Plywood scarf joints vs butt joints vs FG joints
Giuseppe Bianco
methods to make long plywood panels. I've experience on butt block only but
the FG splices seem the best (read simplest) method. Do FG splices leave a
bump on the hull? Are they strong?
KF4call
In my opinion, the reputation for difficulty that scarph joints have (at least
for 1/4 ply) is undeserved. I have built only one boat and I have virtually no
other woodworking experience. Using instructions provided by Tracy O'Brien (a
west coast US boat designer) I was able to produce 4 invisible scarph joints
without spoiling any of them.
In boatbuilder magazine (July/Aug 1991) David Carnell sets out a
specifications and a program of testing fg butt joints for strength. That
approach seems very positive as well, but I have no experience with it. Best
regards, Warren
Dave Carnell
Giuseppe Bianco wrote:
> I would like to get comments on advantages and drawbacks on each of these
> methods to make long plywood panels. I've experience on butt block only but
> the FG splices seem the best (read simplest) method. Do FG splices leave a
> bump on the hull? Are they strong?
Made right they are stronger than the plywood in bending. They are easily
faired so that they are invisible.
Dave Carnell <http://home.att.net\~DaveCarnell>
John Milne
rest of us?
tia
jm
Jane Sheridan & John Rankin
used information from Dave Carnell which I attach below. While it may be
easy enough to do scarph joints on 1/4 and 3/8 inch ply because there are
various ways to make quick and accurate cuts, on 1/2 inch ply it's more
difficult. In my case the scarphs were 54 inches wide and that's a lot of
sanding.
So I did fiberglass butt joints--no butt blocks, and when I tested a scrap
to destruction the wood broke beside the joint. The joint itself was
strong.
It is really easy to do if you follow Mr Carnell's advice. I did use tape
rather than cut cloth and it worked out perfectly, the only place I strayed
from his directions.
Re fairing. I used a little West fairing epoxy compound, and about 20
minutes of work, and then after I had glassed the panels you can not see the
join, but you can just, only just, appreciate a slight fullness with your
hand over it. When painted I expect it to be invisible, and I would be
happy to finish it bright.
I am so tickled with this technique I can't imagine doing it any other way.
PS did I say this was my first attempt at joining panels?
Herewith Dave Carnell's excellent advice.
Epoxy Knowhow
INVISIBLE BUTT JOINTS:
In 1978 I first used a plywood butt joint of fiberglass cloth and
epoxy resin to avoid having to make a scarph joint (not easy and loses
length)or use butt blocks (hard to work around). The joint is so thin that
careful feathering of the edges makes it invisible.
In 1986 I wrote about the joint in Small Boat Journal. About the
same time "Dynamite"Payson wrote in Boatbuilder about a similar joint
concept. Years later I discovered that Joe Dobler had used the principle
well before our publication, as had Jack Chippendale in England.
A piece of plywood bent around the side of a boat is carrying most
of the load in its outer and inner plies. The load (stress) on the convex
side (usually the outer) is a tension load trying to pull the wood apart.
On the concave side (usually the inner),
the load is compressive-the wood is being pushed together. The invisible
butt joint makes two pieces of plywood one by building a skin of fiberglass
and epoxy on each side. When you flex the joint, the load is carried
entirely by those two skins
you have built.
I made joints in various thicknesses of plywood and tested them by
breaking them in flexure with the maximum stress applied at the joint.
Joints that passed were ones where the plywood, not the fiberglass-epoxy
resin joint broke.
My design basis for invisible joints in plywood is: for 1/4"
plywood, 1 layer of 6 oz. fiberglass cloth on each side; for 3/8", two
layers on the top (outside of bend) and one layer on the bottom; for 1/2",
three layers on top and two on the bottom; for 3/4", four layers on top and
two on the bottom. Make the first fiberglass strip on top 2" wide and each
succeeding one an inch wider. On the bottom side make the first strip 2"
wide and the second one (if used) as wide as the widest strip on the top
side. After you lay up the joint cover it with a piece of 4 mil
polyethylene film and squeegee or roll it out. This presses the cloth
layers together and feathers out the excess epoxy onto the plywood. Peel the
film off after the joint cures and the surface is smooth and faired so that
very
little filling or sanding is required. If you use woven tape instead of
pieces cut from cloth, the selvage may make a ridge at each side of the
joint. A joint with a single layer of 6 oz. cloth on each side is about
0.020 in. thicker than the plywood at its thickestpoint and tapers off to
zero at each side. Two layers on each sideadds about 0.030 in.
at the thickest point. The joint in 3/4" plywood with four layers outside
and two inside is only about 0.045 in. thicker at its thickest point.
These joints are designed to use a minimum of material to get the
ultimate strength.
I would only make them with epoxy resin (not polyester resin) because:1)
epoxy bonds the glass cloth to the plywood in a stronger joint that will not
peel apart; 2) epoxy will always eventually complete its cure; 3) there is
no fire hazard with epoxy; and 4) there is less of a toxic hazard with
epoxy.
Originally, both Payson and I made the joint on one side and turned the
piece over to complete the joint. The turning over is fraught with danger
of destroying the joint that is very weak at that point.
I have gone to laying polyethylene film on a smooth surface, laying the
wetted out fiberglass tape (I use cloth to avoid the selvage) on that, epoxy
coating the face of the plywood that goes against that, laying the plywood
on the wet tape, filling any least void between the plywood edges with
thickened epoxy (this is critical, as any voids between the butting plywood
edges can make the joint weak), epoxy coating the upper plywood joint
surface, laying on fiberglass and wetting it out, covering with poly film,
laying on a smooth board, and weighting the assembly with concrete blocks.
In fact, the last time I did it I laid up a sandwich of two 16' by 20"
pieces for the side planks of a sailing skiff and cured them all in one
operation.
If you are making joints in plywood thicker than ¼", make the bottom
side of the layup the one with the fewer number of fiberglass strips.
EPOXY IN A NUTSHELL
This is a distillation of my experience in using epoxy for 30 years
and improving my techniques. I started using epoxy for boatbuilding in
the 1960s. This was before Gougeon came out with their West® system. I was
using generic epoxy from Defender and an amine hardener that was mixed 1:10
with the resin. Later I switched to Epon® resin and Versamid® hardener from
a surplus outfit in CA. This was a 2:1 mix and far easier to use.
Then, as now, all resins and hardeners were made by a few major
chemical companies. The companies selling products at retail develop their
formulations from commercial products.
Resin and hardener are ingredients that have to be mixed in the
correct proportion to cure to a solid with the desired strength and
hardness. If you want the mix to cure faster or slower, you pick a
different hardener. You don't change the mix ratio.
Epoxy is far superior to polyester resin because it sticks to just
about all materials, while polyester is not even a reliable adhesive for
laminating glass cloth to wood.
If you mix your epoxy in the correct ratio it will eventually cure.
If the catalyst you add to polyester does not kick it off, it will never
cure.
Epoxy resin and hardeners have shelf lives of many years. I am still
using a two-part surplus military epoxy putty that was manufactured almost
25 years ago. The only exception to unlimited shelf life I have found with
epoxy resins is that the hardener for 1:l mix systems thickens and cannot be
used after about a year.
GLUING & LAMINATING:
The most important use of epoxy resin is as glue, including gluing
fiberglass to wood. Its advantage over most other glues is that it will
fill gaps; in fact, there always has to be some gap. If you clamp too
tightly the epoxy will be squeezed out so that the joint will be weak.
Adding filler to epoxy used as glue makes stronger joints, perhaps because
the filler keeps too much resin from squeezing out of the joint. One-inch
boards edge glued will break apart in the joint when it is
flexed; add about 20% pulverized limestone and the glued joint breaks in the
wood. I add about 10% limestone to resin when laminating fiberglass onto
wood, also.
FILLERS:
Fillers are added to epoxy resin to make putties for two kinds of
uses that have greatly different requirements. Those used for structural
joints alone or in combination with fiberglass should be as
strong as possible. Putties used for filling and fairing must sand easily.
The best filler for structural uses is pulverized limestone (flour
fine, not gritty as ground limestone is). It mixes to a putty that doubles
the resin volume and is dense and strong. It is universally available as a
fertilizer material at under a nickel a pound. It is difficult to sand.
Portland cement is pretty much equivalent. Talc, another mineral, is almost
as strong and sands easily. It also
is thixotropic (the putty does not flow, but will spread). It is available
from fiberglass supply houses at around a dollar a pound.
For filling and fairing applications hollow bead type fillers sand
most easily because they are hollow and break. There are three kinds of
beads: thermoplastic (Microlight®) which can soften with heat; phenolic,
which are usually dark-colored; and glass (Scotchlite®, which are white.
The glass beads make the lowest density filler and are the lowest cost.
White wheat flour from the kitchen is a pretty good filler for finishing
putties.
STRUCTURAL JOINTS:
A fiberglass-epoxy butt joint of plywood can be as strong as the
plywood itself. See "Invisible Butt Joints" above.
Right-angle joints in ¼" plywood for rowing seat boxes, etc. can be
made with just a ¼" radius bead of epoxy putty on the inside of the joint.
I tack such a box together with brass brads and then make the epoxy fillet
joints.
For angle joints such as chines in ¼" plywood a 1½" fiberglass strip
laid over a ¼" radius epoxy fillet on the inside and a 1½" strip on the
rounded outside edge gives a joint that breaks by pulling the plywood apart.
Bulkheads secured by a 1½" glass strip over a ¼" radius epoxy fillet on each
side fail in the plywood. All joints must have the weave of the glass cloth
filled smooth for maximum strength.
Many designs specify much more glass than needed. Make up short
specimens of your joints, cure them, and test them-in a vise, by standing or
jumping on them, or by running your truck over them. If the joint holds and
the material breaks, your joint is strong enough.
SAFETY:
The principal hazard of working with epoxy resins is from skin
contact. The hardeners are the offenders. As a general rule, the lower the
mix ratio, the less the hazard (2:1 is less apt to irritate than 4:1), but
you should avoid all skin contact and wash thoroughly after any contact.
Wash
thoroughly before eating, drinking, or going to the bathroom. Gloves and
clothing help protect you, if they are clean.
ENCAPSULATION:
You will note that I have not mentioned epoxy encapsulation; i.e.,
coating both sides of everything with several coats of epoxy resin. It has
no advantages and is a waste of money and time that adds useless weight. It
won't turn lauan underlayment into marine plywood, though it will make it
cost nearly as much. It does not keep the water out of the wood boat that
lives in the water and a dry sailed boat doesn't need it.
KF4call
I went out to the garage/ workshop/storage area today to try to find the
O'Brien Scarphing instructions. I could find every other document set by him
EXCEPT the scarphing instructions. That probably means I put them somewhere
"safe" and will never see them again.
As I recall the instructions call for an easily made jig to use on the
circular saw and directions for maintaining the alignment while fastening the
sections. I will continue to search for the instructions and provide more
informaiton when available---Warren
John Milne
Thanks for the garage search. You have the same idea of 'safety' as me. I've
no idea why that happens. However - for the benefit of anyone else is
reading this and until the O'Brien material turns up - John Kohnen wrote to
me pointing out a couple of good articles on plywood scarfing his nautical
and
boatbuilding page:
Giuseppe Bianco
the
> circular saw and directions for maintaining the alignment while fastening
the
> sections. I will continue to search for the instructions and provide
more
> informaiton when available---Warren
Actually such a jig exists off the shelf. It's manufactured by Gougeon
Brothers and named "Scarffer", and if I remember well is #875 of their
catalogue, sold at about 50 $. It is supposed to easily cut scarfs on
plywood up to 9 mm thick when attached to a standard circular saw. Sam
Devlin says good things about it on his book. By the way, the responses to
my original question are about 50% in favour of scarfs and 50% for the FG
splices. Zero votes for plywood butt joints. I'm surprised a bit. Glen L.
Witt's says that butt joints are as strong as scarfs, and certainly much
easier and fast to do...
Chris Crandall
: By the way, the responses to
: my original question are about 50% in favour of scarfs and 50% for the FG
: splices. Zero votes for plywood butt joints. I'm surprised a bit. Glen L.
: Witt's says that butt joints are as strong as scarfs, and certainly much
: easier and fast to do...
They all have their advantages and disadvantages. I like the butt joint,
myself, in traditional plywood boatbuilding. In stitch-and-glue, it would
be a nightmare. In trad plywood (either pw on frame or tack and tape), a
butt joint is an easy, powerful, and even useful tool. You can use the
butt joint to line up a frame, for example.
Gary Zwissler
deck pieces, laminate the butt block to one piece, leaving it stand
proud where its matching piece glues to it. Install the deck piece with
the laminated butt block, then line up and lay the matching deck piece.
Clean and easy.
Gary Z.