How to Repair
plywood boats is, fortunately, fairly
both with and without epoxy
has been a popular
method for boat building for quite a few years and it has produced many
and beautiful boats.
Unfortunately, as with any method, there comes a time when repairs are
Fortunately, the procedure is not that difficult.
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“When a great adventure is launched with a powerful thrust, fatigue in the muscles and doubts in the mind are swept away by a fullness that moves life along like a breath from the depths of the soul.” (Bernard Moitessier)
Many home builders have been convinced that it is possible to use
ply sealed with epoxy.
This type of
is laminated with
non-waterproof glue and will have significant voids as soon as the
coating is damaged in any way it will soon start to rot and delaminate. plywood
If you suspect that a non-marine grade of ply has been used you should
replacing the whole sheet rather than patching.
But don’t despair, it is actually quite a simple job to replace
On a small boat replacing the entire hull bottom is as easy as using a
The first objective is
to identify and cut away
all the damaged plywood.
If you mark out and cut
the offending area along
straight lines it will be easier to cut a replacement patch.
The particular shape
doesn’t really matter, however a square or rectangle will be
easier to cut and replace.
It is normal in plywood
construction for the ply
to be epoxied to any frames, stringers and bulkheads.
To repair plywood you
will have to cut around
these then remove the glued on ply later.
As it is easier to cut
from the outside the hull,
the positions of all the frames etc will have to be marked on the
From the inside drill a
few fine holes along side
the framing so thy can be seen from the outside then draw a line to
join them up.
To avoid cutting through
or damaging any frames,
stringers or bulkheads use a skilsaw or router with the depth set to
the hull thickness.
If the boat has been
skinned with fiberglass, you
will need to use a carbide bit in the router.
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Now the ideal patch to
repair plywood with would
be one with beveled (scarfed) sides which would fit perfectly into a
Easy to say but very
difficult to execute well
It will be much easier
to get a good fit if the hole and patch are cut with edges at right
angles to the surface.
You will probably have
to use a chisel or
Japanese saw to finish and trim the corners.
This same method can be
used to cut away whole
chines, although on a small boat it may be simpler to replace the full
sheet rather than patch.
Where the joints have
been made by filleting, it
will be necessary to cut away the old fillets so that the joint can
again be stitched and glued.
If the damage was caused by
rot, you might also want to think about
the surrounding wood, to kill off any stray rot spores.
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If you are using this method to
insert a patch to repair plywood it will need to be supported on the
So, the inside surrounding
edge of the hole will need to
be sanded back to bare wood.
If it is possible cut the
vertical edges of the hole so
they center on underlying frames.
Once the damaged ply is cut
away the remaining ply on the
frames etc will have to be chiseled off.
A belt sander will get rid of
most of the glue on the
exposed frames and tidy up any edges.
But before you think of gluing
in a patch the wood will
require preparing properly.
A saw blade turned on its side
can be used to scrape
gashes into the wood to help create a good mechanical bond.
It may be time to get out your
right angle sander/grinder
with some 24 grit and take everything back to bare wood.
Then reseal all the wood with
If any of the wood has been
contaminated with oil or grease, this will have to be thoroughly
If the oil has soaked in it
will require repeated washing
with degreaser and solvent over enough time to allow the oil to come to
Warming the wood might help
bring the oil to the surface
and fine sawdust will help to absorb it.
But it will take time for all
the oil to come out before
it can be successfully glued.
If you are replacing large
areas on a small boat, the
boat will need checking for any twist before gluing up.
This can be done by using a
straight edge and eyeballing
to make sure everything is true.
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The Repair Plywood
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A patch to
repair plywood should be the same thickness as the original plywood.
Make a template of the cut-out
as a pattern for the
Cut out the patch so that it
fits the hole in the hull
perfectly and lies flush with the surface of the hull.
Now if this patch were just
edge (butted) glues in place
it would not be very strong.
So, it will need to be
supported with but
Before assembly the patch and
backing blocks should be
given several sealing coats of epoxy.
In the case of a small patch
to repair plywood, this
could be a backing plate larger than the patch which, has a good
Where the patch and its
backing plate/but blocks can be
inserted from within the hull these should be glued up on the bench
can be tightly clamped while the glue cures.
Use masking tape to make
up of excess glue easier.
If you can’t get to it from
inside, then the patch
and backing supports will have to be assembled on site.
As it will be impossible to
clamp the patch, through
fasteners will have to be used to hold the patch tightly in contact
butts and the butt blocks and the hull, all the way around.
Again I recommend using
masking tape to make the clean up
of squeezed out epoxy easier.
The fasteners can be removed
once the epoxy is set and
the holes filled.
Though personally, I would
leave them so long as they
were not obtrusive.
When the epoxy is set, you can
then begin fairing and
finishing and glassing.
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method described above is fine to repair plywood on a flat area.
However, on the hull of a boat
you will most likely have
to patch areas where the original ply has been bent.
Bending the original large
sheet of ply was probably
But bending a small
replacement patch will be virtually
If you were to use solid wood
you could use a piece that
was thick enough to allow for fairing after installation.
Unfortunately, with ply if you
try this you will cut
through the plies which will substantially weaken it and open them up
possible water ingress.
The solution is to laminate
the patch using several layers of thinner ply to gradually build it up
Cut out the damaged area as
Then fasten several vertical,
behind the opening.
These will act as formers for
the fiches as you lay
Treat the end batons as but
blocks which are going to
remain in position.
Any temporary batons you may
wish to remove later should
be coated with wax or wrapped in polythene to prevent them sticking.
Again, because of the
impossibility of clamping you will
have to use through fastenings to hold everything tightly in place as
When the patch has cured, the
temporary batons can be
removed and horizontal butt blocks laminated in place.
Remember that when laminating
with epoxy (or for that
matter any time you repair plywood) using more is not necessarily
that applies anytime you repair plywood.
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Small Boat Repair.
The following video shows how
easy it is to repair small plywood boats by replacing panels on a boat
has been built over a wooden frame.
Mark the position of the
frames by drilling as described
Cut away the old ply then
proceed as if building from
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Rot repair methods Great thorough article on repairing rot.
You may find this article on repairing localised rot using epoxy.