LOA (Length Overall) and LBP (Length Between Perpendiculars) measure a ship’s length in two genuinely different ways, and using the wrong one in the wrong context produces a real calculation error.
LOA: The Absolute Maximum Length
Length Overall is exactly what it sounds like — the total length of the vessel from its most forward point to its most aft point, including any bow or stern overhangs, bulbous bow projections, or other extremities. This is the figure relevant for practical clearance purposes — fitting into a berth, a drydock, or determining overall vessel size for port fees.
LBP: The Design Reference Length
Length Between Perpendiculars is measured specifically between two defined reference points used in the ship’s original design — the forward perpendicular (typically at the point where the design waterline intersects the stem) and the aft perpendicular (typically at the rudder stock centerline, or the aft end of the waterline). LBP is always shorter than LOA, since it excludes bow and stern overhangs beyond these reference points.
Why LBP Is What Stability Calculations Actually Use
LBP, not LOA, is the reference length used in most hydrostatic and stability calculations (block coefficient, TPC, and similar formulas) — because these calculations are fundamentally about the vessel’s underwater hull form and displacement, which relate to the design perpendiculars, not the extreme overall structural length including overhangs that may not even be in the water.
Why Using the Wrong One Matters
Using LOA where a stability calculation specifically requires LBP (or vice versa) introduces a genuine, meaningful error into the calculation — these aren’t interchangeable figures for a working officer, even though both are legitimately called “the ship’s length” in different contexts.
Quick Reference
- LOA — for berthing, drydocking, port fees, general “how big is this ship” descriptions
- LBP — for stability, hydrostatic, and design calculations
Exam Relevance
The LOA/LBP distinction, and knowing which applies to which calculation, is a core, frequently-tested Ship Stability and Ship Knowledge topic in deck officer competency exams.