Of the three services, the Navy's structure is the most flexible and indefinite. The Navy can be roughly divided into ships and shore establishments, a shore establishment being a naval base on land. Apart from the obvious material differences, however, there is little difference between the two. A naval shore establishment is commissioned just like a sea-going vessel and, just like a sea-going vessel, is known as Her (or His) Majesty's Australian Ship. Although they have different jobs to do, the personnel at a naval base are also organised in a similar manner to those aboard ships, having daymen, watchkeepers and duty watches.
A flotilla is composed of two or more ships: these can be destroyers, submarines, mine-sweepers, corvettes or light patrol craft. In the case of destroyers, a flotilla has a rigid structure. There are nine vessels in the flotilla, which are divided into two divisions of four, with another vessel leading. The divisions are further divided into two sub-divisions of two vessels. There is no fixed organisation for flotillas of the smaller vessels. In the case of three or more vessels, they can also be divided into divisions and sub-divisions. A flotilla is usually commanded by a captain, and a division by a commander. Destroyers and submarines are usually commanded by a lieutenant commander, and smaller vessels by lieutenants.
A naval squadron is composed of two or more capital ships. Capital ships are large powerful vessels - battleships, cruisers or aircraft carriers - usually commanded by a captain. Squadrons of more than three ships are generally organised into smaller units known as divisions and sub-divisions. A squadron can be commanded, depending upon its size, by a commodore, rear admiral or vice-admiral.
A task force, or task group, is a number of ships grouped to carry out a specific task such as the escort of a convoy or the protection of an aircraft carrier. There is no set structure for a task force and the number and type of vessels allocated depends upon the tasks the group is expected to carry out. A task force is usually identified by a number. Task Force 44, which served in the Pacific during the Second World War, consisted of the Australian cruisers HMAS Australia, Canberra, Hobart, and two United States Navy ships, the destroyer USS Perkins and the cruiser USS Chicago. During 1991, the RAN task force sent to enforce the naval blockade of Iraq, known as Task Group 627.4, consisted of the guided-missile frigates HMAS Darwin and Adelaide and the supply ship HMAS Success. A task force is usually commanded by a commodore or a rear admiral.
An amphibious force consists of a number of vessels grouped together to land an army from the sea on enemy territory. It has no fixed composition and usually consists of transport ships, landing ships and landing craft, as well as a variety of armed ships tasked with the protection of the landing from enemy sea or air attacks, and the bombardment of enemy positions ashore. An amphibious force also includes small units of specialists to survey beaches and select feasible landing spots, and to control and guide the landing.
A fleet is the largest naval structure and would usually contain a number of squadrons, flotillas and/or task forces. Again, its structure is flexible. Often a fleet is organised to work in a specific geographic area, such as the British Pacific Fleet during the Second World War. The RAN has never had enough ships to form more than one fleet, so the fleet, in terms of the RAN, equates with the entire sea-going navy. The United States Navy, however, has many fleets. A fleet is commanded by an admiral.