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Sheet
Plywood Construction
As
plywood comes in large dimensionally stable sheets it is possible
to plank large areas of a hull quickly and easily. However plywood
can only be bent into a compound curve with great difficulty, and
there are limits to how tightly it may be bent. Therefore almost
all plywood boats have a join, known as a chine, between the bottom
and the side panels of the hull. This lead to many early single
or hard chine boats having rather ugly box like hulls.
Multichine
construction allows a plywood hull to more closely approach a round
bottom hull. Combining plywood with other construction methods,
such as cold moulding or strip planking, eliminates chines and allows
almost any hull shape.
Plywood
on Frame Construction
Plywood
boats are sometimes constructed upside down on a jig consisting
of both permanent and temporary frames and bulkheads. Chine logs,
sheer clamps, keelson, and stringers are set into the frame and
faired to the hull shape. Plywood sheets, cut approximately to shape,
are then nailed, screwed or glued to this framework. Shaping of
the edges of the sheets at the chines will be necessary to ensure
the sheets butt up against each other neatly. Once the planking
is complete the boat is turned right side up to finish the decks,
superstructures and interiors.
Particular
care must be taken to seal the edges of plywood sheets, as the end
grain readily soaks up moisture. This could lead to the plywood
swelling, or development of rot. The edges of chines also need to
be protected from wear and damage. Fibreglass tape can be laminated
over the edge, or a batten can be fixed along the length of the
chine to act as a rubbing strip.
A
plywood boat will be greatly improved by using Bote-Cote throughout
its construction. Coating the sheets helps fill the grain and stops
cracking and crazing of the surface veneers. Several coats of Bote-Cote
applied to the end grain will ensure that it is permanently sealed.
Bote-Cote thickened with Gluing Filler not only fills any gaps,
e.g. between sheets along the chines, it is also a fully structural
glue which strongly bonds planking and frames into a uniform engineered
structure, and finally is used as a fillet to neaten and strength
all joints. Bote-Cote is used with fibreglass tape along the chines
to seal and strengthen them, and to protect them from abrasion.
It is often advisable to fibreglass sheath larger boats, at least
up to the waterline, and again the glass cloth is laminated to the
hull using Bote-Cote.
Stitch
and Glue Construction
Stitch
and glue is a derivative of plywood sheet construction. The major
advantage over conventional plywood construction is the elimination
of framing, which thus speeds construction.
Plywood
sheets are cut accurately to predetermined sizes and shapes, and
small holes are drilled in the edges of the plywood along all the
joins. Copper wire or nylon electrical cable ties are threaded through
each matching pair of holes and tied off, drawing the edges of the
plywood together along the joins. The boat is thus drawn up into
shape, and can then be glued by filleting the internal joins to
bond the whole structure together. The joints are then fibreglass
taped both inside and outside to complete the hull construction.
All fitting out takes place as for any other construction method.
Most
early stitch and glue designs used polyester resin for fibreglassing
the joints. Far stronger and more long lasting joints can be obtained
using Bote-Cote, and in addition, Bote-Cote would now be used for
soak coating all the plywood to improve its qualities.
Stitch
and glue construction is most common in dinghies, but there are
some designers who have extended its use right up into cruising
yachts.
Tortured
Ply Construction
It
is very difficult to bend plywood into compound curves (i.e. in
two directions at once), and it can only be achieved with thin plywoods
up to about 5 mm thick. "Torturing" the plywood in this manner stiffens
it, thus reducing the need for internal frames and stringers, so
long as the ply is held in its tortured shape. It is a popular method
of construction for small to medium catamaran hulls and kayaks.
Precoating
of plywood for torturing may be omitted, as Bote-Cote stiffens the
ply and may make it more difficult to bend. For particularly complex
profiles, the plywood can be pre-wet with water, which makes it
easier to bend around the shape. It can then be allowed to dry out
before Gluing permanently.
Alternatively
two or more layers of thin ply can be laminated together using a
vacuum bag to apply enough pressure to force and hold them in the
curved shape while the glue sets.
Constant
Camber Construction
Constant
Camber is a hull moulding technique developed in USA by multihull
specialists John Marples and James Brown. The hull is designed to
be formed by joining a number of separate panels all made on the
same curved mould, the panels being formed by laminating alternating
layers of veneer or plywood strips to the necessary thickness and
then consolidating them under a vacuum bag. Simply put, it makes
curved plywood panels that are joined together in the same way as
making any plywood boat. The method utilise only one mould, eliminates
framing, and minimises finishing and fairing work.
STRIP
PLANK CONSTRUCTION
This
method is a development of carvel construction. The hull is planked
with narrow, edge fastened strips of timber, eliminating the need
for caulking. Prior to the introduction of epoxy glues, very accurate
tight fits were needed between the strips. Waterproof glues to edge
glue the strips made the process easier, and the use of modern wood
epoxy systems such as Bote-Cote has turned this method into one
of the most convenient methods for planking hulls.
The
size of the boat and the shape of the hull govern strip size. Strips
must be small enough to bend around the curves in the hull, but
thick enough to give the hull adequate strength. The combination
of these two factors is almost the only limitation on the size and
shape of hulls that may be built by strip planking.
Temporary
frames are set up on a strongback, at close enough intervals to
hold the strips in correct shape and position, and the first strip
is tacked lightly to these frames. The starting point on the hull
should be chosen carefully so as to minimise the amount of edge
bend which can develop as strips are added progressively. The lie
of later strips should be tested by measuring off appropriate spaces
along the edge of each frame from the starting strip and fitting
a test strip. Adjust the position of the starting strip to obtain
the least edge bend, combined with the best appearance if the boat
is to be clear finished (strips parallel to the sheer look best).
Some strips may be tapered at the ends to allow for changes in girth
between bow, amidships, and stern, or short tapered strips called
'cheaters' may be inserted to take up greater girth and avoid edge
bend. It is also possible to start planking from two directions,
e.g. the sheer and the keel, and to make a smooth join of the planks
at the meeting point.
Traditional
Strip Planking
Each
strip is glued and edge nailed to the preceding strip. For this
technique, permanent internal frames are required to provide the
necessary transverse strength to the boat, as timber has relatively
low cross grain strength compared with that along the grain. This
method of strip planking has been almost totally replaced.
Modern
Strip Planking
This
is one of the most popular boat building techniques, it is suited
to both amateur and professional builders, and creates a strong
well shaped hull with very few limitations of shape and size. The
main timber used is Western Red Cedar, which has both a high strength
to weight ratio (along the grain), and a low density. This allows
strips to be comparatively thick without adding excess weight, which
creates a stiff hull at minimum weight. Other timbers can, of course,
be used. Strip planking is also used for making other curved structures
such as hatch and cabin tops, and even light weight masts and spars.
Instead
of edge nailing strips together and onto frames, a fibreglass sheathing
both inside and outside the hull is used to provide transverse or
cross grain strength, replacing ribs and frames. The glass sheathing
also strengthens the surface against damage. The structure is, in
effect, a sandwich panel, that is the cedar is a stiff core within
two hard, strong layers of reinforced epoxy plastic. Unlike other
core materials such as plastic foams, the cedar contributes considerable
strength and stiffness to the composite. Internal structures such
as bulkheads, thwarts, bunks, benches and cupboards would all be
structural, and contribute to the strength and stability of the
hull. On smaller boats such as canoes and dinghies, no other internal
framing is required to provide adequate strength to the boat, since
the composite shell of the strip planked boat has such high strength
and stiffness.
Each
strip is only nailed temporarily to the building frames, with a
gap filling glue mixed from Bote-Cote with light weight Fairing
Compound being applied to each strip as they are assembled. Purbond
adhesive is an alternative, but the gaps will require later filling
prior to fairing. When the process is complete, the nails are pulled,
the holes filled, the outside of the hull faired and the glass is
laminated on. The weight and type of glass depends on the size of
the craft and should be specified by the designer. The hull is now
quite stable and can be removed from the building frames to allow
the process to be repeated inside the hull, followed by fitting
out.
The
problems with strip planking lie in getting the edges of adjacent
strips to lie together as they are bent and twisted around the hull.
Failure to do this means a lot of time spent in removing the resulting
unevenness during the fairing process, and the hull gets thinner
as the wood is sanded away. Strips with concave and convex edges
are sometimes used to obtain a closer fit between adjacent strips,
however they can still slip sideways under bending stress. The builder
must spend a lot of effort forcing them into alignment between the
frames, or fair out the misalignments afterwards.
Both
of the above types of strips must be scarfed before use to attain
the lengths necessary for any boat longer than 5 or 6 metres. This
means a lot of additional work. If they are not scarfed, hard joins
will be evident in the planking.
The
best solution to the problems of misalignment, scarfing, and tedious
amounts of fairing is 'Quickstrip' planking. These planks are milled
with a modified tongue and groove profile, which absolutely ensures
that each adjacent strip is locked into the previous one, but at
the same time allows each to twist relative to its neighbour to
accommodate the changing shape of the hull.
Using
BoatCraft's Purbond polyurethane adhesive for edge Gluing, instead
of Bote-Cote epoxy even further enhances the process. Since Purbond
is a single pack glue, it can be applied more easily to the edge
of each strip than a filled epoxy mixture, and is more economical
as it expands into the small gaps in the T & G profile while
it cures. Planks may be joined longitudinally by simple butt joints
either cut square or preferably at 45 deg. The combination of Purbond
with Quickstrip has reduced the time involved for the strip planking
technique by a factor of two or more.
LAPSTRAKE
CONSTRUCTION
Lapstrake
is the modern equivalent of clinker construction, where each successive
plank around the hull overlaps and is fastened to its predecessor.
Traditional hulls used profiled solid timber planks, steamed to
shape, with ribs fastened in to provide cross strength. Vast quantities
of copper nails, clenched and roved, were needed to complete the
fastening of the planks and ribs. The hull relied on water absorption
and swelling to seal it properly, so it was necessary to use only
those prized timbers that had intrinsic rot resistance.
Modern
lapstrake utilises planks cut to shape from readily available plywood
panels, scarfed to length, which do not require steaming to bend
them. The plywood has adequate cross strength, so ribs are no longer
necessary. Modern adhesives, such as Bote-Cote, permit all the planks
to be bonded together, to prevent leaks and eliminate the metal
fastenings which eventually caused weakness and rot in the planks
of the traditional boat (the 'leaky old clinker' syndrome). Bote-Cote
seals and encapsulates the timber to eliminate rot development.
A
lapstrake boat is built on exactly the same temporary frame structure
as described above for strip planking, and the hull is planked up
similarly. The differences lie in there being many fewer planks,
each one being trimmed to shape ('spiled') before fitting, and the
planks are glued along a 12 - 20 mm overlap instead of being edge
glued.
In
lapstrake boats, fibreglass sheathing is not required, indeed it
is difficult to make fabric fit around the many steps or laps without
lifting away. The end result is a perfect replica of a classical
clinker boat, strong but light in weight, with lasting qualities
at least as long as the original one, and needing far less maintenance.
COLD
MOULDING
Cold
moulding begins with construction of a male mould, which can be
solid or consist of closely spaced stringers. This is faired before
hull construction begins. In comparison with strip planking, this
makes for a longer less productive process for a one-off hull, however
it has advantages if several identical hulls are to be built.
Strips
of veneer are laid over the mould and stapled into position. Each
strip is shaped to fit snugly to the previous one, and edge glued
to it. After the first layer of veneer has been cured a second is
laid diagonally over it, and glued to the first, again using thickened
Bote-Cote. Staples in the first layer must be removed as the second
layer is fitted. Between three and six layers of veneer are applied,
depending on the design and thickness of the veneers. The final
layer is usually sheathed externally with Bote-Cote and fibreglass.
In
common with strip planking, cold moulding shares the advantage of
permitting frameless construction to produce a light, stiff hull.
Indeed the two are sometimes combined, two diagonal layers of veneers
can be laminated over a strip planked hull instead of sheathing
with fibreglass for transverse strength.
A
further variation of cold moulding is to cut the veneer strips from
plywood rather than solid timber. This is more economical, it reduces
the number of layers to be applied, and the plywood strips are more
easily handled than straight grain veneers.
SANDWICH
CONSTRUCTION
The
practice of using a lightweight core sandwiched between two laminates
is well established. Separation of the two laminate skins greatly
increases the stiffness of a panel, and the reduced weight of the
core enhances the stiffness to weight ratio of the composite.
The
core material usually consists of End Grain Balsa, closed cell plastic
foams, or paper or plastic honeycombs (listed in order of increasing
cost). Glass fibre fabrics are the commonest reinforcements for
the skins but many other reinforcements are used. Other combinations
are possible, for instance as pointed out above, strip planking
is a sandwich panel with its cedar core being stiffer and stronger
than say PVC foam.
Wood
fibres exhibit characteristics of strength, impact resistance, and
resilience, which are equal to and often superior to synthetic fibres.
It is not surprising therefore to find wood fibres replacing synthetic
fibres in some forms of this construction. One technique involves
laminating a core between two layers of plywood.
BoatCraft's
Cortech panels are made from End Grain Balsa, which has very high
compressive strength, bonded between sheets of light weight Gaboon
plywood. Such panels weigh much less than solid plywood, and are
many times stiffer. They are particularly useful for large deck
and cabin top panels, such as in catamarans, where they contribute
a great deal to the stiffness and strength without adding very much
weight high up on the boat.
Sandwich
panels of any shape can be laminated over a relatively simple mould.
The individual layers can be fastened in place while the glue sets
(like strip planking), or the whole assembly can be laid down at
once in a press or by vacuum bagging to consolidate the layers.
Sandwich construction has the greatest promise for reduction of
hull weights for all methods of construction.
Marine
Timbers
Factory 3,
10 Rutherford Road, Seaford 3198 (facing
Keppler Circuit)
Ph: 03 9775 0006 Fax: 03 9775 1776 |