A double hull is a ship design where two separate watertight layers of steel form the bottom and sides of the vessel, with empty space between them. If the outer hull is punctured in a collision or grounding, the inner hull acts as a second barrier, keeping oil or other cargo from leaking into the water. This design became the global standard for oil tankers after a catastrophic spill in 1989 and is now required by both U.S. and international law.
How a Double Hull Is Built
A single-hull vessel has one layer of steel plating between its cargo and the ocean. A double-hull vessel adds a second inner shell on both the bottom and sides, creating a gap that runs the full length of the cargo-carrying area. This gap typically holds ballast water, which the ship takes on or pumps out to maintain stability when cargo tanks are empty or partially loaded.
The required width of that gap depends on the size of the vessel. For tankers of 5,000 deadweight tons and above, U.S. regulations require the double sides to be at least 1 meter (about 39 inches) wide, scaling up with vessel size to a maximum of 2 meters (79 inches). The double bottom follows a similar formula based on the ship’s beam width. Smaller vessels operating only on inland or short coastal routes can have narrower spacing, with a minimum of about 61 centimeters (2 feet) on sides and bottom. These dimensions ensure enough cushion to absorb impact energy before the inner hull is breached.
Why Double Hulls Became Mandatory
The turning point was the Exxon Valdez disaster in March 1989, when a single-hull tanker ran aground in Alaska’s Prince William Sound and spilled more than 11 million gallons of crude oil. The environmental devastation and public outrage pushed the U.S. Congress to pass the Oil Pollution Act of 1990 (OPA 90), which required all new oil tankers operating in U.S. waters to have double hulls and set deadlines for existing single-hull tankers to be retrofitted or retired.
The international community followed. The International Maritime Organization added new regulations to MARPOL, the main treaty governing ship-based pollution, making double hulls the standard for new tankers worldwide. A phase-out schedule for existing single-hull tankers began in 1995, initially giving vessels up to 30 years of service life. That timeline was tightened in 2001 and again in 2003, accelerating retirements. By 2005, single-hull tankers were banned from carrying heavy-grade oil. The last exemptions for older single-hull vessels expired by 2015.
How Effective Double Hulls Are
Double hulls don’t prevent accidents, but they dramatically reduce how much oil escapes when one occurs. A study of U.S. Coast Guard accident data from 2001 to 2008 found that double-hull tanker ships spilled 62% less oil on average than comparable single-hull vessels involved in similar accidents. For tank barges, the reduction was about 20%.
The protection also scales with ship size. Earlier estimates projected that oil cargo spillage could be reduced by 30% for a 40,000-ton tanker and up to 70% for a very large crude carrier of 240,000 tons. Larger ships have proportionally more space between hulls, giving the inner shell a better chance of surviving intact even in severe groundings.
The Tradeoffs
Double hulls cost more to build and maintain. Construction runs about 16 to 18% more than a comparable single-hull design, primarily because of the additional steel. The maintenance burden is even steeper: the inspection area roughly doubles, and the amount of steel surface exposed to corrosion and metal fatigue triples. That means significantly more steel replacement over the vessel’s lifetime, driving up operating costs.
The space between hulls also creates confined areas that are harder to ventilate and inspect. Corrosion can develop in the ballast spaces without being immediately visible, which is why international regulations require enhanced survey programs for aging double-hull tankers. The design is not a perfect solution, but the environmental benefits have been large enough that no serious effort has been made to reverse the requirement.
Where Double Hulls Are Required
Under U.S. law, all tank vessels over 5,000 gross tons carrying oil in American waters must have double hulls. The Oil Pollution Act also directed the National Academy of Sciences to evaluate whether any alternative designs could match the protection of a double hull. No alternative has displaced the requirement.
Internationally, MARPOL Annex I applies the double-hull standard to oil tankers globally. Flag states had limited authority to grant extensions for certain categories of older vessels, but those extensions were capped and have since expired. Today, virtually every oil tanker in commercial operation worldwide is double-hulled. The design is also increasingly common in chemical tankers and liquefied gas carriers, though requirements vary by cargo type and vessel class.