Free surface (shipping)

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A free surface describes the surface of a liquid in a partially filled tank of a ship . Their effect is important for the calculation of the ship's stability .


On seagoing ships, tanks are either driven empty ( bilge ) or completely filled in order to avoid open surfaces. On the one hand, there are various operational reasons, such as the travel consumption from the bunker tanks or, for example, unavoidable partial filling of holds or ballast tanks , which oppose this, on the other hand, a ship can also suffer a water ingress as a result of a collision or a grounding, one or more holds or Ballast tanks partially fill.

If the ship is in a static position, free surfaces only form a horizontal surface within the tank. If, for example, the ship rolls , the liquid within the tank flows to the side of the heel , reduces the initial metacentric height and increases the heel. The reduction in the metacentric initial height depends on the latitude moment of inertia of the liquid surface, the density of the liquid, but also on the displacement of the ship.

A baffle that halves the surface in the middle reduces the moment of inertia to two sixteenths and thus has a considerable effect. Smaller openings in the foot area, which enable the level adjustment and allow the pump to be completely emptied, are not a problem because not all of the liquid can flow to the other side and back through these passages within a few seconds of a rolling period.


For the unavoidable free surfaces in normal operation, constructive precautions are taken; They can be kept so low by structural and operational organizational measures that they have practically no measurable effects.

The situation is different if unplanned amounts of water penetrate the hull; be it through leakage or wave impact. Ferry ships that require vehicle decks relatively close above the swimming water line are particularly at risk here . So that the vehicle traffic (rail and / or road vehicles) can maneuver unhindered, one tries to avoid separating fixtures. A number of accidents with total loss were mainly caused by free water on the vehicle deck, which was able to slosh practically unhindered from one side of the vehicle deck to the other at the same time as the ship moved, and ultimately led to capsizing . Only a central longitudinal bulkhead helps against this; possibly even several longitudinal bulkheads along the lane, even if this slightly reduces the transport capacity.

See also


  • Johannes Müller, Josef Krauss: Manual for the ship's command . Volume 3, seamanship and ship technology, part B. Ed .: Walter Helmers. Springer, Berlin 1980, ISBN 3-540-10357-0 , pp. 16-17 .