# Fleet submarine

A fleet submarine is a submarine with the speed, range, and endurance to operate as part of a navy's battle fleet. Examples of fleet submarines are the British First World War era K class and the American World War II era Gato class.
The term has survived in Britain to refer to modern nuclear-powered attack submarines. In the United States Navy, the term came to be used primarily for the long-range submarines that served in World War II.

## Examples
### United States

The term was used by the United States Navy to distinguish submarines suitable for long range patrols in the Pacific Ocean from earlier classes such as the United States S-class submarines. The initial goal, pursued with frequent interruptions since the AA-1-class (aka T-class) launched 1918–19, was to produce a submarine with a surfaced speed of 21 knots to operate with the Standard-type battleships of the surface fleet. Most of the nine "V-boats" launched 1924–33 (V-1 through V-6) were either attempts to produce a fleet submarine or were long-range submarine cruisers. Eventually, a long range of 11,000 nautical miles (20,000 km) was combined with high speed, beginning with the Salmon-class launched in 1938, to allow sustained operations in Japanese home waters while based at Pearl Harbor. These qualities also proved important in the Pacific commerce raiding of World War II, but the 1922 Washington Naval Treaty's prohibition on unrestricted submarine warfare precluded inter-war planning in this area. Although the Gato-class was considered the fully developed archetype, the earlier Porpoise, Salmon, Sargo and Tambor-classes were incrementally improved prototypes distinctly different from the two contemporary experimental Mackerel-class coastal submarines. The Tambors were fully developed and similar to the Gatos except for diving depth and separation of the engines into two compartments.

### Japanese

Japanese I-boats were a conceptually similar long-range differentiation from smaller "medium" or "sea-going" Ro-boats, although some I-boats had features like aircraft hangars and large-caliber deck guns more often associated with submarine cruisers.

### British

In order to get the speeds – over 20 knots while surfaced – required to match their capital ships and to be able to screen ahead of the fleet or flank the enemy, the British initially used steam propulsion. The K-class entering service in 1916 were large for their time. Although able to reach 24 knots the complexity of shutting down boilers and stowing funnels made them slow to dive.
As the speed of capital ships increased, the United Kingdom abandoned the fleet submarine concept following completion of three 21-knot River-class submarine submarines of the early 1930s using supercharged diesels, because the size required for range and surface speed decreased maneuverability.

### Others

Continental European nations sometimes used the terms "ocean-going", "long-patrol", "type 1" or "1st class" submarines, generally referring to Atlantic or Indian Ocean operations in the absence of anticipated need for Pacific patrols, and often without the speed for fleet operations.

## Comparison of World War II submarines

| Name | Type | Nation | Surface Displacement | Submerged Displacement | Speed | Torpedo Tubes | Crew |
| Gato class | fleet submarine | United States | 1,525 tons | 2,415 tons | 20 kt | 10 | 80 |
| Thames class | fleet submarine | United Kingdom | 1,850 tons | 2,723 tons | 22 kt | 8 | 61 |
| Kaidai class | fleet submarine | Japan | 1,833 tons | 2,602 tons | 23 kt | 6 | 80 |
| Type IXD2 | ocean-going submarine | Germany | 1,616 tons | 1,804 tons | 19 kt | 6 | 57 |
| Redoutable-class | ocean-going submarine | France | 1,570 tons | 2,084 tons | 17 kt | 9 | 61 |
| Kaichū type | medium submarine | Japan | 1,115 tons | 1,447 tons | 19 kt | 4 | 80 |
| Type XB | minelayer | Germany | 1,763 tons | 2,177 tons | 16 kt | 2 | 52 |
| Cagni class | submarine cruiser | Italy | 1,461 tons | 2,136 tons | 18 kt | 14 | 85 |
| Type B1 | submarine cruiser | Japan | 2,584 tons | 3,654 tons | 23 kt | 6 | 100 |
| O 21-class | medium submarine | Netherlands | 888 tons | 1,186 tons | 19 kt | 8 | 55 |
| Type VIIC | medium submarine | Germany | 769 tons | 871 tons | 17 kt | 5 | 44 |
| Pietro Micca | minelayer | Italy | 1,371 tons | 1,883 tons | 15 kt | 6 | 66 |
| 600 series | medium submarine | Italy | 615 tons | 855 tons | 14 kt | 6 | 41 |
| S-class | medium submarine | United Kingdom | 715 tons | 990 tons | 14 kt | 6 | 44 |
| Grampus class | minelayer | United Kingdom | 1,520 tons | 2,157 tons | 15 kt | 6 | 59 |
| Minerve class | medium submarine | France | 662 tons | 856 tons | 14 kt | 9 | 41 |
| Narwhal-class | submarine cruiser | United States | 2,730 tons | 4,050 tons | 17 kt | 6 | 90 |
| Surcouf | submarine cruiser | France | 3,250 tons | 4,304 tons | 18 kt | 12 | 118 |
| Argonaut | minelayer | United States | 2,710 tons | 4,080 tons | 15 kt | 4 | 89 |
| S-boats | medium submarine | United States | 840 tons | 1,150 tons | 15 kt | 4 | 42 |

## Modern boats
### United Kingdom

Since the mid-1960s, the Royal Navy has used the term Fleet Submarine for its nuclear-powered attack submarines, distinguishing them from the ballistic missile submarines (and, in the 20th century, from the diesel-electric submarines, which it termed 'Patrol Submarines').
The classes of Fleet Submarine currently in service are the Astute-class and Trafalgar-class.