A corresponding vertical line is drawn at the stern, designated the after perpendicular or AP. When there is a rudder post the AP is located where the after side of the rudder post intersects the DWL. In figure above, the AP is drawn at the centerline of the rudder stock, which is the customary location for merchant ships without a well defined sternpost or rudder post. In the case of naval ships, it is customary to define the AP at the after end of the vessel on the DWL. Such a location is also sometimes chosen for merchant vessels- especially vessels with a submerged stern profile extending well abaft the rudder. Figure below shows the various locations of the AP here described.
An important characteristic of a ship is its length between perpendiculars, sometimes abbreviated LBP or Lpp. This represents the fore-and-aft distance between the FP and AP, and is generally the same as the length L defined in the American Bureau of Shipping Rules for Building and Classing Steel Vessels (Annual)'. However, in the Rules there is included the proviso that L, for use in the Rules, is not to be less than 96 percent and need not be greater than 97 percent of the length on the summer load line. The summer load line is the deepest waterline to which a merchant vessel may legally be loaded during the summer months in certain specified geographical zones.
When comparing different designs, a consistent method of measuring ship lengths should be used. Overall length is invariably available from the vessel's plans and LBP is usually also recorded. However, for hydrodynamic purposes, length on the prevailing waterline may be significant; alternatively, an "effective length" of the underwater body for resistance considerations is sometimes required.
One useful method of determining the after end of effective length is to make use of a sectional area curve, whose ordinates represent the underwater cross sectional area of the vessel up to the DWL at a series of stations along its length. The effective length is usually considered as the overall length of the sectional area curve. However, if the curve has a concave ending, a straight line from the midship-cross-sectional area can be drawn tangent to the curve, as shown in Figure above. The intersection of this straight line tangent with the baseline of the graph may then be considered to represent the after end of the effective length. On many single-screw designs it has been found that the point so determined is close to the location of the AP. Such an effective length ending might then be used in calculating hull form coefficients. A similar definition for the forward end of effective length might be adopted for ships with protruding bulbous bows extending forward of the FP.
It is important that in all calculations and measurements relating to length, the method of determining the length used, and the location of its extremities be clearly defined.