How do you calculate the nautical draught of a ship?

The nautical draught of a ship indicates how deep it is in the water. The more and the heavier a ship is loaded, the deeper it will be in the water. As a result, the deck thus gets closer and closer to the surface of the water. For the safety of the crew and the ship, the nautical draught must not exceed the maximum load of the ship. This requires a correct calculation of the ship's maximum load. In addition to the amount of cargo, the fuel, provisions, equipment and drinking water on board and the density of the water (salt, fresh or brackish) also play a major role. In salt water, ships are less deep, so they have a shallower nautical draught.

On its stem or on its stern?

How deep a ship is in the water is not always the same everywhere. The difference between the bow and the aft of the ship is called the trim. Vessels may be "on its stern" (tilting backward or trim by the stern), "on its stem" (tilting forward, trim by the head or with a negative trim) or "even keel" (lying straight). But not every trim is equally ideal. Preferably, a ship should lie "a foot on its stern." That meants it's about thirty centimeters less deep in the front than in the back. While sailing, the ship will then straighten itself out. A less ideal trim indicates the moment when a vessel lies forward tilted (with a negative trim), since it is then less steerable ánd the captain's shower does not drain properly.

Deep, but how deep?

Pumping ballast (water) allows you to determine the trim of the ship. For instance, you can pump water into the forepeak ballast tank or into the aft ballast tanks to get ships "on its stern" or "on its stem". The steerability of the ship affects the amount of ballast. Empty ships once require more ballast water than loaded ships to trim the ship. Ship type and voyage also play a role, as trim varies according to the placement of cargo on board.


To determine height or depth measurements in our port, we use a general reference model, the Second General Leveling (or Tweede Algemene Waterpassing). The average sea level at low tide in Ostend is the datum ('zero level').The safety of a ship depends on the available water depth, the nautical draught and the Under Keel Clearance (UKC), or the minimum space between the deepest point under the ship's keel and the bottom of the fairway. The UKC also varies with the sailing area (15% of the nautical draught at sea to 10% of the nautical draught in the channel of a lock). In the port, the UKC is usually 1 meter on the fairway and 0,6 meter at the berth.

Plimsoll lines

Various cargo lines are welded or painted on the hulls of ships. These Plimsoll markings show the different load lines for the maximum load of the vessel. The sailing area (tropical areas), the season (winter or summer) and the density of the water (fresh water or fresh water) also influence the calculation of the maximum safe loading capacity. The markings ensure that the ship is never overloaded and reduce the risk of capsizing or flooding on board.

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