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Monday, February 4, 2013

Manufacturing of FRP Boat


Most yachts and boats today are constructed of glass-reinforced plastic (GRP). This strong and resilient material is a combination of a resin (which is made to harden or set chemically) and a strong reinforcement material, usually fibres of glass, which gives the material its common name of fibreglass.
The resin may comprise polyester, solvents, catalysts and other additives. The reinforcement is either glass fibre cloth (a smooth woven fabric), roving (a coarse, basket-like woven fabric) or mat (a random combination of many shortfibre strands of glass).
Production begins with the formation of a smooth female mould (itself usually made of GRP laminate) over a precisely constructed wooden plug, which establishes the hull shape. The colour of the hull is established by gelcoat resin, sprayed against a parting agent, previously applied to the surface of the mould.
Glass and resin are then combined in a hand lay-up process to produce the hull structure. Thickness can be varied by the composition and number of layers and is determined by the correct compromise between the strength and lightness required over different sections of the hull. The deck is produced in the same way.
Thereafter the real skill lies in the fitting out. This includes the construction of bulkheads (athwartships hull-stiffening panels), joinery of the interior and the proper connection of all elements of GRP, wood and metal. 'Sandwich' construction involves laminates of GRP enclosing a core of closed-cell foam or balsa wood. This provides a stiffer structure, weight-for-weight, but has reduced impact resistance. Fittings have to be attached to sandwich hulls and decks carefully, so as not to allow water to seep in and degrade the core.
Increasingly more advanced materials are entering the boat building trade. Epoxy resins and graphite aramid fibres such as KevlarTM, carbon and other new reinforcements promise remarkable strength, stiffness and structural weight-efficiency.
Wooden boat construction
Since the beginning of time, wood has been the traditional boatbuilding material. Ancient ships, and until the nineteenth century, trading and naval vessels, were constructed of wood. Interest in yachting and speed under sail led to lighter but strongly built and carefully designed timber structures. Even 100-year-old yachts, if properly designed, built and cared for, can still be serviceable.
Carvel has always been the most common form of wooden construction. Generally, a skeleton of steam-bent oak is formed to support planks from stem to stern. These are made of light wood in small boats and harder woods, such as elm, in larger craft. The seams between planks are packed with caulking to make the structure watertight.
Clinker/lapstrake construction, common for small boats in the past, is a method where relatively thin, shaped planks overlap each other at the seam. Mechanical fasteners (often copper rivets) join the plank edges, both to seal against leaks and to secure the shell to internal stiffening pieces.
A smoother finish can be achieved by using a moulded wood construction, which involves the fabrication of a single glued-ply unit for the entire hull.
Wooden boatbuilding has had something of a revival in recent years with an increasing number of people rediscovering the joys of more traditional craft. Complete with rigs derived from the working boats of times past, original construction techniques are being enhanced with the use of modern materials, such as epoxy resins, to increase strength and, perhaps most importantly, reduce maintenance while retaining good looks.
The choice of good building materials and a close fit between members are essential to the durability of yachts. Quality construction will go a long way in keeping out water and preventing rot.
Metal boat construction
The strength and durability of metal construction are appealing, particularly for larger yachts. Except for small craft, the traditional riveted connections for steel or aluminium have given way to welded hulls.
Two types of hull framing are possible. Transverse framing involves curved, angled or T-section stiffeners inside the hull in the same pattern as the conventional framing of a traditional wooden hull. Longitudinal framing runs fore-and-aft, itself supported by bulkheads.
Welding is used initially to tack shaped and curved hull plates to the framing grid and to position plate edges to each other. Welding passes are then made to fill all butts and seams, for hull strength and water-tightness. Further selective welds are made to ensure satisfactory connection of the hull plating to framing and stiffeners.
During welding, shrinkage of the weld metal as it cools is a critical issue for the ultimate shape. Therefore, a proper welding sequence, from port to starboard and deck to keel, over the plating must be followed to prevent distortion of the yacht's shape from the desired geometry and to prevent the building up of internal stresses, which may limit the external load-carrying ability of the hull structure.
Skillful lofting - the full-sized drawing out of hull-shape lines, accurate forming of frames and set-up - plus a proper welding sequence, can achieve an accurate, reasonably smooth hull surface. Nevertheless, for a proper yacht-quality hull surface, a layer of fairing compound is required over the metal plating. After a priming coat to inhibit corrosion and ensure proper bonding, the filler material is trowelled on and finally hand-sanded to the desired accuracy and smoothness using long, flexible sanding planks. Conventional or sprayed polyurethane paint coating finishes the job.
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