An auxiliary ship is a naval ship which is designed to operate in any number of roles supporting combatant ships and other naval operations. Auxiliaries are not primary combatants, although they may have some limited combat capacity, usually of a self-defence nature.
Auxiliaries are extremely important for navies of all sizes, as without them, the primary fleet vessels can not be effective. Thus, nearly every navy maintains an extensive fleet of auxiliaries. However, the composition and size of these auxiliary fleets varies depending on the nature of each navy and its primary mission. Smaller coastal navies tend to have smaller auxiliary vessels focusing primarily on littoral and training support roles. Larger blue-water navies tend to have large auxiliary fleets comprising longer-range fleet support vessels designed to provide support far beyond territorial waters.
One of the most direct ways that auxiliaries support the fleet is by providing underway replenishment to major fleet units. This allows the fleet to remain on station, with the replenishment vessels bringing up fuel, ammunition, food, and supplies from shore to the fleet wherever it is operating. Oilers are vessels specifically designed to bring fuel oil to the fleet, while the earlier Colliers supplied coal burning steamships. Tenders are specifically designed to support a type of smaller naval unit, most often submarines or seaplanes, providing a mobile base of operations for these units.
Supporting forward operating bases requires immense transportation capacity. Transports are often converted merchant ships simply commissioned into naval service. Tankers are transports specifically designed to ship fuel to forward locations. Transports are often employed not only carrying cargo for naval support, but in support of all forces of a nation's military. In particular, troopships are used to carry large number of soldiers to operational theatres. Some transport ships are highly specialized, like the ammunition ships employed by the US Navy.
Repairing ships at sea or in forward areas is important as it allows these units to return to service quicker, while also increasing the chance of survival for ships critically damaged in battle. Repair vessels range from small equipment ships to floating dry docks.
Harbor support is a critical support role, with various types of vessels including tugboats, barges, lighters, derricks, and others, used to move ships and equipment around the port facilities, and depot ships and tenders to service ships currently in the harbor. These vessels also help maintain the harbor by dredging channels, maintaining jetties and buoys, and even providing floating platforms for port defense weapons.
A wide variety of vessels are employed for research and survey, primarily to provide a navy with a better understanding of its operating environment, or to assist in testing new technologies for employment in other vessels.