A ballast tank is a compartment built into a ship or submarine that gets filled with water or emptied of it to control the vessel’s stability, balance, and how deep it sits in the water. Every large vessel has them. They’re one of the most fundamental pieces of engineering in maritime transport, solving a problem that’s existed as long as humans have moved cargo by sea: how to keep a vessel stable when its load changes.
How Ballast Tanks Work
The basic principle is straightforward. Water is heavy, and by pumping it into or out of tanks positioned strategically around a ship’s hull, the crew can adjust how much the vessel weighs and where that weight sits. This controls three things: draft (how deep the hull sinks), trim (whether the bow or stern sits lower), and list (whether the ship leans to one side).
Ballast tanks are typically positioned at the lowest point of the hull and distributed around the ship so weight can be shifted as needed. When a cargo ship unloads its freight at port, it suddenly becomes much lighter. A lighter ship rides higher in the water, which sounds fine until you consider that the propeller may no longer be fully submerged, the hull catches more wind, and the vessel becomes dangerously unstable in rough seas. Pumping seawater into the ballast tanks replaces that lost weight and keeps everything balanced. When the ship takes on new cargo, the ballast water gets pumped back out.
Fuel consumption creates a similar challenge. As a ship burns through its fuel over a long voyage, it gradually loses weight on one side or at one end. Ballast tanks compensate for this in real time, letting the crew redistribute weight to keep the vessel level even as conditions change.
Ballast Tanks on Submarines
Submarines use ballast tanks for something even more dramatic: controlling whether the vessel floats, sinks, or hovers in place. A submarine on the surface has its ballast tanks filled with air, making the sub’s overall density less than the surrounding water, so it floats. To dive, the crew floods the ballast tanks with seawater while venting the air out. The sub becomes denser than the water around it and sinks.
Holding a specific depth requires a delicate balance. Submarines carry smaller trim tanks in addition to the main ballast tanks. By adjusting the mix of air and water in these trim tanks, the crew achieves neutral buoyancy, where the sub’s density matches the surrounding water exactly and it neither rises nor sinks. Water can also be shifted between bow and stern trim tanks to keep the submarine level rather than tilting nose-up or nose-down.
Surfacing reverses the process. Compressed air stored in onboard flasks is forced into the ballast tanks, pushing the seawater out and making the submarine buoyant again. In an emergency, high-pressure air can flood the tanks rapidly, bringing the sub to the surface fast.
The Invasive Species Problem
Ballast tanks created one of the most significant ecological problems in modern shipping. When a cargo ship takes on seawater in one port and discharges it thousands of miles away in another, it carries along whatever was living in that water: bacteria, plankton, larvae, small fish, and algae. Ships move an enormous volume of water this way, and the biological consequences have been severe.
Zebra mussels arrived in North America’s Great Lakes through ballast water discharged by transatlantic cargo ships. They’ve damaged commercial and recreational fisheries and clogged city water supply infrastructure. In Australia and other coastal regions, ballast water introduced dinoflagellates that cause toxic red tides and mass fish kills. European green crabs, spread partly through ballast discharge, have devastated mollusk and crustacean populations in areas they’ve invaded.
Regulations and Treatment Systems
The International Maritime Organization now requires ships to treat their ballast water before discharging it. The Ballast Water Management Convention, which has been strengthened through multiple rounds of amendments (the most recent entering force in 2025), mandates that ships install approved treatment systems. Commissioning testing of these systems is also mandatory.
Treatment technologies fall into a few categories. Physical methods like filtration and ultraviolet light are the simplest and most cost-effective. Chemical methods use disinfectants but produce byproducts that raise their own environmental concerns. More advanced systems combining heat and electric fields work regardless of the water’s salt content or temperature, but they cost significantly more. Most large commercial vessels now carry some form of onboard ballast water management system.
Construction and Corrosion
Ballast tanks are built from carbon steel, which presents an obvious problem: they spend their working lives filled with saltwater. The standard protection is an epoxy coating on the tank’s interior surfaces, backed up by sacrificial zinc anodes. These are chunks of zinc metal attached to the steel structure that corrode preferentially, sparing the tank walls from rust. The zinc slowly dissolves over time and needs periodic replacement.
Newer approaches aim to extend tank life to 25 years or more. These include more durable experimental coatings, aluminum anodes designed to last a vessel’s full economic lifespan, and construction using corrosion-resistant steel that reduces the need for anodes entirely. Corrosion protection matters because structural failure in a ballast tank compromises the ship’s hull integrity.
Why Ballast Tanks Are Dangerous to Enter
Ballast tanks are classified as enclosed spaces, and entering one is among the most hazardous tasks a crew member can perform. The IMO has issued specific safety recommendations in response to ongoing deaths from enclosed space entry aboard ships. The dangers are largely invisible. Rust formation inside the tank consumes oxygen, and biological decomposition of sediment or residues can produce toxic gases. A crew member who opens a hatch and steps in without testing the atmosphere first may encounter air with dangerously low oxygen levels.
The risks aren’t limited to oxygen depletion. Depending on what cargo the ship carries and what residues have accumulated, the atmosphere inside a ballast tank can also be flammable or contain toxic vapors. One deep breath of pure nitrogen, which can accumulate in enclosed shipboard spaces, is fatal. Entry requires atmosphere testing with gas detectors, continuous ventilation, rescue equipment standing by, and a trained observer stationed outside the tank at all times.