Ship draft (also spelled “draught”) is the vertical distance between the waterline and the lowest point of the hull beneath the water. It tells you, in simple terms, how deep a vessel sits in the water. A small recreational boat might have a draft of just a few feet, while a fully loaded cargo ship can have a draft of 50 feet or more. This single measurement affects where a ship can safely travel, how much cargo it can carry, and whether it can enter a port or pass through a canal.
How Draft Is Measured
Draft is measured from the water’s surface straight down to the lowest fixed point on the vessel, which is usually the bottom of the keel. But it can also be measured to whatever hangs lowest, including propellers and rudders. When measured to the absolute lowest projecting part, it’s called “extreme draft.”
Ships don’t always sit perfectly level in the water. The front (bow) and the back (stern) often have slightly different depths, so the industry uses a few specific terms. “Forward draft” is the depth at the bow, “aft draft” is the depth at the stern, and “mean draft” is simply the average of those two numbers. The difference between forward and aft draft is called “trim.” A ship with a deeper aft draft than forward draft is said to be trimmed by the stern, which is common for most cargo vessels under normal conditions.
You can actually read a ship’s draft yourself. Large vessels have draft marks painted or welded directly onto both sides of the hull, near the bow and stern. These are large numbers that indicate the distance from that marking down to the bottom of the keel. As a ship loads cargo and sinks deeper, the waterline rises against these marks, giving a direct readout of the current draft.
Why Water Type Changes a Ship’s Draft
The same ship carrying the same cargo will sit deeper in freshwater than in saltwater. This happens because saltwater is denser, roughly 1,025 kg per cubic meter compared to freshwater’s 1,000 kg per cubic meter. That 2.5% difference in density means saltwater pushes up on the hull with more force, so the ship floats a bit higher. When a vessel moves from the ocean into a river or a freshwater port, it sinks slightly, increasing its draft. This matters enormously for ships navigating shallow channels or entering port.
Temperature also plays a role. Warmer water is slightly less dense than cold water, so a ship will sit marginally deeper in tropical waters compared to cold northern seas, even if the salt content is identical.
The Plimsoll Line and Load Limits
Every large commercial vessel has a set of markings on the hull called Plimsoll lines (or load lines), named after Samuel Plimsoll, the 19th-century British politician who championed them as a safety measure. These marks show the maximum depth a ship can safely be loaded to under different water and weather conditions.
There are six standard markings, each representing a scenario where buoyancy and sea conditions differ:
- TF: Tropical Fresh Water
- F: Fresh Water
- T: Tropical (saltwater)
- S: Summer (saltwater)
- W: Winter (saltwater)
- WNA: Winter North Atlantic
The Winter North Atlantic line is the most restrictive, allowing the least cargo, because rough seas and cold conditions demand the most freeboard (the distance between the waterline and the deck). The Tropical Fresh Water line allows the deepest loading because warm freshwater is the least dense environment, meaning the ship needs the most room to sink. If a ship were loaded to its summer saltwater line and then sailed into a freshwater river, it would sink below the safe mark. The different lines prevent exactly that kind of miscalculation.
Air Draft: The Other Measurement
While draft measures how deep a ship reaches below the water, air draft measures how high it extends above the water. Specifically, air draft is the distance from the waterline up to the highest fixed point on the vessel, such as a mast, antenna, or the top of stacked containers. This measurement is critical for passing under bridges, power lines, and other overhead obstructions.
The U.S. Coast Guard considers air draft a critical safety factor and requires vessel operators to determine whether their vertical clearance is sufficient before approaching a bridge. Failing to account for air draft has led to collisions with bridges and power lines. Notably, as a ship loads cargo and its water draft increases, its air draft decreases, since the hull sinks lower and the above-water portion shrinks. Unloading has the opposite effect.
How Draft Limits Where Ships Can Go
Draft is one of the primary factors that determines which ports, channels, and waterways a ship can access. Every harbor, canal, and river has a controlling depth, and vessels with drafts exceeding that depth simply cannot enter.
The Panama Canal is a well-known example. The Neopanamax Locks currently allow a maximum authorized draft of 49 feet (14.94 meters), though this limit fluctuates based on the water level in Gatun Lake, the freshwater reservoir that feeds the locks. During droughts, the Canal Authority reduces the allowable draft, sometimes significantly. A previous adjustment brought the limit down to just 48 feet. Ships that exceed the limit must either lighten their cargo or wait.
Ports also enforce safety buffers between a ship’s draft and the actual seabed. This buffer, called under keel clearance, ensures the hull doesn’t scrape the bottom even with waves, tides, and the slight squat effect that occurs when a ship moves through shallow water. Many ports require a minimum under keel clearance of 10% of the vessel’s draft, and some apply a 30% safety factor on top of the ship’s dynamic draft to account for real-world conditions like swell, currents, and changes in water density.
What Affects a Ship’s Draft
The most obvious factor is cargo weight. Every ton loaded onto a ship pushes it deeper into the water. Naval architects calculate exactly how much a vessel’s draft will change per ton of cargo added, a figure specific to each ship’s hull design. Fuel and ballast water also contribute. A ship departing port with full fuel tanks sits deeper than the same ship arriving after a long voyage with nearly empty tanks.
Ballast water is used specifically to control draft and stability. When a cargo ship unloads at port and has no freight to carry, it takes on seawater in ballast tanks to increase its draft and keep it stable. Without ballast, a lightly loaded ship sits too high in the water, exposing more hull to wind, reducing propeller efficiency, and making the vessel harder to steer.
Hull design matters too. Wide, flat-bottomed vessels like barges have shallow drafts relative to their cargo capacity. Deep-V hulled sailboats and warships tend to have proportionally greater drafts. Container ships and bulk carriers fall somewhere in between, with loaded drafts typically ranging from 30 to 50 feet depending on size class.