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CORE INSTALLA
BY PETER WENKERT
I
n an effort to satisfy the
boating public''s growing
appetite for high performance,
builders have
increasingly looked to
sandwich technology—core
construction—for hulls,
decks, and superstructures.
Cores can reduce weight,
extend range, add thermal
and sound insulation, and
improve a boat''s overall
economy of operation.
Most builders rely on one
sure and through the expansion
and contraction of
freeze/thaw cycles. Think of
the bond line as a new roll
of tape," he continues. "It''s
difficult to peel at first, but
once started, it comes apart
easily. As soon as you lose
the bond, you''ve lost the
structure."
John Collamore of Hulls
or more of three types of
lightweight core: end-grain
balsa (made by Baltek Corp.); crosslinked
PVC foam (trademarked as either
Divinycell or Klegecell); or linear PVC
foam (sold as Airex). With the jump in
demand for high-performance boats,
however, has come an enlarged menu
of available materials. Builders can now
also choose from aluminum, paper, and
plastic honeycombs; fabrics of woven or
spun synthetic fibers; and prelaminated,
finger-jointed balsa/mahogany strips
(DuraKore).
Growth in the market for core materials
has focused attention on the
relative merits of each type and generated
intense debate, sometimes dubbed
"Core Wars." Nevertheless, the similarities
among the cores are many: Each
provides varying degrees of stiffness,
weight savings, and insulation; more
important, each is only as good as the
care and quality of its installation.
"I tell builders who are converting
from single-skin to sandwich construction
that they are better off using cardboard
for the core—and installing it
right—than using our most expensive
core and installing it wrong," says Phillip
Giles of Barracuda Technologies, the
U.S. manufacturer of Divinycell.
Adds Kenny Robertson of Robert36
PROFESSIONAL BOATBUILDER
Gel times and shop temperatures are especially critical in core
installations. This large clock-timer-thermometer array was used at
Able Marine (Trenton, Maine) as a crew cored a 91'' one-off sailing
yacht.
son''s Custom Boats (Port Manatee,
Florida), "I''ve used every kind of core
on the market, and the most critical
thing I''ve learned is that it''s not so much
which core you use, as how you use it."
Rick Rust of Westport Shipyard
Unlimited East (Deltaville,
Virginia), a builder of fiberglass
commercial vessels,
knows just what Barer is
talking about. "A number of
(Tacoma, Washington) concurs. "Most
core failures," he says, "are really bondline
failures, which result from poor
installation procedures. Installation is
definitely the key to successful core
constaiction; that''s why we vacuum bag
all our cores."
Vacuum bagging is arguably the
most effective method of assuring
strong and uniform bonds between the
core and its FRP (fiber-reinforced plastic)
skins. But the vast majority of
builders still rely on hand-layup procedures
for core installations. "Excellent
results can be obtained through hand
layup," states Brian Barer of TillotsonPearson
(Warren, Rhode Island), but he
cautions, "The key is preventing voids
in the bond line.
"A void between the core and skin
years ago," Collamore recalls, "we
consulted with the Navy on the repair
of a patrol boat on which large areas of
the cored FRP superstructure had
delaminated. There was evidence that
the core had never been completely
rolled out into the [bed coat of] mat, so
when water penetrated a small fracture
in the outside skin, it caused large-scale
delamination.
"We couldn''t get to it from the inside,"
Collamore goes on to say, "because
of equipment and fixtures. So we
had to remove the outer skin with a
panel saw. We then chiseled out the
damaged core and replaced it by bedcling
new core against the inside skin,
using epoxy. We completed the job by
laminating a new outside skin against
the core, fairing it out, and finishing it
with gelcoat."
Recognizing the importance of
is a stress riser," says Barer. "It''s the
perfect place to start a laminate tearing
apart. Voids eventually fill with water
due to condensation, which promotes
delamination through hydrostatic presquality
control in the use of their products,
core manufacturers have paid a
great deal of attention to developing
and refining their installation procedures.
While Baltek continues to advocate
a resin-rich bed coat of 1½-oz
chop or chopped-strand mat for bonding
balsa, the past few years have seen
the foam-core suppliers switch to
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