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For most craft where a durable deck
surface is required, teak provides the ideal properties. Teak can
enhance the visual appearance of any deck and can provide good grip
characteristics especially when wet. In the dry state teak is easy to
work and takes adhesives well. Teak is classified as a 'heavy' hard wood
with a density of approx 930kg/m3. It shows exceptional grain strength
and stiffness, along with excellent wear and durability characteristics.
Deck Thickness
Deck thickness for any type or size of
boat is largely determined by structural requirements, the
considerations being building material, frame spacing and target weight.
On wooden yachts, even those of 50ft. in length, decks are seldom more
than 19mm thick and 12-13mm is considered sufficient for most boats up
to 40ft. long.
Most
alloy yacht decks are finished in teak and a 60ft. hull may have a
10mm-12mm deck laid over 3-4mm plating. GRP decks of many production
boats commonly incorporate a lightweight core. Usually a 15-25mm balsa
core is sandwiched between GRP skins, each 1-2mm thick. Teak is often
used to finish the GRP deck surfaces particularly on the more up-market
craft. Teak is laid using one of three methods:
Pre Fabricated - method used by Teak
Marine where decks are created from a template. Fabricated on a lofting
floor off site. Crated and shipped to location where it is installed at
an average rate of about two square feet per man hour. Most efficient
and easy method. See other article on web site for how to install.
The traditional method - whereby the
teak is sawn into strips up to 19mm thick and applied directly to a
metal, GRP or wood surface.
The thin veneer method - here the teak
is only 5-6mm thick (before finishing) but the total deck thickness is
made up by laying on to a plywood base (perhaps 8-10mm thick) using
adhesive. This method saves weight since only a thin layer of teak is
used. However, life expectancy is still over 15 years even with heavy
use.
Adhesives
A
two-part solvent-free epoxy adhesive is the most effective way to bond
teak, whether in veneer form or thicker section.
The
same adhesive should be used for all bonding operations but if the thin
veneer method is employed, an 8-10% addition (by volume) of Graphite
powder will give the necessary colour for the adhesive that is seen
between the planks. This addition will also give the glue some
ultra-violet protection.
General Teak Preparation
Before
any bonding commences, determine the layout pattern of the teak strips.
They can be either parallel to the centerline or parallel to the
sheerline. The former method is more commonly used since the grain of
the teak will be oriented in the most effective way to resist deck
compression loads; it is also the quickest procedure and probably the
only choice if the traditional planking method, using thick sections, is
used.
The
teak strips should be 'quarter sawn', a technique which ensures that the
edge grain is on the flat surfaces. Teak cut this way not only wears
more slowly and more evenly than 'plain cut' teak but also has the least
expansion and contraction. Strips 40-50mm wide are the normal width for
ease of handling and the length should not exceed 3m.
When
cutting teak for the thin veneer method the strips should be cut no more
than 6mm thick so that after sanding the finished thickness is 3-5mm. If
the thickness exceeds this figure the inevitable expansion and
contraction which occurs, and which is normally constrained by the epoxy
adhesive, may exceed the bond strength of the adhesive causing it to
split at the wood/adhesive face.
Preparation of Surfaces
1. Teak
After cutting the teak all bonding
surfaces should be sanded with a coarse abrasive paper (40-60 grit)
across the grain in order to obtain a good key, followed by cleaning
with liberal quantities of a fast epoxy solvent).
2. Other Wood Surfaces
All screw holes and other surface
imperfections should be made good with epoxy filler. Polyester-based
fillers should be avoided as they may inhibit the cure of the epoxy
adhesive. After sanding with a coarse grit paper the surfaces are
cleaned thoroughly with a fast epoxy solvent). Ideally the wood or
plywood surface is coated with 2 coats of an epoxy system using a fast
hardener to give adequate protection against moisture. If possible both
sides should be coated. After allowing at least 24 hours to cure the
surfaces are wiped with cleaning fluid and sanded using 60-80 grit paper
prior to bonding the teak. Particular attention should be given to
sealing all end-grain if plywood is used as a base.
3. GRP
All surface coatings must be removed
(paint, non-skid coatings, etc.). If the deck has a moulded-in tread it
should be ground away with a coarse sander down to the laminate. A small
angle grinder with a flexible sanding pad is ideal. Before bonding the
teak all sanding dust should be removed and the surface cleaned with a
fast epoxy solvent. Acetone is not recommended as it may contain
undesirable contaminants. If bonding to a new GRP surface it is
important to determine whether the laminate is fully cured, since epoxy
will not bond satisfactorily to a less than fully cured polyester
laminate. From the time the boat was first started the elapsed period
for this is usually between 2 and 4 weeks at normal temperature. A small
preliminary test is advised to ascertain when a good bond can be
obtained.
4. Aluminum
It is important for the best adhesion
that all bare aluminum is pretreated with an Aluminum Etch Primer before
bonding. It is likely that some filling will be necessary which can be
done using the chosen epoxy system with the addition of glass bubbles
and colloidal silica. After sanding and solvent cleaning using a fast
epoxy solvent, bonding can commence. If the deck shows any significant
distortion it is simpler to adopt the thin veneer method using a plywood
base which will serve to fair the surface.
5. Steel
Steel surfaces coated with an epoxy
holding primer should be coarse sanded and cleaned thoroughly with a
fast epoxy solvent.
Application Methods
1. Conventional Decking with Teak
This method of construction should not
be confused with 'traditional' method of planking, where the planks are
laid directly over deck beams and notched into the king plank down the
centre and meet a covering board down the sides. Here no adhesive is
used (the only fastening method being screws or nails) and the seams are
caulked and payed with a marine stopper or hot marine glue. The
conventional method differs in two important respects. Firstly, a
flexible caulking compound is used between the seams and an adhesive is
used to fasten the strips down to a solid base (which may be plywood,
GRP or metal). Secondly, the teak strips can be thinner (but not as thin
as the veneer method), perhaps 19mm thick, and they are screwed down to
the base material (in the case of wood) or bolted (metal or GRP). Since
plywood is lighter than teak, some weight saving can be made over the
traditional method and the deck is likely to be stronger and leak-free.
On production GRP or metal hulls a flexible elastomer adhesive sealant
such as 'Sikaflex' is sometimes used instead of epoxy adhesive to bond
the teak directly to the GRP substrate and the same product is used to
lay the seams. Being apparently no less effective than using the more
rigid adhesive technique it may be more convenient for manufacturers,
since the preparation is far less intensive. After all of the teak
planks have been fastened into position the gaps between are payed with
a flexible sealant, either a polyurethane elastomer type such as 'Sikaflex'
or polysulphide rubber type such as 'Arbocol'. Teak plugs should be
glued into all screw or bolt holes and left proud of the surface.
2. The Thin Veneer Method
This method uses a suitably thick
plywood base, pre-coated with epoxy system, upon which are fastened teak
veneers no thicker than 6mm. An epoxy adhesive mix, pigmented black by
addition of graphite powder, is used to both glue the strips down and
fill the gaps between the strips. The strips are usually held in
position by stapling every 10-20cm and the gaps between the strips
maintained at a constant width (usually 3-4mm) by the insertion of
spacing pieces, usually of plastic.
The use of a temporary clamping bar
will ensure the minimum number of staples piercing the teak veneer. The
bar, usually of wood approximately 10mm thick and protected with
polythene, is laid at right angles across a number of strips at a time.
Staples are then driven through it into the plywood base at a spacing
corresponding to the gaps between the teak strips, thus bedding all the
strips into the adhesive at the same height. When the glue has
sufficiently cured the bar is removed. Sufficient adhesive should be
applied to the teak and bare plywood to squeeze out and fill the gaps
between the teak strips.
Finishing
When
the sealant and the adhesive mix has cured all surfaces are sanded with
50 grit paper. On large open surfaces a floor sander is best but a hand
operated belt or rotary sander is adequate. With the conventional method
of teak decking, only the minimum of wood is removed but in doing so the
excess flexible sealant is removed and the teak plugs are faired in.
With the teak veneer deck, sufficient teak should be removed by sanding
to bring the thickness down to a finished thickness of no more than 5mm.
Any remaining filling of the gaps can be completed at this stage with
additional graphite epoxy mix and a final sanding with 80 grit paper
used to finish the deck. |