Boat/Yacht Materials
Gordon Ringberg
Senior Underwriter
Boats have been built using just about every natural or manmade material
imaginable. Ancient mariners used bundled grass, dug out trees, animal skins,
tree bark, log rafts and sawn timbers to build the craft that carried them down the
rivers and across the seas. Today’s boats are built predominantly using fiberglass,
aluminum and steel, however in 2008 a raft built from 15,000 recycled bottles,
fishing nets, and a Cessna cockpit, successfully sailed from California to Hawaii.
So, with a proper design and care to work within the limitations of the material,
just about anything can be made into a boat.
No material is the ideal medium for all boats. The intended use of the vessel
usually determines the choice of the boat building material. The material used to
build a commercial fishing boat will be very different from the material used for a
boat competing in the upcoming America’s Cup races.
The following are materials that we see most often as marine underwriters.
Wood
Wood Construction – People have built, paddled and sailed wooden boats for tens of thousands of
years. Although today wooden boats are seen by many as old fashioned and undesirable, there are still
good reasons to build a wooden boat. Wood combines strength and resilience in a lightweight, easily
worked form, and wood is one of the most affordable materials to use in building boats.
There are many variations in how wood boats are constructed. But an important distinction is how the
planking is fastened. Is it fastened only to the framework (as in carvel planking), fastened to the
framework and its neighboring planks (lapstrake and strip planking), or glued up from thin veneers,
creating an almost one piece hull (cold-molded and some modern plywood hulls)? This is important not
just from the standpoint of building a boat, but also largely determines how repairs should be carried
out, and suggests what one should look for when evaluating the condition of an older boat.
See also:
www.smaalders.net/yacht_design/gobboatconst.html , www.boatbuildingplans.us/wooden-boat-building/,
www.krauscravenboatbuilding.com, www.woodenboatco.com/WOODENBOATCO.htm
Steel
Steel - is strong, inexpensive and easily repairable worldwide. The strength/weight ratio of steel is
high which creates hulls of competitive performance, if top racing is not the target. Modern paints
applied properly can offer long term protection against rust.
Steel is used in the construction of commercial work vessels, older houseboats, “expedition” yachts, and
some long range cruising sailboats. An audio gauge to measure plate thickness is required as part of
the survey when underwriting steel hulls over 15 years of age.
See also:
www.kastenmarine.com/steelboats.htm, www.dixdesign.com/dix38pil.htm,
www.allseasyachts.com/, www.benford.us/articles/boatbuildingmaterials.html
Aluminum
Aluminum - gives excellent strength/weight ratio and does not corrode (but can be wasted by
electrolysis). Painting is optional. With the right choice of the rest of the metallic elements on board and
the right cathodic protection, the hull can remain as new without maintenance. Aluminum can be used
to build lighter superstructures on steel hulls fastened by electrically isolating bolt joints.
Aluminum is used to construct houseboats, fishing boats, air boats and high quality luxury sail and
motor yachts. An audio gauge to measure plate thickness is required as part of the survey when
underwriting aluminum hulls over 15 years of age.
See also: www.kastenmarine.com/aluminum.htm, www.adrenalinmarine.ca/photos.html, www.edcmc.org/news031406BurgerMJS.html, www.blankyachtdesign.de/yachten-jachten/en/3-bootsbau-yachtbau-boatbuildingcruiser.php?id=news10
Reinforced Plastic Composite
Composites are materials made up of two or more individual components whose combined physical strength exceeds
the properties of each individually. The reinforced plastic composite consists of a fibrous reinforcing network embedded
in the cured resin matrix. The thermosetting type resin (polyester, epoxy or vinyl ester) is a plastic that cures from a
liquid to a solid through a chemical reaction of its two components. Once this reaction occurs, the material can not be
reformed.
Composites can be easily formed into complex shapes, have superior resistance to most environments, and they can be
used by most individuals without a major investment in equipment.
The physical properties of composites are fiber dominant. This means that when resin and fiber are combined, their
performance remains most like the individual fiber properties. For this reason, fabric selection is critical when designing
composite structures. Three types of reinforcing materials can be used to construct a project; fiberglass, carbon fiber,
and Kevlar®. All three have their attributes and short comings, and are available in numerous forms and styles.
Resin selection is based on fabric compatibility, service conditions, and the desired characteristics of the finished part.
There are two common types of thermosetting resin to choose from; epoxy and polyester. Epoxy is the higher
performance and higher priced system. It is used in weight critical, high strength, and dimensionally accurate
applications. Polyester resins are less expensive, offer more corrosion resistance, and are more forgiving than epoxies.
For this reason, they are the most widely used. Certain resins are not compatible with all fabrics. For instance,
Kevlar often exhibits adhesion problems, so epoxy or the highest grade polyester should be used.
Fiberglass
Fiberglass is the most widely accepted and least expensive reinforcement used in composite
construction. It has been used successfully in many applications since the 1950’s, and much is known
about its properties. It is relatively lightweight, has moderate tensile and compressive strength, is
tolerant of both damage and cyclical loading, is easy to handle and machine.
Fiberglass is actually a textile. It comes on a bolt like a big, white bed sheet. The stuff that makes it
hard (like a boat) is the resin. Resin is a liquid and is applied as a coating (like paint) to the fiberglass.
Unlike most household coatings, the resin requires a hardener. The hardener is also a liquid which,
when added to the resin, causes a reaction which allows the resin to cure. When the resin and
fiberglass harden, they bond mechanically and chemically, forming a composite material.
For most purposes, the terms “reinforced plastics”, “composites”, “fiberglass”, “FRP” and “GRP”
can be used interchangeably.
See also: www.fibreglast.com/product/LC_008
Kevlar & Carbon Fiber
Kevlar®, the most common aramid type fiber (aramid fibers are a class of heat-resistant and strong
synthetic fibers), is another composite reinforcement option. Kevlar exhibits the lowest density of any
fiber reinforcement, high tensile strength for its weight, and superior toughness. It is priced favorably
between fiberglass and carbon fiber. Kevlar is puncture and abrasion resistant, making it the
reinforcement of choice for canoes, kayaks, and leading edges of airfoils. On the down side, Kevlar is
difficult to cut and machine during part fabrication. A pair of sharp scissors should be dedicated solely to
cutting Kevlar. It also has a low service temperature and poor compressive properties. It is possible to
combine Kevlar with other materials creating a hybrid laminate to compensate for the shortcomings.
Carbon fiber is a modern reinforcement characterized by extremely low weight, high tensile strength,
and high stiffness. The material handles easily and can be molded much like fiberglass. However, some
advanced techniques are necessary to achieve the maximum properties of this material. Carbon fiber is
also the most expensive of the reinforcing fibers. This fact often limits its use to parts needing
selective reinforcement or high stiffness with the least weight.
See also: www2.dupont.com/Kevlar/en_US/uses_apps/composites.html
Sandwich Composite Technology
Cross-section through a fiberglass hull showing typical sandwich
construction with a closed-cell foam (left) or balsa (right) core. The
fiberglass laminate commonly comprises alternating layers of mat and
woven roving. Often there are two layers of mat immediately beneath
the thin gel coat to prevent “print-through” of the coarse weave of
the underlying roving.
Foam Core is serrated on the left to conform to curves while on the right
is the material on the backside that holds the sheet together. End Grain
Balsa core also comes in flexible sheets
The rising demand for new materials with higher strength to weight ratios has created a dramatic growth
in sandwich composite technology. Sandwich construction employs a lightweight core that has a flexural
strength and flexural modulus far exceeding that of the skin laminates alone.
The normal method of building a composite sandwich is to laminate the outer skin by conventional means
in a mold then lay the core material into the wet laminate. The inner skin is laminated onto the top of the
core material effectively sealing it. Sandwich core laminates of this type are used to stiffen various
composite applications such as boat hulls, automobile hoods, molds, and aircraft panels. By increasing
the core thickness, you can increase the stiffness of the sandwich without substantially increasing
weight and cost.
Ferro-cement
Ferro-cement is a boat building method using steel wires covered with a sand and cement plaster. It is the cheapest and
easiest form of construction for boats over 25 ft. and apart from strip-plank composite construction, it is the only viable
material for large round-bilge boats within the amateur capability, without the requirements of special tools or a weatherproof building.
Although a few boatyards built ferro-cement boats in the 1970’s, most were homebuilt. A professionally plastered hull can
be sufficiently strong for ocean sailing, although the abrasion resistance is not as great as steel or fiberglass. But because
of the wire mesh which is laid over the frames, it’s not an easy job for amateurs to get all the bubbles out of the plaster
when it’s applied. Expansion and contraction due to temperature changes eventually cause cracks to appear in the hull.
Ferro-cement does have a reputation in the US that does not match the comparatively high regard that it is held in other
countries. Some of this is just plain unfair prejudice, but some of this comes from real shortcomings in the materials. A
well-built ferro-cement boat can be a reasonably good cruising boat. But the image of the crudely finished ‘hippie’ built
cement and rust buckets still clouds the perception of ferro-cement for many. Also, it is very hard with non-destructive
survey techniques to tell whether the original work was done well and is in good condition.
See also:
www.kgrawood.com/bb.htm
www.motherearthnews.com/Do-It-Yourself/1972-07-01/How-To-Build-A-Ferrocement-Boat.aspx
www.ferrocement.org/
Fiberglass Over Wood
Fiberglass over wood boats come in two flavors; 1) those where the fiberglass over wood is part of the original
construction method such as; cold molded veneer, strip planking, plywood over frame, or plywood stitch and glue, and
2) traditional build wood hulls that are latter sheathed in fiberglass to extend the life and/or to reduce hull maintenance.
1) As part of the original construction, the epoxy/fiberglass covering adds strength, prevents rot, minimizes leaks and
weight from the absorption of water, and protects against the attack of worms and borers. The fabric used is usually
fiberglass cloth, but polypropylene, Dynel, Kevlar and carbon fiber can be found in high tech applications, and where
weight is a factor. The resin used in construction is almost always epoxy because of it superior strength, adhesion and
water resistance. We consider these boats to be wood construction for rating purposes.
2) Traditional carvel planked hulls swell and shrink as moisture levels changes. Since wood and fiberglass have different
rates of expansion, the fiberglass sheathing will eventually crack and separate from the wood hull. Also, polyester resin
is not waterproof, so water can migrate between the wood and fiberglass to cause rot in the wood structure.
Unfortunately because of the fiberglass sheathing, structural damage is usually not detected until it’s too late. Epoxy
resin is waterproof and may hold the sheathing to the hull longer than polyester resin, but even the epoxy bond could
eventually fail because the hull “works”.
See also: www.glen-l.com/index.html, newboatbuilders.com/docs/fiberglassoverwood.pdf,
1930schoonermistress.com/
Inflatable
uPVC Tube Fabric
Inflatable – Two different fabrics are used in the construction of the inflatable chambers for soft bottom
and rigid bottom inflatable boats. Where the boat will be used can be a deciding factor about which hull
fabric to choose. The choices are PVC coated polyurethane cloth, or Hypalon (neoprene–coated nylon).
Both fabrics are rugged, durable and dependable under virtually all conditions. But if the boat will be used
in tropical conditions, a Hypalon boat will last longer because of its better resistance to UV degradation.
Construction of PVC and Hypalon boats is often different, too, but both are utterly reliable in use. Some
manufacturers use a thermobonding technique to literally "weld" the PVC fabric panels together, producing
airtight seams stronger than the fabric itself. Also, since the PVC coating is more airtight than Hypalon,
inflation pressure doesn't need to be topped off as with Hypalon boats.
Hypalon inflatables (and some PVC boats) are glued together using high–strength, two–part adhesives.
This is a more labor–intensive process than the automated "welding" of PVC and is a common weak point
of "bargain–priced" boats, so it must be done properly.
See also: West Marine - Selecting an Inflatable Boat,
www.hotribs.com/02articles/059-tube-materials/inflatable-boat-tube-materials.asp
Rotational Molded Polyethylene
Rotational Molded Polyethylene is a plastic material that's used to make many of today's mass-produced kayaks and
canoes. Triumph Boats has taken the material, and the rotation-molding process, to a new level by producing fishing
boats from 12 to 23 feet.
Triumph's proprietary Roplene roto-molding process starts with a two-part aluminum mold of the hull and deck, which is
"charged" with marine-grade, powdered polyethylene. The mold is then placed in a large oven and heated to 380
degrees to liquefy the polyethylene. As the mold "cooks," the entire oven is rocked and rotated to ensure even
distribution of the polyethylene. Once the heating process is complete, the hull/deck unit is allowed to cool before being
placed in a special pressurized jig that helps ensure a more uniform product. Total turnaround time from powder to
completed hull is around three hours. Next, foam is injected into the space between the hull and deck for structural
rigidity, sound deadening, and added flotation. Last comes the installation of hardware, fuel lines, wiring, hatches,
console and other features and systems.
The Roplene process has several advantages: it is inexpensive, it creates no harmful emissions, it produces a one-piece
deck/hull unit, it is incredibly rugged and easy to repair, and, since plastic floats, it is inherently buoyant. It can even be
recycled.
See also:
www.triumphboats.com/, www.portlandpudgy.com/
Cardboard
Cardboard, Milk Carton and Recycled Trash boat races take place all around the country, usually as a school project
or to support a charitable cause. Although many are creative and require some skill to build, the inherent vice of the
material makes these boats uninsurable.
See also: Cardboard Boat Races, Big Green Derby, Milk Carton Boat Race
?
www.markelmarine.com