying the glued join is stronger than wood without a join.
> Of course if you cannot procure long stock, you have no choice.
> DonB

For a long time I have been a skeptic when someone says-"An epoxy
glued joint is stronger than the wood". An urban myth in my view.
True on the face of it, but have you ever noticed that the wood fails
right next to the glue?
Saying a glued scarph join is stronger than the wood, is not the same
as saying the glued join is stronger than wood without a join.
Of course if you cannot procure long stock, you have no choice.
DonB

Re: [bolger] Re: urban myth -&quot;epoxy join is stronger than the wood&quot;

Re: [bolger] Re: urban myth -"epoxy join is stronger than the wood"