The term FRP is generally accepted to mean fibre reinforced plastic. The names fibreglass reinforced polyester, resinglass, and glass reinforced plastic (GRP) are also used. This material is a plastic and is unique in that it is made by the user in situ. It is composed of a series of reinforcements and liquid chemicals which when brought together in specific proportions can be formed into strong, solid but flexible shapes. By varying the quantities of the main components, the finished product can achieve different properties to suit the desired application.
The material has been developed over the last forty years to have many varied characteristics. A crude use is the casting of ornaments in slow setting, filled resin with no reinforcement, aviation is at the other end of the technology spectrum where whole wings of military fighters are made of carbon fibre reinforced plastic. At the low technology end the property of faithful reproduction is employed while at the other a very strong lightweight structure is made to exacting standards. In between are manufacturers of motor car bodies, furniture, pre-fabricated buildings and boats who use combinations of the properties of faithful reproduction and strength to varying degrees.
By changing the chemical composition of the resins and varying the reinforcements, properties of finished FRP may be designed to suit different applications. These may be heat or fire resistance, resistance to acids and fuels, or to be taste and odour free for water or fish tanks.
Historically, FRP as a boatbuilding material was developed for military purposes in North America in the late 1940's. Early hulls were made by draping the reinforcement over a pattern or former made of wood (male mould) and painting on the resin. This was an era before catalysts were developed when strong sunlight was used to make the composition harden. This is called “curing”. From these crude beginnings faster curing resins were developed followed by contact moulding with female moulds (materials laid on the inside) for mass production.
Soon decks and interiors were being made of FRP so that boats were no longer hand crafted in wood piece by piece but bolted and bonded together on an assembly line. The cost saving was realized by the boatbuilding industry in the developed world to the extent that today yachts, motorboats and small workboats in FRP are more popular than wooden boats. FRP boats now available range in size from rowing boats of 2 m to naval minesweepers of 80 m length and types from yachts through commercial fishing boats to harbour tugs. In terms of materials and modern production technology Sweden, France, United States and Great Britain have the most to offer.
The prime reason in developed countries for the change to FRP was lower production costs for small boats built in series. Industrialized countries have the advantage of low raw materials costs (resin and reinforcement) but the disadvantage of high labour costs. Therefore the opportunity to change from labour intensive methods to a faster and less skilled productions system was attractive.
In the developing world the reverse is the case where any FRP boatyard must import all raw materials and is therefore subject to attendant problems of foreign currency and continuity of supply. This major disadvantage must be weighed against labour costs and carefully considered before any decision is taken to embark on investment in FRP production.
It must be emphasized that a certain level of technical expertise is needed when attempting to build boats in FRP together with a basic understanding when embarking on a boatbuilding programme.
The fundamental tool for the production of FRP vessels is the mould. The most common type is the female mould which can be described as a reverse or mirror image of the finished hull which allows the FRP materials to be laid up on the inside. It too is made of FRP and is cast from a “plug” which is a reproduction of the hull or deck, faithful in size, shape and every detail.
The plug is the beginning of the whole process and is an exact hand crafted replica of the final hull. Normally it is made from wood and used only for casting the mould and is then discarded. It requires high skill levels to achieve fairness and a smooth finish. But this is reproduced faithfully each and every time a hull is made, so the better the mould, the better the hull.
It can be said that the first hull made from a new mould has been built three times:
WOODEN PLUG→ FRP MOULD→FRP HULL
This gives some idea of costs and effort needed in setting up FRP production. Therefore it is important to choose the most suitable design and to produce at least a minimum number of boats to recover the investment made for the production of the plug and mould.
For larger vessels the tooling-up phase (plug and mould building) is repeated for the deck, wheelhouse and interior moulds which increases further the investment costs before a single vessel is produced. The sequencing of these tasks is complicated and it is emphasized that thorough planning and careful choice of vessel must be undertaken before commitments are made.
The main material components previously mentioned are the reinforcement and resin. The most popular reinforcement used is a form of glass. It is processed into filaments then woven or chopped and supplied in rolls similar to bolts of cloth. The thickness of the cloth varying with the weight of the glass in grams per square metre. The two main types are “chopped strand mat” and “woven roving”. At a working level, there are two main types of resin - “laminating” and “gelcoat”. The former is a translucent liquid of various pale colours with a strong smell of styrene which is characteristic of resins. The latter is a more viscous liquid with a similar smell. The difference is in use where the gelcoat is applied directly to the mould without a reinforcement and is mainly to provide a smooth, coloured finish to the outside of the hull while the laminating resin provides the matrix within which the reinforcement is bedded. These and other components will be explained in the following sections.
Figure 1 Hull plug
Figure 2 Hull mould
(See also Figure 29)
Figure 3 Hull
Figure 1 shows the wooden plug for a 15.0 m Sambuk (typical inshore fishing boat on the Red Sea), Figure 2 the FRP mould laid over this plug and Figure 3 the FRP hull released from mouldwith all internal stiffeners installed.
Reduction of maintenance
No caulking, no leaks. Hulls are one continuous piece of FRP with no joints or gaps to allow water into the hull.
No plank shrinkage when laid up. Wooden hulls suffer from plank shrinkage when brought out of the water and laid up in the sun. FRP does not shrink or swell so leakage and re-caulking are avoided.
Rot proof and resistant to borers. FRP is non-organic and will not rot. As a plastic it cannot be eaten by marine borers.
Corrosion and electrolysis reduced. FRP is inert. As a plastic it will not corrode.
Simpler construction. Once a mould is made, identical copies of a hull can be made many times over and in a shorter time.
Reduction of skill levels required once a basic training is received.
Total dependance on imported materials and foreign currency availability.
Choice of vessel fixed once design is chosen and moulds made.
Must retain core group of qualified technicians.
Fire and health hazards from chemicals.
Large start up investment.
The main materials used in boatbuilding are wood, steel, aluminium, ferrocement and FRP. Each has its optimum use just as each has advantages and disadvantages.
Wood is the best known traditional material but depends on a shrinking forest resource and highly skilled carpenters. Marine grade aluminium is lightweight, long lasting and requires a skilled labour force whereas ferrocement uses low cost materials and a large labour force, each has its applications.
Steel is easier to obtain than aluminium, is more rugged but corrodes if unprotected, it is the prime material for shipbuilding. FRP is the newest arrival and will be discussed at length in the following chapters.
Table 1 and 2 compare some physical properties and costs per unit weight of FRP, wood, aluminium and steel used for boatbuilding. Table 3 compares weights.
|Material||Specific weight||Tensile strength||Compressive strength||Elastic modulus|
|1b/ft3||tonne/m3||kN/m2 × 10||kN/m2 × 10||kN/m2 × 10|
|Material||Cost/unit Weight||Equal tensile strength||Equal bending stiffness|
Note:: Based on the scantlings as given by Lloyd's Register of Shipping for FRP, Rajendran and Choudhury (1969) for wood, Hanson (1960) for steel and New Zealand Ministry of Transport for ferro—cement