Submarines come in two main kinds: boats that run on diesel engines and batteries, and boats driven by a nuclear reactor. Both slip under the sea. They do very different jobs, and they cost very different sums.
This guide compares the two types in plain words. It covers how each one works, what it does well, and where it falls short.
First, some shared ground
Any submarine is a watercraft built for independent operation underwater. By naval tradition, submarines are called boats rather than ships, no matter their size. They were first used widely during World War I, between 1914 and 1918. Today they serve in many navies, large and small. Wikipedia's main article on submarines gives a full history of the type.
The trick to diving is buoyancy. A boat dives and resurfaces using diving planes and by changing the amount of water and air in its ballast tanks. Modern fleets use submarines to attack ships and other submarines, to run quiet spying and reconnaissance missions, to launch cruise missiles at land targets, and to carry out nuclear deterrence patrols. We covered a connected angle in Virginia-Class Attack Submarines: The Navy's Fast-Attack Fleet, As of 2026.
How diesel-electric boats work
A conventional submarine is typically diesel-electric. Diesel engines charge big batteries, and the batteries drive the boat when it runs silent and deep. Sooner or later the charge runs low. A conventional submarine must therefore surface, or come close to the surface, fairly often. Battery or fuel cell powered boats are all electric, and they all face the same basic limit: a mission can only last so long before the boat must recharge.
What nuclear power changes
A nuclear submarine is powered by a nuclear reactor, though it is not necessarily nuclear-armed. Nuclear submarines hold considerable performance advantages over conventional, typically diesel-electric, boats. Nuclear propulsion is completely independent of air, so it frees the boat from the need to surface often. A reactor also makes so much power that the boat can run at high speed for long periods, and the long gap between refuelings gives a virtually unlimited range. The real limits on a voyage are simple things, such as restocking food and other consumables. A nuclear boat can even make fresh oxygen and water from seawater. Wikipedia's article on nuclear submarines explains these advantages in detail.
The trade-off is money and rarity. The high cost of nuclear technology means relatively few of the world's military powers field nuclear submarines. The first of them all, USS Nautilus, was launched in 1954, and American naval reactors have run without incident ever since. Nuclear attack boats are classed as SSN, and they do not carry missiles. Nuclear-powered boats that carry ballistic missiles are the SSBN type. For related coverage, see The U.S. Navy Submarine Fleet by Type: SSN, SSBN, and SSGN Counts Explained.
More boats in more hands than ever
Submarine fleets now span a huge range. Some boats are small craft for one or two people that run for a few hours. Others can stay under for six months, such as the huge Russian Typhoon class, the biggest submarines ever built. More countries are building and operating submarines today than ever before in modern history, and China is leading the construction surge.
Conclusion: quiet patience versus endless reach
The two types answer different needs. A diesel-electric boat costs less, which is why so many navies, large and small, run submarines of this kind. Its weakness is the battery cycle: it must surface or snorkel far more often. A nuclear boat costs far more, so only a few powers field one, but it need never rise for air, can cross oceans at high speed, and stays down for months. That endless reach is why the nations that can afford nuclear boats treat them as the long-range, hard-to-catch end of their fleets.
