Boat/Yacht Materials

Gordon RingbergSenior 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/news-031406BurgerMJS.html, www.blankyachtdesign.de/yachten-jachten/en/3-bootsbau-yachtbau-boatbuilding-cruiser.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 needingselective reinforcement or high stiffness with the least weight.

See also: www2.dupont.com/Kevlar/en_US/uses_apps/composites.html

Sandwich Composite Technology

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.

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

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 weather-proof 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.htmwww.motherearthnews.com/Do-It-Yourself/1972-07-01/How-To-Build-A-Ferrocement-Boat.aspxwww.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

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

uPVC Tube Fabric

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

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www.markelmarine.com