Key Point: With the advent of nuclear power and the guided missile, the seemingly goofy idea of a heavy surface-attack submarine became an impressive and chilling reality.

A century ago submarines presented as much of a challenge for naval war planners as drones do today. Like airborne and surface stealth decades later, underwater stealth was a game-changer—but how to use it?

Until the missile age, a wide variety of naval missions and vessels tried applying the submarine’s advantages to best use. From early on, planners envisioned submarines carrying out commerce raiding, minelaying, shore bombardment and intelligence collection—besides the sub’s obvious role in fleet attack.

The limited undersea endurance of pre-atomic subs shaped design and tactics. At the time, submarines largely cruised on the surface and only submerged for brief periods. Early concepts of operations foresaw subs scouting ahead of the battle fleet, then submerging below an enemy fleet to surface and attack it from behind.

During World War I, sub-surface torpedo attacks on warships and merchant vessels became the submarine’s preferred attack technique. However, many engagements involved substantial surface combat.

Deck armament often equaled that of coastal and riverine gunboats—one or two artillery pieces plus several machine-gun mounts for anti-aircraft and surface attack. During these engagements, a five- or six-inch deck gun could wreak havoc on upperworks, steering gear or shoreline structures. Machine guns could rake decks and lifeboats.

But convoys of armed merchantmen and their destroyer escorts were another matter. So was stealthy shore bombardment of coastal bases and defenses. These missions inspired truly remarkable undersea ‘cruisers.’ These were truly big-gun submarines.

Undersea Cruisers

The British Admiralty appears to have kicked off the idea with its extraordinary response to reports of the Kaiser’s new U139-class subs, which packed 5.9-inch guns. In 1916, the Committee on Submarine Development chose to mount 12-inch guns—battleship guns—on submarines.

The exact military requirements for such a weapon shifted as the concept developed. Originally envisioned as stealthy coastal attack platforms, the “submarine monitors” became high-power anti-ship weapons. Or would have, except they never saw wartime use. Perhaps the Admiralty realized the threat such subs with guns posed to their own Royal Navy if their use proliferated.

Four of the disastrous K-class steam submarines—K18 through K21––were reconfigured into the diesel-powered M-class. The giant subs were more than 295 feet long and were 24 feet in diameter, and each carried a single surplus 12-inch naval rifle from the Formidable-class battleships in a watertight turret.

While the M-class boats mounted four 18-inch torpedo tubes with a reload apiece, doubts about the torpedoes’ efficacy buttressed arguments for the guns. From a submerged posture, the subs could lob 850-pound shells over the better part of a mile.

Targeting and firing the gun proved both remarkably crude and ruggedly simple. Crews called it the “dip-chick” method.

An M-class monitor lined up on her target at periscope depth then surfaced to expose some six feet of her weapon’s barrel. Using the periscope as an optical sight, the commander aligned a simple bead sight on the barrel’s tip—like aiming a submerged rifle using its bead sight and a pair of binoculars—and ordered the gun fired.

The sub then immediately submerged, the whole operation taking only 30 seconds. An all-up ammunition magazine and lift supplied 50 rounds—but the gun could not be reloaded while submerged.

That such odd limitations severely constricted the M-class subs’ wartime effectiveness was moot. None of the subs saw action in World War I, but inter-war experimentation within treaty restrictions continued throughout the 1920s and 1930s. Surprisingly, the “submarine cruiser” concept survived the war and reached its baroque apex in some very large . . . and strange subs.

X-ed Out

By the time the Royal Navy’s X-1 launched on November 16, 1923, the 1922 Washington Naval Treaty had dramatically reshaped the post-war balance of sea power. The treaty restricted the size of guns permitted aboard submarines, and banned the targeting of merchant vessels.

Into these waters sailed the X-1 with its twin dual 5.2-inch turrets, high speed and long range. The largest, most heavily armed sub in the world when launched, her main batteries complemented six torpedo tubes. Rate of fire and ammunition supply were problematic. Targeting used a nine-foot-wide retractable rangefinder behind the conning tower. The sub required 58 men to crew the turrets alone.

Despite crippling mechanical shortcomings, especially her terrible engines, the X-1 made a great submarine cruiser—a type of warship the Washington Treaty specifically forbade. Consequently, the British Government remained very cagey about the X-1. Her poor maintenance record didn’t help. After the vessel literally fell over in a drydock in 1936, the Admiralty scrapped her.

The M-1 suffered equally bad luck—a Swedish freighter accidentally knocked her turret off and she sank with all hands in 1925. After the loss of the M-1, the Royal Navy removed the turrets from M-2 and M-3 and scrapped M-4 altogether. M-3 became a large stealth minelayer with capacity for 100 sea mines mounted in a deployment rack running along the back of the vessel.

The M-2 received a watertight hanger, a crane and a collapsible floatplane and explored aircraft-launching operations between 1927 and 1932. In 1932, M-2 sank with all hands during training maneuvers.

Les Sous-marines Corsaires

Even as the British tried to operationalize the big-gun sub, the French navy doubled down on the concept with its planned “corsair submarines,” a thoroughly dashing Gallic term. Only the first of the three, the Surcouf—named for a famous French privateer—was ever built.

But she was a monster.

Like the X-1, the French giant sub was the largest in the world when launched in October 1929—over 363 feet long, almost 30 feet in diameter, with a range of 7,800 miles at 13 knots. Surcouf could bring to bear her twin eight-inch guns on targets and track them with a 16-foot rangefinder from nearly seven miles away.

To supplement her reconnaissance and gunnery range, Surcouf—like M-2—carried a collapsable floatplane in a watertight hanger. Though never used in combat, the small Besson MB.411 seaplane could call in long-range ship sightings and gunfire corrections to the submarine.

Surcouf‘s leisurely submergence rate and poor submerged handling contrasted with her “corsairing” qualities—including two auxiliary boats and a brig equipped for 40 prisoners. A cruiser this big and complex required a 118-man crew, enough for three regular subs.

Appropriately perhaps for such a quasi-piratic vessel, Surcouf led a troubled life. She participated in the seizure of some small Vichy-controlled islands off Newfoundland before disappearing with all hands under mysterious circumstances early in World War II.

The later, even larger Japanese  I-400-class submarine aircraft carriers extended the aircraft-as-long-range-artillery concept to its mid-century conclusion. The huge subs mounted five-inch deck guns and anti-aircraft machine guns, but relied on their embarked aircraft and their weapons for main firepower.

With the advent of nuclear power and the guided missile, the seemingly goofy idea of a heavy surface-attack submarine became an impressive and chilling reality. A sub-launched ballistic missile armed with multiple hydrogen bombs represents the acme of long-range naval fire. Most recently, sub-launched cruise missiles have taken out Libyan air defenses and Syrian rebel strongholds.

Navies around the world are arming their undersea forces with missiles filling the roles guns filled long ago—shore bombardment, anti-ship and anti-air. Will energy weapons perhaps eventually fill this longstanding role in unexpected ways?