How Core Materials Make Better Boats
1. WHAT IS SCRIMP(TM)?
SCRIMP(tm) is a process called
resin infusion molding. SCRIMP(tm) involves using a
vacuum to force resin through a laminate at a controlled ratio.
Below is some text, courtesy of Tillitson-Pearson regarding the
scrimp process they developed in the mid eighties.
Environmentally responsible, SCRIMP(tm) is a completely
closed system that traps VOC emissions instead of sending them
up the stack. Minimal need for solvents reduces VOC emissions
by as much as 90% over open molding processes. SCRIMP's closed-mold
technology brings styrene levels down well below today's stringent
standards, and eliminates the need for the costly exchange of
heated air. In fact, measured levels of VOCs are lower than 10ppm.
Because lay-up is performed with dry materials,
workers are not exposed to wet resin. Not only does this eliminate
the need for masks, gloves, and protective clothing, but it means
a cleaner, healthier production setting and environment.
SCRIMP(tm) saves labor and time
with dry lay-up. Direct fiber placement of material to each specifically
designed molded part is assured. Workers can apply material to
a mold much more easily when not encumbered by respirators, gloves
and resin suits. Engineering can check fiber orientation prior
to infusion. Unlike RTM, SCRIMP(tm) requires only one
tool side in conjunction with a flexible bag. With virtually no
size limitations, the SCRIMP(tm) process can produce
large and small components, as well as complex, multidimensional
trussed parts. SCRIMP made composites exhibit the same material
properties as those produced by more expensive processes.
Because the SCRIMP(tm) system achieves
equilibrium resin content (50% to 70% fiber weight, depending
on fiber architecture), unlike most composite processes, it is
inherently repeatable. Controlled bagging of performs and repeatable
resin infusions enable any licensed fabricator to perfectly produce
and reproduce specifically designed, high-quality precision parts
with consistent dimensional accuracy.
The SCRIMP(tm) process can be
used to infuse thick laminates with the same high quality results
as a simple 1/8" laminate. SCRIMP composites, with or without
a gel coat, exhibit near-perfect surface quality. Void-free surfaces
do not require filler in painted applications or expensive rework
associated with "egg shell" effects. SCRIMP(tm), unlike prepreg, does
not require refrigerated storage, time-consuming debulking, breathers,
bleeders, and porous releases, or the scrap associated with bridging.
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2. COMPOSITE CONSTRUCTION USING EPOXY
Twenty-five years of advances in construction
techniques using epoxy-bonded materials have revolutionized boat
building and set new standards for performance and reliability.
Leading builders are building stronger, lighter, more durable
boats with epoxy composite construction.
THE EVOLUTION OF BOAT CONSTRUCTION
For centuries, artisans shaped, fitted and assembled timber into
wooden boats. Builders developed sophisticated wood construction
methods, but never overcame wood's susceptibility to rot and significant
maintenance requirements. The development of fiber reinforced plastics
(FRP) offered apparent solutions in new materials and techniques.
The explosive growth of fiberglass-polyester boats over the last
thirty years was built on the perception of low maintenance and
easy fabrication. However, as with wood, polyester resins have been
plagued by the effects of moisture penetration. The problems of
rot and softening were replaced by hydrolysis, blisters and delamination.
The solution to these problems lies in epoxy composite construction.
WHAT IS EPOXY COMPOSITE CONSTRUCTION?
Epoxy composite construction consists of bonding all of the materials
and parts of the boat together with epoxy resin. The resulting structure
has physical characteristics superior to the components by themselves.
Composite construction includes a variety of building methods that
use epoxy to protect the materials from moisture as well as hold
the materials together. Epoxy resins, the key ingredient, are among
the most versatile of thermoset plastics. They bond exceptionally
well to a wide range of materials and are highly moisture resistant.
Compared to polyester resins typically used in fiberglass boat construction,
epoxies have greater strength, less shrinkage, better moisture resistance
and better fatigue resistance.
A NEW INTEGRATED MATERIALS TECHNOLOGY
Combining the best of wood technology with the advances in FRP materials
and processes, leading builders have turned to composite construction
to produce durable, distinctive boats. Builders use the moisture
resistant qualities of epoxy to take advantage of wood's strength,
stiffness, light weight, resistance to fatigue, insulating ability,
availability, cost, and beauty. Epoxy's excellent adhesion to balsa
and plastic foam cores, glass, aramid and carbon fabrics, allows
the builder the advantage of selectively integrating these materials
into the boat's structure. Designers, builders and owners have more
choices available. Through epoxy composite construction, the builder
can offer boats in a wide range of designs, materials and construction
methods.
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3. WHAT ARE THE ADVANTAGES OF EPOXY COMPOSITE CONSTRUCTION?
VERSATILITY
The builder using composite technology can build boats with a range
of materials, designs, and construction methods that are perfectly
suited to the boat's use and the customer's needs. Everything from
strip canoes to work boats, high performance multihulls to offshore
racing powerboats have been built using epoxy composite construction.
Composites can be uncomplicated structures of wood and wood veneer
or complex vacuum laminated hybrids incorporating glass fabrics,
aramid, or carbon fibers.
LOWER MAINTENANCE
All of the components in a composite boat are protected by an
epoxy moisture barrier. Since the moisture content is stabilized,
the maintenance problems associated with wooden boats - rot, joint
cracks, structural members swelling or shrinking, and surface
checking - are eliminated. Epoxy provides a stable base for paints
and varnishes, reducing the frequency of refinishing. In glass
laminated boats, epoxy's superiority to polyester resisn as a
stable moisture resistant adhesive reduces the possibility of
delamination and gelcoat blistering caused by moisture penetration.
A HISTORY OF SUCCESS
Epoxy composite construction techniques for boat building were
first developed over thirty years ago. Over the years, thousands
of composite recreational and working boats have been built and
the earliest are still going strong. Composite construction has
proved itself at the top levels of competition in sail and powerboat
racing, in the harshest environments and under the toughest working
conditions. Epoxy composite boats have set a standard for performance,
reliability and beauty.
Copyright ©1998. All rights reserved. WEST
SYSTEM and PRO-SET are registered trademarks of Gougeon Brothers,
Inc., PO Box 908, Bay City, MI 48707-0908, U.S.A. Email comments
or suggestions to webmaster@gougeon.com
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4. HOW CORE MATERIALS MAKE BETTER BOATS
By Eric W. Sponberg, Naval Architect
The following article first appeared in American Sailor magazine.
It has been edited and updated to reflect some of the latest core
material technology.
Most sailboats racing IMS, PHRF, and one-design
are built with at least one kind of core in their hull and deck
laminates, and for good reason. Cores make the hull of the boat
stiff and light. Stiffness means the hull does not flex out of shape,
which would increase hydrodynamic drag and slow the boat down. Lightness
means less weight to move through the water, so speed is faster.
In addition, cores insulate the hull against heat and cold, dampen
vibration from slamming seas, and deaden the sound of chugging engines.
Thanks to cores, boats have better performance and enhanced creature
comforts.
Cores are made from a variety of materials which have different
strengths and stiffnesses. Since cores are an integral part of the
boat's structure, the designer must take proper account of these
properties so that hull and deck laminates won't fail. By understanding
how a boat's structure is designed and how it works, you can compare
these core materials for yourself.
A boat's hull laminate is stiffened by internal members such as
bulkheads and longitudinal stiffeners. These subdivide the hull
laminate into panels. Each panel experiences water pressure and
wave impacts from the sea. Under these loads, the panel bends and
experiences stresses within the laminate.
The panels of a boat's structure must be designed to withstand a
number of different stresses. The worst loads are imposed by wave
impacts against the sides and bottom of the hull.
Panels are usually analyzed by looking at
a strip of laminate that is the shortest distance between two internal
structural members such as longitudinal stringers or bulkheads.
The deflection of the strip is greatest in the middle, but the stresses
there are only half what they are at the ends of the strip.
At the ends of the strip, the inside surface of the laminate is
in compression, the middle region is in shear, and the outside surface
is in tension. In a single-skin laminate, the outside surface is
in tension, the inside surface is in compression, and the middle
is in shear. Tension and compression are generally easy to visualize.
Shear is the tendency of the inside and outside halves of the laminate
to slide against each other in opposite directions. Shear is highest
right in the middle of the laminate. To resist all these stresses
without fracturing, a single-skin laminate must be relatively thick
and heavy.
In a cored-skin laminate, the outside and inside skins experience
the tension and compression stresses, and the core experiences the
shear stress. Because of this separation of duties, the skins together
can be less thick than the total thickness of its single-skin counterpart.
Cores, however, must be quite thick, and so the total thickness
of the cored-skin laminate is more than a single-skin laminate.
This makes the cored-skin laminate stiffer. And because cores are
very lightweight, the cored-skin laminate weighs less than the single-skin
laminate.
The most common core materials utilized in building boat hulls
and decks include balsa wood, PVC (polyvinyl chloride) foam, SAN
(styreneacrylonitrile) foam, and honeycombs made from aramid (Kevlar®),
plastic, and paper. Most widely used throughout the world, balsa
core is made with the wood grain running from skin to skin and
is termed end-grain balsa. The primary developer and major manufacturer
of end-grain balsa core is Baltek Corporation of Northvale, NJ.
Baltek supplies core in densities of 6.5, 9.5 and 15.5 lbs./cu
.ft. Just recently (late 1999), Baltek has announced the availability
of SuperLite®, a range of lightweight balsa cores from 4.9 to
8.7 lbs./cu. ft.
PVC foam cores come in two varieties: cross-linked and linear
(non-cross-linked). The cross-linked is brittle and, if bent too
much, it breaks. It is available in a broad range of densities,
from 3 lbs./cu. ft. to 25 lbs./cu. ft., with 5-10 lb. densities
being the most common in boat building. Cross-linked PVC cores
are Divinycell® and Klegecell®, both marketed by Diab Group of
DeSoto, Texas. Diab also markets an end-grain balsa core called
ProBalsa® in densities of 5.6, 9.7, and 13.8 lbs./cu. ft. Linear
PVC is not chemically cross-linked and does not break when bent.
It comes in only two densities, 3.8 and 5.5 lbs./cu. ft. The leading
brand of linear PVC core is Airex®, marketed by Baltek Corporation.
SAN foam is a new core material that has been developed since
this article was first written ten years ago. The one product
available is called Core-Cell® which is manufactured and marketed
by ATC Chemical Corporation of Buffalo, NY. Basically, it combines
the linear, non-brittle features of Airex in a broad range of
densities like Divinycell, from 3-12 lbs./cu. ft.
Honeycombs used to be pretty expensive (some still are), and as
such, were rarely used in production boats and only in the most
expensive one-offs. They are extremely light and usually resemble
a bee honeycomb without the honey. They are made of a number of
materials:
- Nomex®, with aramid fiber, is made by Hexcel Composites, Dublin,
CA
- Plastic, Nida-Core®, from Nida-Core Corporation, Hoboken,
NJ
- Paper, called Tricel®, from Tricel Corporation, Gurnee, IL
Of these, Nomex is the most expensive and
is used primarily in custom boats. Plastic cores are being used
increasingly as structural cores in hulls and decks, and in interior
joinery. Paper core is becoming increasingly common in boat interior
joinery, but is not recommended for hull and deck structures. If
paper core gets wet, it goes all mushy, just like cardboard left
out in the rain.
Since cores resist shear stress, designers compare different kinds
primarily by their shear strength and shear modulus. Modulus means
stiffness— the higher the modulus, the stiffer the material. In
general, balsa and aramid honeycomb are stronger and stiffer in
shear than the other core materials. Properties are from manufacturers'
published data for approximately 6.0 lb./cu. ft. density. Two
densities of balsa are shown. Properties vary significantly with
density— the denser the material, the stronger and stiffer it
is.
High compressive properties are necessary to resist the crushing
loads of through-bolts wherever hardware such as winches, tracks,
and cleats are mounted. Balsa core and the higher densities of Divinycell
and Klegecell have sufficient compression properties to resist such
crushing loads, but the other cores must be removed and replaced
with solid wood or structural putties in areas where through-bolting
occurs.
Like mechanical properties, prices go up with increasing density.
Note, however, that the heavier balsa core is less costly than the
lighter balsa. This is because more labor is require to hand-select
the correct densities of balsa stock to make up the core panels.
The bond of the skins to the core must be perfect. In production
boat building, the laminating crew lays the outside skin into
a female mold first. Before it cures, they press the core blindly
into the wet laminate. The laminators cannot see if the core is
in perfect contact to the skin on every square inch of surface.
To achieve total contact between core and outer skin, most builders
nowadays use vacuum bag techniques, employing the same sealing principle
that keeps packaged bologna fresh in the supermarket. To vacuum
bag a core, laminators first lay a bleeder cloth over the core,
and over that they drape a large sheet of plastic film— the "bag"—
and seal its edges all around with a special thick, sticky, sealing
tape. An air suction pipe is installed into the bag, and the air
between the bag and core is sucked out through the pipe by a vacuum
pump. As air is removed, the outside air pressure presses the core
uniformly into the entire surface of the skin. The bag is left in
place until the outside skin cures with the core stuck to it. The
inside fiberglass skin presents no problem when laminating because
the laminators can see the core surface as they lay down the transparent
wet fiberglass laminate.
Balsa, PVC and SAN foams require special putties to assist in bonding
the core to the outside skin. The putty is spread over the skin
before it cures, then the core is pre-coated with resin and pressed
into the putty. Baltek's putty is called Baltek-Bond® and is used
with its balsa core products. Diab Inc. markets putties called Divilette®
(for Divinycell), K-Lite® (for Klegecell), and ProBond® (for ProBalsa),.
ATC Chemicals markets a putty called Core-Bond® for their Core-Cell
core.
With honeycombs, particular care must be taken with the bond surface
because the skins bond only to the paper-thin edges of the honeycomb
cell walls. This manner of bonding is very difficult to achieve
on every single cell wall. Just enough wet fiberglass or bonding
material must be used for a good bond, but excessive amounts of
resin will fill up the honeycomb cells, making the boat way too
heavy. Nida-Core has solved this problem with their polypropylene
core by using a polyethylene film to fuse a non-woven polyester
scrim to both sides of the core. Simply wet out the scrim and lay
up your polyester or epoxy laminate directly onto the scrim. The
scrim and film provide the appropriate bond to the honeycomb.
How thick should the core be? This depends directly and entirely
on the size of the boat, the size of the hull panels, and the anticipated
loads. The designer must solve this engineering problem for every
single panel in the hull and deck. A stronger and stiffer core such
as balsa can be thinner, and the weaker cores must be thicker. Note,
too, that core mechanical properties increase significantly with
density— the denser the core, the stronger and stiffer it is.
How strong should the skins be? Again, the designer must engineer
the answer. The thicker the core, the thinner the skins can be,
and so the lighter the laminate. The skins should be as thin as
possible, consistent with strength, stiffness, ease of lay-up, and
overall hull durability.
It is a mistake to design and build skins that are too thin. Polyester
resin composites are porous, so water can readily seep into the
laminate by osmosis. This leads to serious blistering, delamination,
and even considerable weight gain, particularly in honeycomb hulls
where water can fill up all the open cells. Also, thin skins tend
to buckle as the laminate bends. A thin skin can bend out of line
so much that it breaks away from the core, causing radical delamination.
Finally, thin laminates are not very durable against minor impacts
such as the hull bumping against a dock.
In summary, core materials make boats stiffer and lighter. In most
cases, all cores can be made to do the same job. The choice as to
which core is best for a particular design usually reduces to which
core is the least expensive for the greatest strength and stiffness.
The weight of different cores is less important because they tend
to be only a small portion of the total laminate weight. And, depending
on the engineering circumstances, a low-density, thick core could
have the same weight as a high-density thin core for the same shear
strength and stiffness. You must also account for the weights of
the skins that result from any given core material and thickness.
Thin-cored laminates generally have thicker skins, which means more
fiberglass, more labor, and so more cost for the lay-up. Generally,
you cannot isolate the core out of the laminate for comparison.
You must look at the total ply stack of the laminate schedule when
comparing core and laminate weights, strength, stiffness, and cost.
Finally, which core material is the building crew most experienced
with? Each kind of core requires slightly different laminating techniques,
and different building crews tend to have developed their own preferences.
The basic guidelines that designers and builders
must follow are:
- Use proper engineering and design procedures.
- Keep the laminate simple.
- Follow the core manufacturer's instructions.
- Use the core only as the manufacturer intended it to be used.
Most cored/laminate failures can be traced
to a violation of one of these rules. But a cored-skin boat designed
and built with care will last for years, while you enjoy the extra
performance and creature comforts.
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