The Yard - Modern Boating Materials

by OCEAN Media on 11 Oct 2007 Powerboat-world provides this modern boating materials feature, by Guy Waddilove, courtesy of OCEAN Magazine, the luxury marine journal from Australia.

Technological advances in composite materials have helped superyacht owners maintain their investment value.

'Cruising yachts can be constructed from a variety of materials including wood, steel, ferro-cement, aluminium and fibreglass composites and the choice of building material is governed by factors including price, performance, maintenance and longevity, although not necessarily in that order of preference.

Ferro cement is usually restricted to the domain of the backyard builder due to its undeserved reputation for inconsistency and unpredictable failure. Because of its weight, steel is normally limited to applications where performance is not a governing factor, whether it be sailing or motoring although steel is still used extensively in superyacht construction for large motor yachts.

Wood has been used for centuries in boatbuilding, but with the development of composite and metal construction it has been relegated mainly to specialist boatbuilding applications using cold moulding or strip planking techniques to form hulls. The hulls are usually extremely strong and fairly stiff, but tend to be quite labour intensive and expensive to build. Aluminium and composite are now the two predominant materials used in the majority of yacht builds today because of the advantages they offer in their weight to performance ratio.

Composite materials have been around in the marine industry for some forty years. During this period, understanding of the materials’ properties and the development of different construction techniques has allowed designers, engineers and builders to get optimum performance from the materials in terms of weight, stiffness and strength.

With respect to yacht construction, the term composite embraces a variety of different materials. Basic composite formation involves the forming of the laminate ‘skin’ from two components: the matrix, which is a resin (polyester, vinylester or epoxy) and their reinforcements (fibres of glass, carbon or Kevlar). The fibres are infused with the resin to form a strong structure that will bend without cracking. To provide structural strength and stiffness the thickness of panels is increased using a sandwich construction with a foam, honeycomb or balsa core between the outer skins.


Because of the huge variety of different materials that can be used for the matrix, the reinforcements and the core, the components of the laminate can be very accurately designed and engineered to a specific strength for a specific purpose. Even within the varieties of fibre, different weaves of cloth can be specified to produce different strength results depending on the properties desired. For example, around the chain plates on a sailing yacht, a stronger laminate can be formed by using unidirectional carbon fibres to provide the extra strength required to take the high loads from the rig. Areas requiring less strength can be formed with less expensive reinforcements that have lower strength properties.

The thickness of the structure can also be varied where increased stiffness or strength is required with a thicker core or more layers of laminate. The strength, weight and stiffness of sections of laminate can be accurately mapped out for different areas of a hull and deck, and the ability to accurately engineer the design in this manner means a yacht can be built to meet an exact specification, whether it be driven by price, weight or performance, or a combination of these factors. It also means the designer is not constrained by the need to design from a uniform plate thickness or standard set of scantling dimensions. This helps to minimise the cost of the project and optimise weight savings.

The use of composites in building yacht hulls allows the boatbuilder to build to very accurate dimensions. Because of the way the hull is formed in a mould, very little, if any fairing is required so the use of fillers to smooth the outer surface is minimised. The moulds for some decks are now being formed in sections of foam cut by computer numerical control (CNC) cutters which receive input directly from the designer’s digital drawing files.

Composite construction is used extensively in production boat manufacture and it is the technique used by the majority of production boatbuilders worldwide because of the ease of replication. When the hull and deck moulds are formed, any number or hulls and decks can be built as exact duplicates so production costs decrease.

Composite hulls and components are formed in a variety of ways to optimise production time and cost. Originally composite forms were created using a wet lay-up with resin rolled onto the fibres over a male or female mould. Modern techniques now used for laying up composites include vacuum bagging, resin infusion and the use of fibres pre-impregnated with resin. These techniques allow the boatbuilder to build with an accurate and uniform resin-to-fibre ratio resulting in a laminate without voids that weaken the structure.
Another advantage of composites is that corrosion is avoided.

Metal hulls must be protected by paint systems to prevent electro-chemical corrosion. The integrity of the paint must be maintained to keep metal surfaces isolated from corrosive salt in the marine environment. If not, the metal will be gradually eaten away losing structural strength. Composites are not subject to electro-chemical corrosion in this way and therefore do not need to be protected: they can remain unpainted in a marine environment.

Early technology polyester fibreglass hulls often suffer from degradation through osmosis, where the underwater surfaces of the hull absorb water. This water, when soaked into the fibres, gradually breaks down the fibreglass structure by destroying the bond between resin and fibres. In extreme cases osmosis can lead to complete failure of the structure. Resin developments has meant osmosis is no longer a problem for owners of more recent composite yachts. These resins are now impermeable.

Aluminium is the second most common metal in the world. Pure aluminium is a very soft metal so it is normally alloyed with other metals such as copper, silicon, magnesium and manganese to produce a harder, more corrosion resistant material for marine purposes.

Aluminium is approximately one third the density of mild steel, but as it is only about half the strength of mild steel in relative terms, scantling sizes (the width and thicknesses of the shell, frames and stringers), need to be larger when compared with steel construction. However, a significant weight saving is still achieved.

Aluminium does not however offer the same weight advantages as composite construction. Because of the huge variety of materials used in composite construction, it is not possible to give a precise indication of the weight savings involved. But if you were to compare a ‘non-exotic’ composite lay-up with aluminium, you would expect a weight saving of around ten per cent.

Construction Material Overall Structural Weight (%)
Aluminium 100
Steel 150 - 180
Single Skin Polyester/glass 105-120
Polyester/glass skin with balsa core 90 – 95
Epoxy/carbon skin with foam and honeycomb core 55 – 65
(Information Supplied by Gurit/SP)

Aluminium specified for marine use does not rust the way steel does, and can theoretically be left bare in the marine environment. The surface of untreated aluminium oxidises when exposed to air forming a hard air-tight surface that prevents oxidisation reaching the base metal, so once the surface has oxidised the metal will not corrode. Some smaller yachts leave the aluminium of their hulls and superstructures unpainted, however the underwater surfaces are