Are Submarines Faster Than Ships? How Their Top Speed Compares

Submarines are among the most specialized vessels in a country's navy, thanks to their inherent characteristics which favor stealth, surveillance, and ambushes. Submarine warfare has been a staple of naval operations since well before World War II, with the perpetual arms-race between submarines and anti-submarine warfare countermeasures taking center-stage during the Cold War. There are several means by which a submarine stays alive in a confrontation with the enemy, though historically, mobility's never truly been one of them. In fact, the fastest ever submarine to make it beyond the prototype stage, the Soviet Union's Alfa-class, boasts an underwater cruise speed of about 42-43 knots – almost as fast as some of the fastest larger naval vessels. This speed easily exceeds that of the Nimitz-class carriers, which operate at over 30 knots – though these typically operate as part of a larger carrier battle group. Such a battle group's speed, therefore, is limited to the maximum cruise speed of its slowest craft.

All that is to say that, historically, U-boats and submarines of similar vintage used to be quite slow. But modern submarines, despite the added friction of being surrounded by water, are actually quite fast, matching or even exceeding many large surface ships. Take a U.S. Navy's Seawolf-class, for example, which has a maximum speed of 35 knots. Vessels like these and the Alfa-class belong to a subdivision called fast-attack submarines, which specialize in covert and shock operations such as insertion of special-operations troops, covert surveillance of enemy battle groups, and ambush warfare. Speed, however, isn't a submarine's primary means of defense. Rather it's a submarine's stealth which distinguishes and protects it, and so while submarines can be fast, they typically aren't actually piloted at those speeds in combat conditions, and here's why.

Comparatively-speaking, submarines are far more difficult to detect than surface vessels, and a lot of that stealth relates to how they're piloted. That's because, at full-speed, a submarine will produce an effect known as cavitation. Basically, cavitation is what happens when a propeller rotates quickly enough to produce bubbles. Those bubbles then generate a noise when they rupture, and that noise is detectable through passive acoustic sensors such as passive sonar arrays.

Cavitation occurs differently at various depths – the deeper you are, the faster you can turn the propeller. However, operating at such depths also poses its own challenges, such as not being able to use a periscope, the potential for there to be a thermocline layer which can reflect sonar emissions, and depth hazards like sandbars when operating closer to shore. As such, when operating close to the surface, submarines typically move at a fraction of their top speed to maintain a stealth advantage – even more so when running silent. This is because silent running also disables the boat's active reactor cooling systems, meaning it can't push its engines if it wanted to.

A Seawolf, meanwhile, seemingly circumvents many of these issues, boasting a silent-running speed of 20 knots. It's still slower than a typical aircraft carrier, for instance, and certainly on the mid-to-slower end in a drag race against a surface vessel, but that's not the point of the submarine, fast-attack or not. Rather, the submarine's true value is measured in how fast it can move while being as quiet and unassuming as possible – to date, no submarine can move as fast as a surface vessel while meeting those criteria, not even a Seawolf, though it comes close.