Morphing and adaptive structures are experiencing a surge in research due to their ability to optimize the operational envelopes of complex vehicles. Adaptive structures have been researched heavily and put to use extensively in the Aerospace Industry. This paper examines a possible use and benefit associated with morphing structures with Unmanned Underwater Vehicles (UUVs). The growing résumé of applications associated with UUVs as well as their unique abilities make them an excellent subject of study for morphing concepts. This paper focuses on the possible uses of flexible hull morphing, specifically the benefits associated with such adaptability in terms of range, endurance, and speed. This research proposes a UUV system which has a flexible hull wrapped around a standard pressure hull. The annulus created between the pressure hull and the flexible hull is used for the storage of expendable energetics, in this case diesel fuel. Storing fuel in the newly created annulus alsoeliminates the need to waste space within the pressure hull storing fuel and thus the pressure hull may also be shrunk. This paper focuses on the benefits of morphing where the shape of the flexible hull remains the same but the diameter of the outer hull is shrunk in concert with the consumption of fuel thus decreasing the drag profile of the vehicle, resulting in increased vehicle range. Two different UUV missions are outlined: 1) slow speed scan and 2) slow speed scan with high speed dash. A commercial semi-submersible was used as a baseline. For the slow speed scan mission, semi-submersible heavy vehicles carrying in the range of 50,000kg of fuel could realize increases of range close to 40% when compared to a similarly sized vehicle without the adaptive capabilities. Small vehicles on the other hand, show limited benefits of diameter morphing, gaining less than 5% inrange over the non-adaptive system. Dash missions during which a scanning operation must be augmented with some sort of high speed maneuver show more significant benefits with a morphing system. The results show that diameter morphing is clearly not viable in all situations, yet the concept shows promise for certain long range/long endurance UUV missions, as well as missions that require sporadic high speed operation.