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The Basics


For any ship, the hull form chosen by the designer determines the stability and floatation (hydrostatic) characteristics. Knowledge of the geometry of ships is necessary for understanding ship hydrostatics.

Location of Points Within a Ship. Because a ship is a three dimensional object, references must be established for locating points in, on, and about the ship. The position of any point in the ship can be described by measuring its position from reference lines and planes.

Reference Lies and Planes. The reference lines and planes used to locate points on ships are:

  • The Forward Perpendicular (FP): A vertical line through the forward extremity of the design waterline, this is the waterline at which the ship is designed to float.
  • The Aft Perpendicular (AP): A vertical line at or near the stern of the ship. This passes through the aft extremity of the design waterline (naval ships) or through the rudder post (merchant ships).
  • The Midship Section: A plane passed athwartships halfway between the forward and the aft perpendiculars.
  • The Centerline: A vertical plane passing fore and aft down the center of the ship.
  • The Baseline: A fore-and-aft line passing through the lowest point of the hull.

Location of Points. The position of any point in the ship can be described by measuring its:

  • Height above the baseline or keel.
  • Position to either side of the centerline.
  • Position fore and aft from the midship section or from one of the perpendiculars.

For example, a point may be 10 feet above the baseline, 15 feet to starboard, and 20 feet aft of the foreward perpendicular. Figure 1 shows where this point lies and how the three coordinates describe its exact position.

Ship Geometry.
A three dimensional object such as a ship is usually represented geometrically in three forms that are similar to the side view, top view, and front view of standard drawings. In naval architecture these three views are known as the body plan (transverse cuts), the sheer plan (longitudinal cuts), and the half-breadth plan (waterlines). Schematically, the three views are shown in Figure 2. The planes that are parallel to the midship section plane, shown usually at either ten or twenty equal intervals along the ship's length, are called station planes, and the true shapes of their intersections with the hull are referred to as stations. Stations are identified with numbers, starting with zero at the bow and increasing aft (the convention in the US and UK), or with zero at the stern and increasing forward (the convention in Europe and Asia). Only half of each station is shown in the body plan, since the other half is symmetrical. By convention, stations from the bow to the midship section are drawn to the right of the centerline, and those from amidships aft are drawn to the left side. Half-spaced stations are often added at the forward and aft ends where the waterline shape changes more rapidly. In the body plan shown in Figure 3 for a destroyer type hull, bow and stern profiles have been superimposed to provide a simplified graphical expression of the hull shape; this is frequently used in technical reports as a substitute for the complete lines drawing. A complete set of lines for a Mariner class hull is shown in Figure 4.

Table of Offsets.
A table of offsets is basically a digitized form of the lines drawing. After a ship has been designed and its hull form determined and graphically described as above, it is customary to set up a matrix system for numerical calculations. This matrix, arranged in tabular form is called the table of offsets.