Page 18 - WestlawnMasthead14_Sept.10.pub

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By Nicholas Di Matteo
Instructor, Westlawn Institute of Marine Technology
Why are the Forward Perpendicular (FP) and Aft Perpendicular (AP) shown at different locations on different
hull lines drawings? On some drawings they are shown at the extreme ends of the design waterline (DWL).
other drawings, they are at the extreme ends of the hull structure at the stem and the transom.
The reason for the differences is that these terms have their origin in the days of wooden boatbuilding. As
ships changed to iron and then steel, Naval Architecture adjusted. Today, the FP is at Station “0” at the forw
end of the bow at the design waterline. The AP is located either through the rudder post, or relative to the ru
der post, based on the design standards and classification society rules. Alternately, the AP is the vertical lin
through the DWL where the stern cross the waterline.
Whenever the length between perpendiculars (LBP), or any dimension or measurement is required for calcul
tions involving the perpendiculars, it always refers to the perpendiculars located at the ends of the waterline.
Westlawn has been teaching boat design since 1930 when almost all boats were made of wood. Some of th
old drawings of wooden boats show the AP and the FP at the extreme ends of the stem and transom. Most b
builders were accustomed to seeing the perpendiculars at the extreme ends of the hull for many years. This i
why some of
these wooden
boat drawings
show the perp
diculars at the
extreme ends
the hull structu
To be consiste
with modern d
sign terminolo
and avoid conf
sion, the perpe
diculars at the extreme ends of the hull structure should be renamed: “B” for BOW and “S” for STERN. This wi
allow the use of AP and FP at the ends of the DWL (as indicated on the drawing) if calculations are required i
volving the “Length Between Perpendiculars,” or for classification societies.
For your information:

Length between perpendiculars LBD is used for large pleasure boats and small ships. LPP is used for large ships.

Navy ships: The placement of the perpendiculars is at the ends of the DWL (Summer Load Line.)

Merchant ships: The FP is at the forward end of the DWL at the stem. The AP is through the rudder post, or relati
to the rudder post.

Transom hung rudders: The AP is at the “Pintle” for transom hung” Pintle and Gudgeon” arrangements.

Classification societies: The AP and FP control the locations of the first and last frames in metal boats, dependin
the size of vessel and classification society.

Longitudinal Stability calculations for large vessels utilize the modern AP and FP at the ends of the DWL.

There are no standard regulatory or class association calculations using the term “ LBP” between an AP and FP l
cated at the extreme ends of the hull. This is in fact the LOA (Length Overall), and this term is used throughout th
marine industry.