Submarine Speed: How Fast Do They Move Underwater?

Modern submarines operate in a hidden dimension, gliding through the ocean's depths at speeds that challenge perception. The question of how fast these vessels move underwater does not have a single answer, as it depends heavily on the submarine's class, design era, and operational purpose. From the slow, stealthy patrol of a nuclear deterrent to the near-sonic rush of a naval attack submarine, the underwater speed spectrum is vast and critical to military strategy.

The Measure of a Hidden Force: Understanding Underwater Speed

To discuss submarine velocity, one must first understand the metric used. On the surface, ships often use kilometers per hour (km/h) or miles per hour (mph). However, within the defense and maritime industries, the standard unit of measurement is the knot. A knot is defined as one nautical mile per hour, equivalent to approximately 1.852 kilometers per hour or 1.15078 miles per hour. This unit is universally adopted because it directly correlates with nautical navigation and the resistance encountered by a hull moving through water.

Unlike land vehicles constrained by roads, a submarine's speed is determined by hydrodynamics—the interplay between the hull shape, the power of its propulsion system, and the density of the water it displaces. Water is approximately 800 times denser than air, creating immense drag. Consequently, achieving high speeds underwater requires immense power and sophisticated engineering to manage factors like cavitation (the formation of vapor cavities in a fluid) and hydrodynamic noise, which can betray a submarine's position.

Categories of Submarines and Their Velocity Profiles

The speed of a submarine is intrinsically linked to its role. Navies around the world generally categorize submarines into two main types: ballistic missile submarines (SSBNs) and attack submarines (SSNs/SSOs). Each category has distinct performance requirements that dictate their design and speed capabilities.

Ballistic Missile Submarines: The Slow Giants

Ballistic missile submarines are the quiet giants of the deep. Their primary mission is to act as a second-strike nuclear deterrent, hiding in the ocean for months at a time to ensure they can survive a first attack and retaliate if necessary. Because of this need for extreme stealth and endurance, they prioritize silence and efficiency over speed.

• Typical Surface Speed: Around 15 to 20 knots (28–37 km/h).
• Typical Submerged Speed: Generally between 20 and 30 knots (37–56 km/h).
• Notable Example: The Russian Typhoon class, the largest submarines ever built, are reported to have a submerged speed of approximately 30 knots (56 km/h).

These vessels are often described as "slow" compared to their attack counterparts, but this is a misconception of intent. Moving slowly reduces cavitation and noise, making them incredibly difficult to detect with sonar. For a submarine tasked with hiding for strategic deterrence, silence is indeed golden.

Attack Submarines: The Agile Predators

Attack submarines, whether nuclear-powered (SSNs) or conventional (SSOs), are designed for a different purpose: to hunt. They seek out enemy ships, submarines, and carriers, requiring the ability to rapidly close distance and evade counter-detection. This role demands a significant investment in advanced propulsion and streamlined design.

• Typical Surface Speed: Generally between 20 and 30 knots (37–56 km/h).
• Typical Submerged Speed: Most modern attack submarines can sustain speeds of 30 to 40 knots (56–74 km/h).
• High-Performance Examples: The American Virginia class and the British Astute class are reported to reach speeds in excess of 40 knots (74 km/h). The exact top speed of these vessels is classified, but unverified reports and wargame data suggest figures approaching 45 knots (83 km/h).

The difference becomes stark when comparing the two. An attack submarine can sprint faster than most surface ships, allowing it to dictate the terms of an engagement. This "underwater sprint" capability is a decisive tactical advantage.

Specialized and Historical Context

Within the broad categories above exist specialized vessels with unique performance characteristics. Submarine-launched cruise missile carriers (SSGNs), converted from older ballistic missile hulls, sacrifice some stealth for the ability to carry a large payload of guided missiles. Their speed profile typically mirrors that of a standard SSBN.

Historically, speed records have been pushed by experimental and military submarines. During the Cold War, the Soviet Union developed the Alfa class, a titanium-hulled attack submarine capable of an astonishing submerged speed of approximately 40 knots (74 km/h). While revolutionary for its time, the Alfa's demanding power system and noisy cooling pumps made it less than ideal for its intended role of quiet hunter-killer operations.

In the civilian world, specialized research submarines like the Russian Mir class or the American Alvin are built for deep-sea exploration, not speed. Their top velocities are a mere 5 to 10 knots (9–18 km/h), prioritizing stability, visibility, and precise maneuvering over velocity.

The Mechanics of Underwater Velocity

Why is there such a wide gap between the slowest and the fastest underwater vessels? The answer lies in the fundamental physics of moving through water and the trade-offs inherent in naval engineering.

1. Propulsion Technology: The heart of any submarine is its power plant. Conventional diesel-electric submarines must surface or use a snorkel to run their diesel engines and recharge batteries, limiting their submerged speed and endurance. Nuclear submarines, equipped with a pressurized water reactor, generate steam to power a turbine connected to a shaft and propeller (or pump-jet), providing virtually unlimited underwater range and high sustained speeds.
2. Hull Design: A submarine's shape is a compromise. At low speeds, a rounded, hydrodynamic hull minimizes drag. However, as speed increases, pressure differences and cavitation become major factors. Designs must balance a smooth surface with structural integrity to withstand the immense pressure of the deep ocean. Extra-large submarines face an additional challenge, as their greater volume creates more drag per unit of propulsion power.
3. Noise Abatement: As Admiral Hyman G. Rickover, the father of the nuclear navy, implied in his rigorous standards, a submarine's greatest advantage is its ability to hide. Speed generates noise. Pumps, turbines, and the turbulent flow of water against the hull all create sound. Modern submarines invest heavily in sound-dampening mounts, anechoic tile coatings, and precision-engineered machinery to reduce their acoustic signature. Sometimes, slowing down is the fastest way to achieve a tactical objective.

The quest for speed is a constant duel between engineering prowess and the immutable laws of physics. Every knot gained requires exponentially more power, which in turn generates more heat and noise, presenting new challenges for the engineers onboard.

Surface vs. Submerged: A Critical Distinction

It is vital to distinguish between a submarine's speed on the surface and its speed underwater. For nuclear submarines, the difference is often minimal because they are not constrained by battery life or snorkel operations. However, for conventional diesel-electric submarines, the contrast is dramatic.

• On the Surface: A conventional submarine can use its diesel engine to charge batteries and propel the vessel, achieving speeds comparable to a surface ship, often 15 to 20 knots (28–37 km/h).
• Submerged: Relying solely on battery-powered electric motors, their speed is drastically reduced, typically to 5 to 10 knots (9–18 km/h) to conserve energy. This limitation was a key tactical vulnerability during the World Wars, forcing submarines to surface and making them vulnerable to detection and attack.

Nuclear propulsion revolutionized this dynamic. By removing the need for atmospheric oxygen, a nuclear submarine can remain submerged for the duration of its crew's endurance, cruising at high speeds for months on end. This undersea endurance is the cornerstone of the third-generation nuclear submarine's strategic value.

The Human and Strategic Factor

While the question "how fast can it go?" is technical, the implications are deeply strategic and human. The ability to move quickly underwater affects everything from mission planning to crew safety.

• Tactical Maneuvering: High speed allows a submarine to rapidly close with a target, fire its weapons, and disengage before the enemy can effectively respond. It is also a primary evasion tactic against pursuers.
• Crew Experience: Pushing a massive vessel to its limits subjects the crew to significant g-forces and vibration. Maintaining high speeds for extended periods requires a highly trained crew and a robust vessel.
• Operational Security: Speed is a tool, not a constant state of operation. As former submarine commander and author Captain (Ret.) John R. Klein has noted, the most dangerous moment for a submarine is often when it is moving quickly and making noise. More often than not, the tactic is to move slowly and silently, blending into the ambient noise of the ocean.