Ship Dimensions

Length Between Perpendiculars (LBP)

This is the distance between the forward perpendicular (FP) and the aft perpendicular (AP). It is usually called LPP for large ships. For small vessels, this value is usually equivalent to LWL.

Length on the Waterline (LWL)

This is the distance from the intersection of the waterplane at the bow, to the intersection of the waterplane with the stern. Often, this term is used to mean the design or target waterline length. See also DWL.

Length Overall (LOA)

This is the length of the boat from the tip of the bow to the furthest point aft on the stern.

Aft Perpendicular (AP)

This location is usually at the end of the design waterline (DWL) or located at the rudder post for large ships.

Forward perpendicular (FP)

This is the position on the boat where X=0. Usually this is set to the forward end of the design waterline

Depth (D)

This is a vague distance usually defined as the distance from the bottom of the hull (without appendages) to the highest point on the sheer line. If the program uses this number, it is only for sizing purposes and need not be perfectly accurate.

Freeboards (F)

This term refers to the distance from the waterline to the sheer line. Often, freeboards at very specific locations are needed, in which case you have to be very careful in determining exactly where and how the freeboards are measured. Sometimes the sheer point is not well defined.

Draft (T)

This is the distance from the baseline (Z=0) to the waterline at amidships. Normally, draft is used to mean the distance to the design waterline(DWL), but it often depends on its context of use. In addition, the bottom of the boat is usually located at the baseline so that draft refers to the distance from the bottom of the boat to the waterline. However, if the bottom of the boat is NOT located at the baseline, you still must calculate draft from the baseline.