## 3 Trends in Operational USVs
### Power Sources, Endurance, Propulsion, and Top Speeds  
The distribution of power sources in USVs reveals a significant reliance on non-battery-powered systems, although battery-powered models still constitute a notable portion (Figure 7a). This contrast likely reflects operational requirements, as battery-powered USVs are well suited for shorter missions and environmentally sensitive areas due to their quiet operation and low emissions. Non-battery-powered USVs, which include diesel, gasoline, and hybrid systems, are preferred for longer endurance and higher power demands. The dominance of non-battery propulsion suggests that energy storage limitations in battery technology still pose a constraint for extended or high-power applications. However, the presence of battery-powered USVs highlights the industry's growing interest in electrification, particularly for compact, autonomous systems operating in nearshore and  
Figure 6: Distribution of (**a**) dry weight (kg) and (**b**) payload capacity (kg) across the USVs. These distributions do not include the Armada 78 USV data.  
controlled environments. This trend aligns with propulsion choices, which is discussed in the following.  
The endurance of USVs spans a wide range; thus, we used a logarithmic scale on the endurance axis (Figure 7b). This indicates that while many USVs are designed for missions lasting only hours (first peak), a subset is optimized for extended endurance around one month (second peak). A noticeable zone regards the wind- and/or wave-propelled USVs, which can easily achieve 12 months of endurance. The lower-endurance USVs are likely employed for tasks such as coastal monitoring, port security, and hydrographic surveys, where frequent recharging or refueling is feasible. In contrast, the long-endurance models, often non-battery-powered, are used in persistent oceanographic research, military reconnaissance, and offshore energy applications. This endurance variation reflects the balance between energy capacity, propulsion efficiency, and mission-specific requirements.  
The dominance of electric propulsion in USVs highlights a strong industry preference for efficient and low-maintenance systems, with diesel and gasoline propulsion making up a smaller portion of the fleet (Figure 7c). Electric propulsion aligns with the increasing adoption of battery-powered systems seen earlier, particularly in compact USVs designed for shorter missions. These systems offer reduced operational costs, lower noise signatures, and fewer mechanical complexities. However, diesel and gasoline engines remain  
Figure 7: Distribution of key operational characteristics of USVs. (**a**) Comparison of battery-powered and non-battery-powered USVs. (**b**) Distribution of endurance in days on a logarithmic scale. (**c**) Breakdown of propulsion types, distinguishing electric from diesel/gasoline. (**d**) Distribution of top speeds in knots.  
relevant, especially for longer-endurance and high-speed applications requiring sustained power. The reduced share of internal combustion engines suggests a gradual transition toward more sustainable and technologically advanced propulsion methods. This balance between efficiency and performance influences another crucial factor in USV design--their top speed.  
The speed distribution of USVs is highly skewed toward lower speeds, with most models operating between 3 and 12 knots, reflecting a design preference for efficiency and endurance (Figure 7d). A few models exceed 15 knots, indicating specialized applications such as rapid response or military operations. The clustering around lower speeds suggests that most USVs prioritize energy efficiency over higher speeds, particularly for missions requiring extended endurance. This speed distribution further reinforces the industry's focus on balancing endurance, power efficiency, and operational reliability across various mission profiles.