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Ship Handling

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Inherent Factors that Affect Ship Handling

  • Propeller
    • Screw. The propeller used to drive all modern ships. A screw may have a various number of blades and pitch.
    • Propeller Thrust. The force caused by the displacement of water along the propeller shaft to thrust the ship ahead as the ship moves in the direction of the low pressure area. The after face of the propeller blade creates a high pressure area.
    • Side Force. The force that moves the stern of the ship in the direction of screw rotation. Side thrust produced by the screw's rotation through the water. It is noticeable at the stern of the ship. The twin-screw ships cancel side force created by the rotating the screws in opposite directions.
    • Screw Wash. Turbulence produced by the screws turning against the water. While twisting or operating astern, it negatively effects a short radius turn by decreasing the efficiency of the rudder.
    • Twist.  On multiple screw ships, it is the effect of screws rotating in opposite directions.
    • Pitch. The distance a propeller would travel in one revolution if water were a solid medium.
    • Slip. The difference between the speed of a ship and its propeller.
    • Cavitation. Cavities or bubbles around a propeller which are a result of the pressure on the lower and upper blade surfaces being unequal. It is caused by blade tip speed being excessive or by the vessel riding high in the water.
    • Bow Thruster. A propeller on some ships that is in a fixed transverse tunnel at the bow. It is used as a maneuvering assistance device on during low speed operations.  
  • Rudder. A device used to control ship's course through the water. It is designed to produce the lateral forces used in the control of the ship's heading. The rudder force acts through the rudderstock, pushing the stern to starboard or port as the rudder is angled to the passing water. Angling the rudder to the flow of water creates a high-pressure force on the leading surface and a low-pressure force on the trailing side. This forces the stern in a direction opposite that in which the rudder is set.
  • Freeboard, The vertical distance from the waterline to the weather deck, usually the main deck. It is determined by the ships mean draft.
  • Sail Area. The area above the main deck, which in strong winds will complicate maneuvering and require compensating navigational corrections.