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30 deploy the sonar very deeply, although he is confident it will work well in most of the wave heights operators are likely to encounter in the kind of inland and coastal waters they will want to use it in. The problem for sonars operating near the surface in rough water is the creation of bubbles that interfere with the transmission and reception of sound. “As the sea state builds you get a bubble layer in the sea, a naturally occurring one, and as that layer builds you need to get the sensor head deeper and deeper to get really good data,” Daltry says. “This is more of a limiting factor than the vessel or its motion.” Modern sensors compensate for vessel motion very well anyway, thanks to the use of survey-grade fibre optic inertial measurement units for example, he says. Self-noise, which is that made by the boat’s propulsion system and water passing over the hull, for example, is also minimal with the C-Cat 3, he notes, thanks to the electric propulsion and the fact that at survey speed it is probably using only half the available power. Also, the sonar is held clear of the hulls, and their narrow, deep profile means they don’t tend to push bubbles down into the water, leaving only naturally occurring ones to deal with. Roto-moulded hulls The principal novel technology in the C-Cat 3’s structure is the use of remoulded polyethylene for the hulls. They are produced for ASV by a specialist in this technology, and the material was chosen because it produces hulls that are robust and easily repairable. The basic procedure involves what Daltry describes as a split aluminium mould about the size of a large conference table. This is heated and then injected with polyethylene powder propelled into it by compressed air through an injection port. The mould itself is placed in a machine about the size of a three-storey building. It is spun around its long axis as the plastic is injected, so that it covers all of the mould’s inner surfaces, melting and running to fill every nook and cranny. After the first layer has gone in, a second type of plastic powder is injected to create what Daltry says is like a composite foam. This is followed by another layer of polyethylene to produce a sandwich-like structure. “There is a higher density in the skins and a lower density in the middle, so you get a reasonable thickness and it’s quite stiff and very robust,” he says. “It is the same colour all the way through, so any scratches won’t show.” The roto-moulding process may be essentially simple but it is not easy, and it took a lot of cooperative work between ASV and the supplier to perfect the process for the hulls. For example, Daltry notes, ensuring that the part does not distort during cooling is something of a fine art. “The supplier does clever things like machine the thickness of the mould so that the heat is transferred in different ways to different parts of the boat. That’s what took a lot of time to get right,” he says. The mould was made by an Italian subcontractor of ASV’s chosen roto- moulding company and was milled from a solid billet of aluminium on a large, February/March 2018 | Unmanned Systems Technology The C-Cat 3 on its custom-made trailer. The hulls are roto-moulded in polyethylene by a specialist contractor, while the gondola is moulded in-house from hand-laid GRP (Author’s image)