HullMaterial

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Hull Material

Summary

The primary materials for hulls are fibreglass, steel or aluminium, wood or ferro-cement. Wood is beautiful but high maintenance, and difficult to repair. Use wood only if you are a confirmed traditionalist. Ferro-cement has a long successful wartime history pre-dating fibreglass. It uses welded or woven wire mesh, or expanded metal lath covered with a sand and cement plaster. It is suited to amateur construction; and has very low resale value.

More exotic materials are being used in super yachts. The Juan KKelvin, SI unit of thermodynamic temperature Super Maxi, a 115-ftFoot monohull sail boat, is built from carbon-fibre. In this example, a carbon-fibre hull was 66%percent lighter than the equivalent aluminium hull. Of course, hull weight is not as critical in a displacement hull.

For a small trawler, fibreglass and steel are your materials of choice. Don’t consider anything else, unless you want to go to sea in a cast-iron bathtub.

In general fibreglass must be three times the thickness of steel for the same tensile and compressive strength but has a 40% lesser weight. Thus fibreglass is better suited than steel to a semi-displacement trawler or a smaller displacement trawler where steel might be too heavy.

Comparison of Strength Fibreglass vs.Steel
Material	Specific Weight lbPound weight/ft3	Tensile Strength kNKiloNewton - Unit of force/mMetre, SI unit of length2x10	Compressive Strength kN/m2x10	Elastic Modulus(Young's modulus) - Tensile elasticity, the tendency of an object to deform along an axis kN/m2x10
Fibreglass	94	100	100	6
Steel	485	210	190	200
Source: http://www.fao.org

Fibreglass

Fibreglass, also called fibre-reinforced plastic (FRPFibre-reinforced plastic), fibreglass-reinforced polyester, glass-reinforced plastic (GRPGlass-reinforced plastic) or resinglass, has become the material of choice for production boats because of the lower labour cost. It is less expensive than steel and costs less to maintain. It is suited to limited series production using a mould; whereas steel is more labour-intensive and better suited to one-off construction.

Fibreglass, unlike steel, is not subject to corrosion, or galvanic action. It has a smooth, high-quality finish. Colour can be added to the gel coat, eliminating the need for painting. Small holes can be patched readily using supplies found in marine and automotive centres globally. Glass grades are type AAmpere (amp), SI unit of electrical current, E or SWetted surface but only E-glass (electrical glass) should be used in a boat. In terms of materials and modern technology Sweden, France, UK, and USA have the most to offer.

Fibreglass is more difficult than steel to make a hull leak-proof because hulls and decks are separate components that must be joined, bolted and bonded.

In general, fibreglass is not as strong as steel; although multilayer NIDA sandwiches are very strong indeed. It is more easily damaged. It abrades easily, and cannot withstand pounding when grounded. The gel coat can blister, which may or may not be serious, and powder from oxidization and ultraviolet light. When fibreglass is penetrated, water can enter its structure through osmosis, making large sections of hull spongy inside.

Fibreglass by its polyester nature will absorb some water. The use of either vinylester or isopthalic resin below the waterline will in-crease resistance to blistering. When sections are cut from the hull for portlights and hatches, ask the builder to test them in boiling water for 100 hours to see if they blister. If they don’t, the construction quality is excellent.

Blistering is caused by water-soluble material (WSMWater-soluble material) trapped in resin or matt layers next to the gel coat. The WSM ‘attracts’ water, which penetrates the hull through the process of osmosis. Since the WSM has now expanded, the resulting pressure forms a blister. The solution in the blister becomes acidic from chemical reactions between various materials and the water.

Fibreglass construction requires special techniques and skills. There are several techniques. Basic boat building requires a plug or a mould. Fibreglass is laid up by hand using cloth, sprayed using a chopper gun, or vacuum-bagged. It can be built up as a single skin or a foam-core sandwich; and reinforced with aramid fibre or carbon-fibre.

Fibreglass comes in chopped strand mat (CSMChopped strand mat), continuous roving (CRContinuous roving) and woven roving (WRWoven roving). Strand is not structural, so it is best avoided for strength. Experience has shown that alternate layers of CSM and WR are best with extra CSM near the outside. CSM glass cloth or tape is also available for small repairs.

There are two types of resin: laminating and the previously mentioned gelcoat. “Marine General Purpose” Isophthalic or Neopentyl Glycol polyester resin is used commonly.

Using a Mould

To make a mould, you start with a plug in the exact shape of the hull or hull component. The plug is disposable, and can be made from a variety of materials. The exterior of the plug is highly finished.

Next, a fibreglass mould is built up on the plug, using alternate layers of cloth and matt, a felt-like material. Each layer is impregnated with a synthetic polymer resin that cures and hardens. The exterior of the mould is reinforced, and the plug is removed and discarded. The best range for working temperatures in the mould shop is 18-21 °Cdegree Celsius, SI unit of temperature with humidity less than 80%. Direct sunlight should be avoided.

Finally, the inside of the mould is coated with a wax-like release agent, and a fibreglass hull or other component is built up inside, starting with the coloured gel coat, a layer of mat and then woven roving. Putting mat, a sort of filler, next to the gel coat prevents print-through of the roving. Anecdotally, putting mat next to the gel coat increases the propensity for blistering. After the piece is finished and cured, it is removed from the mould for finishing. The mould is then ready for re-use. Gel coats should be >greater than30 mm and incorporate protection from ultraviolet (UVUltraviolet) light, which damages plastics.

Moulds are typically used for limited production. For series produc-tion, construction is done using just a plug. With a plug, fibreglass is laid up over the outside of the plug, with a roving layer last, and then hand finished with a gel coat. Today, plugs can be cut using multi-axis robotic milling machines driven by CADComputer-aided design cutting files.

Hand Lay-Up

Hand lay-up is not common today, for reasons of efficiency and environment. Special guns that spray resin and chopped glass fibres into the mould have replaced hand lay-up. A more recent technique, vacuum infusion, uses a vacuum bag over the mould to en-sure consistent quality.

Fibreglass components are made with either a single-skin or a sandwich construction. Single-skin construction uses alternate layers of continuous and woven roving that are saturated with resin and allowed to cure. Typical glass-to-resin ratios are 30:70. Unidirectional and biaxial reinforcing materials can increase strength while eliminating stress deformation, reducing weight and increasing the glass-to-resin ratio to 50:50, which is better.

Sandwich Construction

Sandwich construction was introduced to reduce material cost. It uses a foam-core material, plywood or balsa wood between two layers of fibreglass. Wood of any kind is not recommended because it readily absorbs water, which will delaminate the sandwich.

Although introduced for cost reasons, a sandwich is stiffer than a single skin, and lighter for the same stiffness. Interestingly, the stiffness is not imparted by the foam-core but by the equal separation of the two skins, and the effect this has on bending forces. Another advantage of a foam sandwich is no extra insulation is required inside the hull. Usually the final paint is applied in the gel coat.

A sandwich is not as durable as solid 'plastic', partly because more skill is required to ensure the skins bond solidly to the core without voids. Incomplete bonding and voids cause delaminating and failure of the component. These risks can be overcome with vacuum infusion. Biaxial fibreglass material is recommended with sandwich construction. Some experts advise against any kind of core below the waterline; although reasons have not been well articulated. Perhaps this was based on experience with early foam-cores.

Early foam-cores were open cell, which out-gassed and was vulnerable to water absorption. Newer foam cores, such as Airex®, Core-Cell®, Divinycell® and NIDA have closed cells. Note that some like Airex® are susceptible to heat and not suited for use in decks or engine rooms.

Several types of foam core can be used in a sandwich:

• Cross-linked Poly Vinyl Chloride (CL PVCPoly Vinyl Chloride) is typically used in decks, superstructures, hull bottoms and sides, bulkheads, and transoms.
• Styrene Acrylo Nitrile (SANStyrene Acrylo Nitrile) is used in areas where high impact is prevalent, such as hull bottoms and sides, e.ggram., Core-Cell.
• Non-cross-linked or "linear" PVCPoly Vinyl Chloride (Llitre PVC) e.g., Airex® R63, is used in the same areas as SAN.
• Polyurethane (PURPolyurethane) and polyisocyanurate (PIRPolyisocyanurate) foam are used in acoustical and insulation panels, structural insulation panels (SIPStructural insulation panel) and parts like stringers. These foams are water resistant, meaning they will absorb some water, around 5%, and encourage mildew.

NIDA, an extruded polypropylene honeycomb covered in polyester, is in a class by itself. It is an extremely strong core material. Divinycell (diab core) is an inter-penetrating network (IPNInter-penetrating network) of PUR and PVC. A honeycombed core such as over-expanded Nomex can be used in dinghies to reduce weight. Honeycomb is not suited to a displacement hull.

Where cost is not a consideration, aramid and carbon fibres can be used to reinforce fibreglass. Aramid is a family of manufactured nylon fibres, including Kevlar®, Nomex® and Twaron, based on long-chain synthetic polyamides. Kevlar® has been used in some small-equipment applications like paddles and in high-speed boat hulls for the US Navy SEALS. In trawlers, the Krogen 58 has Kevlar-reinforced bottoms in the stem and stern areas.

A similar technology is carbon-fibre composites. These are generally too expensive (and too light) for displacement boat hulls but are found widely today in aircraft and automotive parts like wing assemblies, monocoques and body panels where strength and light weight are important, or in super yachts like the aforementioned Juan K Super Maxi. The Maxi is a high-speed sail boat, not a displacement boat, so light weight is important.

Steel

Steel hulls are made of plates of Lloyd’sSecond approved marine quality steel. The steel is pre-primed at the steel mill. Plates are precision laser cut, using computerized design instructions called cutting files. Cutting files minimize wasted steel, and ensure a good fit. Deck plating is typically from 5/32-in to 1/4-in (4-6 mm), and hull plating is from 1/4-in to 5/16-in (6-8 mm) in thickness.

Typically the hull is assembled ground up, by raising web frames and longitudinal stiffeners, and then attaching the hull plates, followed by deck plates and superstructure. A few builders are using transverse instead of longitudinal stringers.

Steel construction requires some heavy equipment; and experienced welders to get a fair finish to the hull. Joints are double welded below the waterline. Above, they can be single welded but it is better to weld them inside and out. After welding, joints are ground smooth and filled in where necessary. A team of three welders might take about 1000 hours to assemble a complete 60-ft hull.

A critical join is between the deckhouse and deck. Although the deckhouse could be simply welded to the deck, it is better to have structural pillars extending up from inside the hull. This can be accomplished by having strong mullions. Extreme waves have been known to tear off deckhouses.

Similarly, the bow, keel and bilge keels should be reinforced in case of an accidental grounding.

If you have a larger budget, you could have a steel hull and aluminium superstructure. This reduces weight above decks and keeps the centre of gravity lower. Galvanic corrosion between the dissimilar metals is avoided by using an explosion-bonded bimetallic strip to join the steel and aluminium. These strips were first developed for the US Navy.

The strips are composites of aluminium and steel, bonded together at the molecular level by an explosive force. The aluminium deckhouse is welded to the aluminium strip, while the steel hull is welded to the steel strip.

Steel will give you a homogenous leak-free hull. It is more difficult than fibreglass to get a fair finish. To prevent corrosion, it must be primed and painted. It must be protected from galvanic action. It is very strong and not damaged easily. It can withstand some pounding when grounded. It will dent before it allows penetration. Any welder anywhere in the world can put a patch on easily.