The technique of drawing molten glass into fine threads has been known for more than three thousand years, and in the last few decades the high tensile strength of these filaments has been put to use in the reinforcement of resins. A fibreglass hull is only partly glass, the rest is polyester resin, so it is better to call this composite material Resinglass. The glass content of a moulded hull may lie between thirty and seventy per cent, from which it follows that the same may be said of the polyester content. The most common form in which glass is laminated or ‘laid up’ is the Chopped Strand Mat (CSM). This is a mat of randomly-laid strands tacked temporarily together by an adhesive which is soluble in the moulding resin. Individual fibres are about one-hundredth of a millimetre in diameter (ten times as thin as a human hair, say). They are made up into strands which in turn can be woven into cloth. A coarse form of cloth, loosely woven from bundles of strands is known as ‘woven Rovings’, which is used by some builders in hull moulding. If all woven rovings were used, the glass would account for some seventy percent of hull weight: if all CSM it would account for thirty per cent. Some moulders use a mixture of the two forms of glass