The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Keels, Stems, and Sheer Clamps
This chapter discusses keel and stem construction, and includes information on keelsons, centerboard and
daggerboard cases, and laminated sheer clamps. It contains only minor changes from earlier editions.
In traditional construction, the keel is the backbone or
major structural element of a boat. It’s usually large and
strong, and ribs and frames are attached to it to form
the skeletal framework over which planking is laid.
In the composite construction methods we use, however,
the integrity of the hull structure is less dependent on
the keel. Loads are distributed so efficiently throughout
the boat that keels are sometimes almost unnecessary.
<strong>The</strong> stem, a second critical element in older designs,
has lost its importance to a lesser extent. Strong stems
protect hulls when they strike objects at sea, so we
continue to use them to minimize damage.
In addition to describing keel and stem construction,
this chapter discusses keelsons, centerboard and
daggerboard cases, and sheer clamps. With the
methods for laminating hulls detailed in later chapters
of this book, we plan these components and some
interior features before planking.
<strong>The</strong> Keel/Stem Assembly
<strong>The</strong> keel, which serves as a natural joining point for the
planking on either side of it, is often the key to making
modern construction methods practical. A keel also
contributes to overall hull strength by distributing high
point loading over large areas of skin. This function
may be more valuable out of the water than in it: when
a boat is hauled, a strong keel may help to support its
entire weight at several points.
<strong>The</strong> size and shape of both the keel and the stem vary
widely from boat to boat and depend on structural
needs and construction techniques. We usually build
the two parts separately, join them with a scarf joint to
form a keel/stem assembly, and then install the unit in
the set-up. A boat with very gentle curve in the stem
could conceivably have a stem and keel made in one
lamination, but we have never found this to be practical.
CHAPTER
19
We make them up separately because keels generally
have only gentle curves, which can be formed with a
few layers of wood, and stems often have at least one
severe curve, which requires many laminations.
Laminating a single keel/stem unit with many layers to
accommodate the curve in one area of the stem would
waste time and material. Most boats have transoms, so
stems are only required at the bow. A double-ender will
need stems at both ends of the keel, however, and in
this situation a three-piece keel/stem complex may be
necessary.
If the keel and stem are to be anywhere near as strong
as the hull skin, special attention must be paid to their
construction. <strong>The</strong> keel/stem unit usually joins the two
halves of the hull skin and while it is unlikely that this
joint will be as strong as the skin, high grain strength is
needed to prevent splitting along the boat’s centerline.
Keels that are less than 1" (25mm) thick can be built of
solid lumber, but we prefer to laminate both keels and
stems, following procedures described in Chapter 11,
because of the structural advantages of the laminate.
<strong>The</strong>re are several ways to prevent splitting at the keel.
Carbon or glass fibers may be bonded between layers of
planking where they meet. We usually add plywood
strips to our keels. On keels less than 1" thick, we begin
with solid lumber, and then bond plywood to what will
be its interior surface in the boat, as in Figure 19-1,
Section B. In large, thick keels, it’s wiser to distribute
thin layers of plywood through the lamination than to
rely on a single strip of heavier ply. Birch plywood is
particularly effective in this application because it is
strong in tension and it bonds well with epoxy. Threeply
1 ⁄8" (3mm) thick plywood is adequate in most cases.
In traditional designs, stems and keels are usually
rabbeted, notched to accept planking and allow it to lie
flush with the outside of the keel or stem. Rabbeting is
time-consuming, and it requires skill and patience. To