Wooden Boat Restoration Repair - WEST SYSTEM Epoxy

2.1.2 The skin
One of two basic types of skins covers the hull and deck structure, providing a
watertight membrane. Variations of planking provided the only alternatives for
covering wooden structures for thousands of years until modern adhesives made the
plywood skin more common and more durable.
Planking
Planking, whether carvel or lapstrake or a variation of these, is more susceptible to
flexing and leaks than plywood. The individual planks run generally parallel to the
centerline of the boat (Figure 2-2). They rely on fasteners at each framing member and
the caulking to resist shear movement (the slight sliding movement between planks
when a hull twists). In lapstrake planking, where each plank overlaps the preceding
one, clinch fasteners or screws along the lap serve to tighten the lap and resist shear
along the lap. Joints between carvel planks were traditionally sealed with cotton and/or
oakum caulking driven into the seams with a caulking iron and filled over with tar or
flexible sealant. Driving the caulking into the seams helped to stiffen the boat initially,
but caulking needed to be redone as the boat’s fasteners worked and the planking
swelled or shrank.
Double planking was usually two layers of carvel, or square seam planking, sealed
between seams and layers with shellac, paint, bedding compounds or glue.
Occasionally canvas, bedded in paint or white lead, would be used between the
planking layers.
Figure 2-2 Typical laid plank deck and carvel planked hull. Figure 2-3 Typical plywood construction.
Inspecting and Evaluating 10
Plywood
Plywood often replaces the many individual planks, greatly reducing the number of
joints that are potential sources for leaks. The framework for a plywood skin is similar
to the framework for planking. But, because the plywood is strong in more than one
direction, the framework for plywood covered structures can be much lighter. The
extra frames required in a planked structure to resist diagonal movement can be
eliminated. The plywood sheets are usually glued to the framework and held in place
with staples, nails or wood screws (Figure 2-3). Multiple layers of thinner plywood are
used on surfaces with compound curves where a single thick layer can’t bend enough in
both directions.
Plywood can also suffer from moisture problems. Moisture can penetrate plywood
through the end grain at seams, causing delamination. On the surface, moisture causes
the top veneer to check. Early plywood decks were protected with canvas laid in a fresh
coat of paint or canvas cement. The fabric may have offered some protection, but
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