Draft marks are the large numbers painted on a ship’s hull that tell you how deep the vessel sits in the water. Reading them correctly comes down to understanding the numbering system, knowing where the waterline falls relative to each number, and checking all six locations on the hull. Here’s how to do it.
How Draft Numbers Are Sized and Spaced
Ships use either imperial or metric draft marks, and the spacing works differently for each system.
In the imperial system, each number is 6 inches tall and 1 inch thick, painted at 12-inch intervals measured from the bottom of one number to the bottom of the next. The numbers represent feet. So the bottom edge of the “12” sits exactly 12 feet above the keel. Numbers may appear as standard Arabic numerals or Roman numerals, but the dimensions are the same either way.
In the metric system, each number is 1 decimeter (10 centimeters) tall, spaced at 2-decimeter intervals from bottom to bottom. The bottom edge of each number represents that draft in decimeters. A mark labeled “60,” for example, has its bottom edge at 60 decimeters, or 6 meters, above the keel.
Where to Take the Reading
The critical rule: the draft reading is taken where the waterline meets the bottom edge of a number. If the water just touches the bottom of the “14,” the draft is 14 feet. If the water covers half of that number, the draft is 14 feet 3 inches (since the number is 6 inches tall, halfway up adds 3 inches). If the water completely covers the “14” and reaches the empty space between “14” and “15,” you estimate how far into that gap the water sits.
This works the same way in metric. Water at the bottom of the “70” mark means 7.0 meters of draft. Water halfway up the “70” means roughly 7.05 meters.
Six Marks, Not Just One
A ship carries draft marks at six positions: port and starboard at the bow (near the stem), amidships, and at the stern (near the stern post). Reading all six gives you the full picture of how the vessel is sitting in the water. A ship rarely floats perfectly level, so the forward draft and aft draft will often differ. That difference is called trim.
You calculate mean draft by averaging the forward and aft readings: add the forward draft to the aft draft and divide by two. This gives a quick approximation of how deep the ship sits overall, though a more precise figure requires checking the midship reading as well.
Checking for Hull Deflection
Comparing the midship draft to the average of the forward and aft drafts reveals whether the hull is bending under its load. If the midship draft is deeper than that average, the ship is sagging, meaning the middle of the hull is dropping lower than the ends, like a hammock. If the midship draft is shallower than the average, the ship is hogging, meaning the middle is rising higher than the ends, bowing upward. Both conditions matter for safe loading and structural integrity.
Reading in Rough Water
Calm water makes draft reading straightforward, but swell and chop complicate things. When waves push the waterline up and down against the marks, experienced surveyors watch for several wave cycles and estimate the mean water level. A common guideline is to place the average at roughly two-thirds of the distance between the lowest and highest water levels observed against the marks. Some surveyors instead record the average of the highest readings and the average of the lowest readings, then split the difference.
When conditions are too rough to physically access the far side of the ship, surveyors read the near side and use a manometer (a device that measures the difference in water pressure between the two sides of the hull) to calculate the opposite draft. Binoculars, a strong torch for low-light conditions, and sometimes a small boat or ladder are all standard equipment for getting a clear sightline on the marks.
How Water Density Affects the Reading
A ship floats deeper in fresh water than in salt water because fresh water is less dense. Seawater has a density of about 1,025 kilograms per cubic meter, while fresh water sits at 1,000. That difference means a vessel entering a freshwater port will sink slightly lower even though her cargo hasn’t changed.
To account for this, naval architects calculate a fresh water allowance. The formula divides the ship’s displacement by four times the “tonnes per centimeter immersion” value at the summer load draft. The result, in millimeters, tells you how much extra the ship will sink in fresh water compared to full-strength seawater. Water that falls between pure fresh and full salt, common in river estuaries and ports, is called dock water, and it requires a proportional correction based on its measured density.
Surveyors measure water density on site using a calibrated hydrometer or a salinity refractometer, sampling the water right at the hull. This correction is essential for accurate cargo calculations during draft surveys.
Draft Marks and the Plimsoll Line
Draft marks work alongside the Plimsoll line, the circle-and-line symbol painted amidships that shows the maximum legal loading depth. The Plimsoll line originated in 1876 when British MP Samuel Plimsoll pushed Parliament to pass legislation against overloading cargo ships, a practice that was sinking vessels and killing crews. The law required a visible mark that would disappear below the waterline if the ship carried too much weight.
Today, the Plimsoll mark includes several horizontal lines indicating maximum drafts for different conditions: tropical waters, summer, winter, and fresh water, among others. When the waterline on the draft marks reaches or exceeds the relevant Plimsoll line, the ship is at its legal loading limit. Reading the draft marks accurately is how officers, port inspectors, and surveyors verify that a vessel hasn’t exceeded that limit before she sails.