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How Core Materials Make Better Boats

1. What is SCRIMP(TM)?

2. Composite Construction Using Epoxy

3. Advantages of Epoxy Composite Construction

4. How Core Materials Make Better Boats


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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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.

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.

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.

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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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.

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.

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.

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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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Lower Falls Landing Box 1, 106 Lafayette St., Yarmouth, ME 04096 Tel 207-846-6775

Boat Building: Processes and technological advancements.

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