Hi-tech sail and mast system
Too technically advanced for bumbling Jack Sparrow of Hollywood film ‘Pirates of the Caribbean’ fame, a new 87.5m sailing vessel, the Maltese Falcon, could well put Sparrow’s Black Pearl to shame. The brainchild of sailing enthusiast and Silicon Valley financier Tom Perkins, this state-of-the-art clipper yacht combines fibre optical sensing with the world’s largest free-standing composite masts.
Insensys, based in Southampton, UK, took on the mammoth task of bringing Perkins’ design to fruition by developing the hi-tech sail and mast system for a vessel that is estimated to have cost US$100m on completion and was built by Italian specialists Perini Navi.
‘It was a sound engineering project,’ says Chief Executive of Insensys Martin Jones, ‘We were dealing with extreme structures in an extreme application.’
The steel-hulled vessel is a modern take on the traditional square-rigged clipper design, with three free-standing masts at a record 57m high and weighing 26 tonnes. Each mast is made from six curved carbon fibre yards and carries five sails that can be individually adjusted to create a single continuous sail with low drag. Hydraulic motors are used to rotate the masts to trim the sails so that they are aligned to the wind for maximum speed. This provides improved aerodynamic efficiency.
This concept – DynaRig – was originally conceived by German hydraulics engineer Wilhelm Prolls in the 1960s. However, at the time, the high strength materials required for keeping the large free-standing masts upright were not available.
Harry Pim, Project Manager of the mast build project at Insensys, says, ‘I am not aware of any other sailing vessels fitted with a DynaRig. [But] most of the big rigs built today are made from carbon fibres, especially in the racing yacht sector. Carbon fibre/epoxy resin laminates are used because of their high strength-to-weight ratio, allowing a mast to be built that would be considerably lighter than a steel or aluminium equivalent.’
Features of Insensys’ composite technology include the ability for each mast to be manufactured in one continuous length. A laminate applies each layer of fabric, which is separately cut and tapered to produce the required thickness and properties along the length of the mast.
‘Typically, each mast section consisted of 120 layers of fabric, shown on an individual sheet, specifying material type, position and trimming,’ adds Pim.
Sticking with the theme of advanced technology, each mast on the Maltese Falcon is embedded with 40 fibre optic sensors installed on 10 cables. These devices provide the crew with reliable information regarding wind speed and forces, the stresses on the masts, as well as data on the driving and drag forces from each rig. This enables the crew to optimise the setup of each rig and the trimming of the sails for maximum efficiency, and allows them to make decisions regarding the amount of sail they can carry and how hard they can push the yacht.
Pim says, ‘Fibre optics have been used in yachts previously. However, this is the first time that the data is fed back continuously through a customised software system. There is a screen on the bridge of the yacht which displays real-time data.’
Laminated to the inner skin of the masts using epoxy and uni-directional carbon fibre tapes, the sensors are designed to withstand the extreme weather changes experienced at sea. They operate using fibre bragg grating (FBGs).
‘These FBGs are a series of stripes written into the fibre using a laser. When light hits the stripes, a certain wavelength of [it] is reflected, depending on the separation of the stripes,’ explains Pim.
‘As the fibre is strained, the pitch of the strip changes and so does the reflected wavelength. By measuring [the latter] you can measure the strain at each sensor’ to an accuracy of one microstrain.
The system operates via Insensys’ Time Domain Multiplexing software, which employs the time-of-flight method to distinguish one sensor from the other.
Pim says, ‘with a single instrument we can interrogate more than 100 sensors in a single fibre, using [one] connector. All other similar instruments use wavelength to differentiate between sensors – each sensor has to be in a certain wavelength slot. This limits them to between five and 10 sensors per cable.’
Given the scale of the project, extensive testing was conducted using wind tunnel studies, smaller models of the yacht with the bending and torsional loads applied, and a radio-controlled version to investigate manoeuvring. A full-scale replica of the structure was also erected and trialled for a year, before the team descended on a shipyard in Turkey where the Maltese Falcon was constructed. Two 500 tonne cranes were required for stepping the masts into position.
‘The dimensions of the mast are around that of a football pitch, so getting the exact lift and tilt was essential,’ says Vice President of Engineering at Insensys Damon Roberts. ‘It was one of the most intense loads ever lifted and there was just a 4mm clearance between the mast and the deck.’
But all the hard work paid off when the Maltese Falcon reached a maximum speed (so far) of 18.3 knots during its trial run this summer, with only five sails set.
‘DynaRig is no longer an experimental concept,’ enthuses Perkins. Insensys is now ready to shift its emphasis and will apply its fibre optic sensing technology to the oil, gas and aerospace industries.