Large yachts are usually built with more than one mast in order to divide the sail area for easier handling. However, the latest developments in spars, rigging, sail making, powered winches and electronics enable yachts to have larger sail areas and still be practical for handling. In 2004, VT Shipbuilding created the world’s largest single-masted yacht ever built, the Mirabella V, and it was designed to have only one mast, meaning it was the tallest mast and largest jib of any sailing craft at that time
The 90 m carbon fibre composite mast weighed 43 tonnes with rigging and sails. In full wind, enormous pressures are exerted on the mast, requiring it to tolerate 1,000 tonnes of compressive load at its base. This required stronger and thicker materials than previous yachts. Whereas smaller masts were designed to avoid buckling under the pressure, the larger mast could fracture, creating different design challenges. Mechanical engineers struggled to meet the mast’s mechanical specification, throwing the whole project into jeopardy. VT Shipbuilding turned to NPL to identify the right material and make sure the mast was strong enough.
NPL made available a user-friendly software package called Composite Design Analysis (CoDA) which could predict the 3D material properties for a synthesised composite material. The synthesised material properties can be used to predict the behaviour of beam and panel structures, joints and flanges. In order to determine the material for the mast with confidence that it would have the correct properties, the CoDA programme was run in reverse. By starting with the desired mechanical properties of the mast, NPL was able to inform the material selection, in particular the fibre properties needed. In order to avoid buckling the size to weight ratio of the mast had to increase, which could mean that fracture in compression was more likely if the wrong material or lay-up was chosen. A commercially available fibre was identified which met the required material specification for the specified laminate lay-up.
NPL then performed mechanical tests on 20 mm thick samples of the carbon fibre composite material using a universal test machine, which applied loads to simulate the 1,000 tonnes load the mast would experience in use. These tests were based on the tests for compression strength as advised in the standards, but modifications were required since established test methods (compared with ISO 14126 drafted by NPL which uses 2 mm thick specimens) were not designed for material sections as thick as these. Materials compression tests showed that if the woven fibre format was too tightly stitched it created weaknesses. Loosening the stitching improved the mechanical performance and these adjustments were made by the fabric supplier.
The yacht was successfully launched in 2003 and remains the largest single-masted yacht ever built. This work is an example of materials evaluation which is required when composites materials are used in new applications. This work informed other examples of ‘thick’ cross-sections on wind turbine blades, under-sea buoys and smaller aircraft propeller blades.
NPL are involved in writing standards, but also giving advice for when there are no standard test method, either because it is a new application, manufacturing method or a novel material. NPL can thoroughly evaluate materials and allow them to be used with confidence.