Lightening holes

  • Large apertures cut in floor plates, side girders, and tank bracket plates.
  • In double bottom vessels they provide an access to the different cells for inspection and upkeep, besides taking weight off the structure, which is their principal object.
  • In a general sense any hole cut in reduce weight without impairing strength.

Margin line

  • An imaginary line drawn 75mm below the bulkhead deck at the ship’s side.
  • It is the highest permissible location on the side of the ship of any damage water plane in the final condition of sinkage, trim & heel.
  • The margin line is the line just below the top of the bulkhead deck, or the highest deck which has watertight bulkheads forming watertight compartments. If the margin line is immersed, the ship is serious danger of sinking from flooding. It is the amount that the ship can be flooded without the risk of the ship sinking

Margin plate

  • The outboard strake of the inner bottom.
  • When the margin plate is turned down at the bilge it forms the outboard boundary of the double bottom, connecting the inner bottom in the shell plating at the bilge.

Oxter plates

  • They are peculiarly curved plates, fitted where the stern frame meets the overhang of the stern.


  • The pulsation in and out of the bow and stern plating as the ship alternately rises and plunges deep into the water.


  • The ratio of water which can enter with the volume of the empty compartment.

Permissible length

  • The length between bulkheads on a ship in order to ensure that it will remain afloat if one, or more, compartments are flooded. The permissible length is some fraction of floodable length.
  • The fraction is called the factor of subdivision. 


  • The pins of bolt that hinge the rudder to the gudgeons on the rudder post.

Pleuger rudder

  • An active rudder in which a small motor driven propeller is incorporated in a streamlined casing.
  • Ship steering at very low speeds is thus possible and the rudder angle can be greater than 35degrees.


  • The impact of the water surface against the side or bottom of a ship hull, whether caused by ship velocity, water velocity, or both.

Rising tanks

  • Double bottom spaces in which the inner bottom is higher at centre line than at sides.
  • This arrangement has the advantage of allowing moisture from the cargo drain into the bilge pockets on each side.


  • The dimension and the thickness of rolled sections and the breadth and thickness of plates, which together compose the ship’s structure or part of same.

Schottle rudder

  • Double right-angled drive mechanism in which the propeller operates within a duct.
  • The propeller and the duct can be rotated in order to steer the driven vessel.


  • The curvature of the deck in a longitudinal direction.
  • It is measured between the deck height at amidships and the particular point on the deck.
  • The forward shear is twice of the aft shear.

Sheer strake

  • The course of the shell plating at strength deck level.

Shoe plates

  • They used to connect the stem to the flat plate keel.
  • The forward end of the shoe plate is dished around the stem, whilst the after end is flattened to connect with the keel plate.

Sole piece

  • The fore and aft piece, forming the lower part of the stern frame in single-screw vessels extending from the propeller post to the rudderpost.

Standard fire test

  • The exposure of a material of a specimen, in a test furnace to a particular temperature for a certain period of time.

Stealer plate

  • At the ends of a vessel, particularly at the bow, the width of the strake decreases and it is often desirable to merge two strakes into one, this being done by a stealer plate.

Cruiser stern

  • A spoon-shaped stern used on most merchant ships, designed to give maximum immersed length.

Transom stern

  • A square-ended stern used to provide additional hull volume and deck space ships.


  • A vessel is said to be stiff if she has an abnormally large metacentric height.
  • Such a ship may have a short period of roll and therefore will roll uncomfortably.

Storm valve

  • A check valve placed at the end of soil or scupper pipes discharging through the ship side near the waterline.
  • It allows the water to discharge overboard but prevents seawater from backing up the pipe.


  • A line in a fluid such that its tangent at any point is parallel to the instantaneous velocity of the fluid at that point.


  • A fore-and-aft girder running along the side of a ship at the shell and also to the outboard strake of plating on any deck.


  • Small water plane area, twin-hull vessel.
  • Twin torredo shaped hulls are fully submerged with streamlined fins or struts supporting the upper platform or deck.
  • It is used for passenger carrying and research vessels because it provides a stable platform.


  • A vessel is said to be tender if she has an abnormally small metacentric height.
  • Such a ship may have a long period of roll but may list excessively in a strong wind and may be dangerous if a hold is flooded following a collision.


  • The slant inward from the vertical of a transverse section of a hull about the designed waterline.

Turbulence flow

  • Fluid flow where the particle motion at any point is rapidly changing both in direction and magnitude.

Turning circle  

  • A circle moved through by a ship when the rudder is placed in its extreme position.
  • It is a manoeuvre carried out on sea trial.


  • When a substance is immersed in the water it will appear to suffer a loss in weight.
  • Since the actual mass of the substance is not changed, there must be force acting vertically upwards to create the apparent loss of weight.
  • This force is called the force of buoyancy, and is considered to act vertically upwards through a point called centre of buoyancy.

Reserve buoyancy

  • The watertight volume of a ship above the water line is called the reserve buoyancy.
  • It can be defined as the buoyancy a ship can call upon to meet losses of buoyancy due to damage of the main hull.
  • Its use in the general working of the ship is to provide a sufficiency of freeboard to make the vessel seaworthy.

Ships vibration types

  • Synchronous or resonance vibration.
  • Local vibration.
  • Vibration due to external forces.