Length defines the measure of the ship in the longitudinal direction. Due to the shape of a ship (i.e., stem rake or sheer line) this can change under varying conditions of load, trim or heel. As such, when performing calculations or comparing vessels, care must be taken to ensure a consistent and appropriate definition of length is used.
Length overall is the distance from the the foremost extremity of the hull to the aft-most extremity of the hull, generally inclusive of all overhangs. Flag poles are generally excluded.
Length of hull is the distance from the aft most extremity of the hull to the foremost extremity of the hull (excluding appendages, such as bowsprits, etc.).
The effective length is the overall length of a vessels sectional area curve (to the DWL). This definition includes portions of the vessel which may not be accounted for otherwise due to submergance, i.e., bulbous bows or stern bulbs. An AP and FP may be assigned to such a curve by considering a line drawn from the point of maximum midship area to the farthest possible tangent fore and aft. Where these lines intersect the baseline provide the longitudinal location of the points.
The load line length is the length assigned by the International Load Line Convention. The load line length is set at 0.85 * D (see loadlines), and generally agrees to the scantling waterline (unless the scantlings have been evaluated deeper, or load line assigned shallower).
Length between perpendiculars is the distance between the forward perpendicular and after perpendicular. LPP is generally the same length as the rule length (though it may vary as per rule length restrictions). When listing trim, it is often described as the amount of trim over the LPP (i.e., 0.768/90.2; this value is unitless (meters/meters)).
Rule length, or scantling length, is the value of length used for classification's scantlings compliance calculations. The length is generally prescribed at the summer loadline, measured from the forward perpendicular to the centreline of the rudder stock; though generally neither greater than 97% nor less than 96% of the overall waterline length. This value can change with rudder construction (or inclusion); as such, it can vary with the design and the classification societies rules must be checked for the exact definition.