82 UVD | Roboat’s 3DFEA USV Hardinxveld there is such a company, by the name of 10XL,” van den Heuvel says. “The structural design needed to be adapted to their printing machinery and processes, and you have to be careful and precise when you’re printing really huge parts running the length and width of something like our ferry design, because there’s a lot of shrinkage of the material as it cools following extrusion. You don’t just hit the print button – you have to pay close attention to see if the material contracts and sets as hoped.” In the end, 10XL successfully printed all of HSG’s parts to the tolerances requested, with the design process for the prototype having taken between two and three months. The smallest 25% of the ferry’s pieces were printed first, and these were then tested using a variety of methods (including destructive testing). These parts included the thruster foundations, as HSG wanted to integrate the thruster with a round flange, and hence sought to investigate how well that flange could be joined to the hull. “To do that, we printed a flange, made it watertight and welded it to a vessel hull part, before carrying out destructive testing of those welds to see how much force and what kinds of forces the material could overcome,” van den Heuvel says. “In the end, we did a lot of testing of the material and the different hull parts before moving to printing and assembling the first complete vessel.” Key standards on testing methods and strength requirements for the structural parts of vessels have been published by groups such as Bureau Veritas and Lloyd’s Register Group, and this provided some guidance for HSG to follow. However, such standards have yet to take strict account of the kinds of materials and processes leveraged in 10XL’s printing. In the case of the Tony ferry, these were recycled polypropylene and fused deposition modelling (also known as fused filament fabrication, or sometimes just material extrusion additive manufacturing), which are still largely new to the boat-building world. “We set up some tests in collaboration with the Dutch Transport Agency, which was responsible for certifying the ferry, because if, like us, you want to carry more than 12 passengers, you have to be certified under their transport rules. We also made use of Bureau Veritas’ rules and regulations related to HDPE [high-density polyethylene], as HDPE has quite similar mechanical and chemical properties to polypropylene,” van den Heuvel explains. For added strength and stiffness, the polypropylene is combined with chopped glass fibres (at a 70/30 ratio of plastic to fibres) and printed as fibreglass. “The glass granulate is also provided by recycling companies, and it sits in a sort of large hopper above the printer, such that, like the plastic, it is partially melted before entering and then leaving the extrusion head,” says van den Heuvel. “The whole hull can be printed at once, in about 10 days, with some parts, such as the thruster foundation, printed separately, and we add some polypropylene brackets to those for welding them to the hull. The welding process was no different to our usual way of working with such plastics, so the fact that we were joining 100% polypropylene brackets to 70% polypropylene-fibreglass composite showed zero adverse effects and merited no changes to our welding technique. “The boat thruster tunnels were made from a polypropylene pipe and welded into the hull. Printing such perfectly round structures is still a bit difficult for additive manufacturing machines, even if you want to print them separately and fit them into your hull afterwards. If an additively printed part cracks or breaks, it can sometimes be repaired via a weld, or in extreme cases we can reprint the part over a matter of days.” Outer hull HSG considered designing the ferry as a catamaran early in the process, as catamarans are very stable at speeds above 15 kph, but as the ferry will mostly travel slower than that, it was determined that a monohull would be better for reducing hydrodynamic drag. “We have a fairly wide, simple monohull, except that we also had to design a shape for the four thrusters to fit under, and as we don’t sail very fast, hydrodynamics don’t play a big part in the energy consumption. In fact, the outer surface of the hull remains a little April/May 2024 | Uncrewed Systems Technology 10XL, as HSG’s chosen additive-printing partner, printed the hull from recycled polypropylene and fibreglass composite (Image courtesy of Holland Shipyards Group)
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