## 1 Introduction  
Marine energy systems can take many forms, such as tidal/ocean current turbines, crossflow turbines, wave point absorbers, and oscillating surge flaps and water columns [1]. Because of the nascent nature of the industry, system designers and developers are yet to converge on a single design philosophy and may never do so because of the broad variety of potential operating conditions (see Figure 1). Many marine energy developers are still very much in the prototyping phases of their designs and system deployments. As a result, there is a great opportunity for low-cost, short-lead-time prototype manufacturing techniques to expedite the deployment of devices for testing and validation. Composite materials, such as fiberglass/epoxy, are an attractive material choice for marine energy systems because of their low cost, high stiffness and strength, and resistance to environmental degradation [2]. Material density and structural mass are of lower importance because the structures are submerged underwater where buoyancy forces offset gravity-induced forces. In fact, marine energy structures are often filled with ballast such as concrete, epoxy slurry, or water to minimize buoyancy loads [3]. Naturally, a large amount of design and manufacturing knowledge has been taken from similar industries, such as wind blade manufacturing and composite boat building. Tidal turbine blades in particular are very similar in design to wind turbine blades, but on a smaller scale and with different loading and stiffness requirements [3]. They also rely on very similar manufacturing techniques and processes; for example, vacuum-assisted resin infusion molding (VARIM) of multiple shells and webs that are subsequently bonded together. The mold making for this process is incredibly expensive and time-consuming [4]. Depending on blade length, molds can cost in the order of hundreds of thousands to millions of dollars, with lead times in excess of 6 months [4], although very little public information is available in this area. This is acceptable on a mass-manufacturing scale in which hundreds of components will be produced from the same set of molds but may be unattainable for small-scale developers in the prototype testing phases of their development, particularly if designs are continuing to evolve. Despite this, large-scale mold manufacturing processes have changed very little in the last 20 years, highlighting a need for faster and cheaper molding practices for large-scale composite structure prototyping and development. A variety of additive manufacturing processes have the potential to fill these gaps in the marine energy system structural design and validation process. In particular, recent advances in large-scale fused deposition modeling (FDM) have shown exceptional promise for reducing costs and lead times for wind turbine blade molds and components [5; 6; 7]. This is particularly exciting for exploring on-site manufacturing and prototyping unique test deployment concepts [8; 9]. Another area of interest in FDM style printing is the co-extrusion of continuous fiber polymer composites [10], allowing for the direct printing of fiber-reinforced polymers with no molding required at all. This technology is still relatively new and only available at a small scale, with low-fiber-volume contents (<35%) [11] and a high probability of manufacturing defects like poor consolidation, but may provide an alternative to traditional composite manufacturing processes in the future. Figure 1: All six reference models developed by the Department of Energy Reference Model project [1]. Graphics courtesy of Sandia National Laboratories. Based on these current advances in additive manufacturing and industry needs, this research hypothesizes that they will allow for novel innovations in marine energy system composite structure prototyping and manufacturing.