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.
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.
Lies and Planes. The reference lines and planes used to locate points
on ships are:
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.
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).
Midship Section: A plane passed athwartships halfway between the
forward and the aft perpendiculars.
Centerline: A vertical plane passing fore and aft down the center
of the ship.
Baseline: A fore-and-aft line passing through the lowest point
of the hull.
of Points. The position of any point in the ship can be described
by measuring its:
above the baseline or keel.
to either side of the centerline.
fore and aft from the midship section or from one of the perpendiculars.
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.
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
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
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