Re: [bolger] RE: Joining Plywood

Thank you all for the information. If I may draw a
conclusion: 
structuraly the differences in the three methods are
trivial.

Particularly if you opt for scarphs in the ned
anyway...

I think 
scarfing wins out in my opinion - it will eliminate
the flat spot and 
the piece of ply on the inside.

Also if you are planing on any kind of clear finish
inside or out, it will look better.

I have never scarfed plywood, but I 
have a power plane which should make quick work of
cutting the 
tapers.

It may prove useful, and as you are probably aware,
there is a specific scarphing tool which uses a plane.
Depending on your wood however, it may tear out the
long edge which is very fragile. If so stop short of
final, and hit it with a power sander, either disk or
belt. If you plan on using a lot of scraphed ply over
your boatbuilding life, you should probably make up a
vacuum saw table jig, it only takes a few hours to
make.

What is the correct taper - 12:1?

8-1 is the usual tapper for ply.

I am thinking to clamp the 
joint between two boards with through screws every six
inches - sound 
reasonable? 

That works, there are a number of better methods. I
scarphed all the ply for my boat full size in a vacuum
bag. there is also a gate horse thing. looks like a
saw horse but uses a second bar with wedges to hold
the edges together. For low uses screws are fine. I
love using vac so much, I even figured out a way for
doing it on sections that couldn't go in a bag.
<

______________________________________________________________________
Only a few days left to file!http://taxes.yahoo.ca

Thank you all for the information. If I may draw a conclusion:
structuraly the differences in the three methods are trivial. I think
scarfing wins out in my opinion - it will eliminate the flat spot and
the piece of ply on the inside. I have never scarfed plywood, but I
have a power plane which should make quick work of cutting the
tapers. What is the correct taper - 12:1? I am thinking to clamp the
joint between two boards with through screws every six inches - sound
reasonable?

I start my Chebacco in two weeks!

Paul

Well put. My concern on structure largely has to do
with circumstances where we are not talking about just
the surface plies. Obviously, in those situations you
could still overlap enough glass thickly enough, over
a large enough area, to have any strength you might
require, but it wouldn't be as fair as a scarph.
Imagine using 7 layer ply as a joinst, where you have
4 layers acting like a regular 2x4, and 3 as webs.

For sheating it probaly works fine, though I do think
there could be problems in certain case where the load
comes perpendicular to the joint (hit dock) The
middle plies on a multi layer are just butted, and the
surface thickness of the glass may not be sufficient
to eliminate a break.

I guess it comes down to this for me: If I could
order it any way I wanted it, would I order it glass
butted? Ain't no way. So basicaly my only motivation
would be related to the process to get the scarphs in,
vs some other method. That part doesn't present any
problem for me.

However, the theory behing the fg butt joint is that
most of the forces 
involved in any plywood failure are on the surface
plys. You could almost 
replace the central plys in the butt joint with fg
wood flour putty, and the 
outer plys with fiberglass. 
 

That is true, for pannels, but not other structural
elements, not witht he Payson process anyway.

Incidentaly, unless you use a lot of fasteners, the
you only have one ply 
glued to your butt block, so you are still sitting
with surface ply failure. 
With FB but joints, you also have failure in the
surface wood under the 

That is true, but you have good back-up to
perpendicular forces, and assuming you are sheeting
the thing anyway with glass, you are covered on the
outside also.
tape. 

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> I don't really think the flat spot is a big deal, just
> fair it in with 410.

I don't want to seem too dense, but what is 410?

I own a 410, but would want to fair anything with it, might make to many
holes. <grin>

Jeff

As mentioned before "stronger than strong enough" is gilding the lily.
My first boat was a 23' center hull for a trimaran. I scarfed several
panels consisting of 3 4x8 sheets of 1/4" ply end to end. I used the
Gougeons scarffer attachment with a good carbide toothed blade and it
was very easily done. There are better gadgets that make cutting the
scarfs even easier. On the few boats I've built since I have used butt
blocks and yes there are flat spots. The best all around joint
considering appearance is the scarf. It is probably the strongest also
but then again stronger than strong enough. With the butt blocks the
flat spots are seen by the builder but probably not by anyone else.
Bob Chamberland

--- In bolger@y..., "prthober" <prthober@p...> wrote:
> I used plywood butt joints on the panels for my Gypsy and now wish I
> had scarfed them or had used fiberglass butt joints - the plywood
> butts make a small but noticable flat spot when the panels are bent.
>
> I thought I read a recent posting on one of these forums that said
> that a glass butt joint is stronger than a scarf or plywood butt -
> anyone have authoritative knowledge about this?
>
> Cheers,
>
> Paul

I agree, I would like to see some extensive test results for scarph v block
v fg butt joints.

However, the theory behing the fg butt joint is that most of the forces
involved in any plywood failure are on the surface plys. You could almost
replace the central plys in the butt joint with fg wood flour putty, and the
outer plys with fiberglass.

Incidentaly, unless you use a lot of fasteners, the you only have one ply
glued to your butt block, so you are still sitting with surface ply failure.
With FB but joints, you also have failure in the surface wood under the
tape.

----- Original Message -----
From: "thomas dalzell" <proaconstrictor@...>
To: <bolger@yahoogroups.com>
Sent: Wednesday, April 24, 2002 12:50 PM
Subject: Re: [bolger] Joining Plywood

> I don't really think the flat spot is a big deal, just
> fair it in with 410. Speaking only for myself, when I
> first started boat building I was so cheap that any
> process that used one ounce of glue one way or another
> was a nightmare for me. But if you think of it from
> the perspective house building, with the mass of
> plaster of drywall fairing (not really fair) compound
> used, why should I worry about a little compound. The
> neat thing about a flat butt, is that all it needs is
> slightly more compound over the flat, and it is fair.
> Nothing like hiding the glass butt joints of a
> non-recessed variety, or even some scarfs. It is an
> additive process that requires no sanding. It also
> goes together lightning fast, and preserves surface
> alignment well.
>
> My only complaint about butts is that they add quite
> bit of weight, and they also add a lot of stiffness
> that isn't usable in certain situations like
> stressformed hulls. The key there is just to design
> them creatively into the structure, where they will
> pick up certain loads like main bulkhead loads.
>
> It really isn't meaningful talking about glass butts
> being stronger. Stronger than strong enough isn't
> useful. You can build either style to be as strong as
> the situation requires. I don't personally believe
> that the Payson joint is as strong as a butt, or a
> scarf (though it is lighter than a butt). So far I
> have seen no published evidence to support its
> strength. We have extensive testing of scarphs and
> possibly butts, but none of the Payson joint that I
> have seen beyond across the knee stuff. So far it has
> been used mostly on planking which is generally a very
> low stressed part of a boat. Until I see some tests,
> or some successful use in higher load applications, I
> am agnostic on the joint. Obviously there is some
> schedule of let-in glass that would be super strong,
> but just a grinder and a little light glass doesn't
> persuade me we are replacing anything more than the
> top veneers. That's enough in certain uses, but it
> doesn't join or replace every fiber in the sheet as
> scarphs and butts do.
>
>
> --- prthober <prthober@...> wrote:
>
> <HR>
> <html><body>
>
>
> <tt>
> I used plywood butt joints on the panels for my Gypsy
> and now wish I 
> had scarfed them or had used fiberglass butt joints -
> the plywood 
> butts make a small but noticable flat spot when the
> panels are bent. 
> 
> I thought I read a recent posting on one of these
> forums that said 
> that a glass butt joint is stronger than a scarf or
> plywood butt - 
> anyone have authoritative knowledge about this? 
> <
>
> ______________________________________________________________________
> Only a few days left to file!http://taxes.yahoo.ca
>
>
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01930, Fax: (978) 282-1349
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>
> Your use of Yahoo! Groups is subject tohttp://docs.yahoo.com/info/terms/
>
>
>

I don't really think the flat spot is a big deal, just
fair it in with 410. Speaking only for myself, when I
first started boat building I was so cheap that any
process that used one ounce of glue one way or another
was a nightmare for me. But if you think of it from
the perspective house building, with the mass of
plaster of drywall fairing (not really fair) compound
used, why should I worry about a little compound. The
neat thing about a flat butt, is that all it needs is
slightly more compound over the flat, and it is fair.
Nothing like hiding the glass butt joints of a
non-recessed variety, or even some scarfs. It is an
additive process that requires no sanding. It also
goes together lightning fast, and preserves surface
alignment well.

My only complaint about butts is that they add quite
bit of weight, and they also add a lot of stiffness
that isn't usable in certain situations like
stressformed hulls. The key there is just to design
them creatively into the structure, where they will
pick up certain loads like main bulkhead loads.

It really isn't meaningful talking about glass butts
being stronger. Stronger than strong enough isn't
useful. You can build either style to be as strong as
the situation requires. I don't personally believe
that the Payson joint is as strong as a butt, or a
scarf (though it is lighter than a butt). So far I
have seen no published evidence to support its
strength. We have extensive testing of scarphs and
possibly butts, but none of the Payson joint that I
have seen beyond across the knee stuff. So far it has
been used mostly on planking which is generally a very
low stressed part of a boat. Until I see some tests,
or some successful use in higher load applications, I
am agnostic on the joint. Obviously there is some
schedule of let-in glass that would be super strong,
but just a grinder and a little light glass doesn't
persuade me we are replacing anything more than the
top veneers. That's enough in certain uses, but it
doesn't join or replace every fiber in the sheet as
scarphs and butts do.

--- prthober <prthober@...> wrote:

<HR>
<html><body>

<tt>
I used plywood butt joints on the panels for my Gypsy
and now wish I 
had scarfed them or had used fiberglass butt joints -
the plywood 
butts make a small but noticable flat spot when the
panels are bent. 
 
I thought I read a recent posting on one of these
forums that said 
that a glass butt joint is stronger than a scarf or
plywood butt - 
anyone have authoritative knowledge about this? 
<

______________________________________________________________________
Only a few days left to file!http://taxes.yahoo.ca

I used plywood butt joints on the panels for my Gypsy and now wish I
had scarfed them or had used fiberglass butt joints - the plywood
butts make a small but noticable flat spot when the panels are bent.

I thought I read a recent posting on one of these forums that said
that a glass butt joint is stronger than a scarf or plywood butt -
anyone have authoritative knowledge about this?

Cheers,

Paul