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A Ship’s Length, Breadth and Depth
Ship dimensions describe different reference lines on a hull. A length, breadth or depth is therefore only unambiguous when it is clear which points it is measured between and which technical definition applies. Ship data may list several valid dimensions side by side that do not directly represent the same thing.
Length overall and length between perpendiculars
Length overall (LOA) runs between the foremost and aftermost points of the hull. Length between perpendiculars (LBP), by contrast, is measured between two defined vertical lines at the waterline. The U.S. Coast Guard (USCG) places this LBP waterline at the greatest operating draft. A bow or stern projection can therefore affect the LOA without changing the LBP to the same extent.
The two values answer different questions: LOA describes the hull’s extreme length, while LBP denotes a distance between specified reference points. Other lengths, such as length at the waterline or on deck, also use their own reference lines. A table that gives only “length” leaves it unclear which type of measurement is meant.
Breadth and depth
The USCG definition of breadth (beam) refers to the ship’s greatest breadth and uses different measuring surfaces depending on the hull construction. The IMO moulded breadth, by contrast, is measured at the ship’s midsection: for metal ships, to the moulded line of the frame; for other materials, to the outer surface of the hull. Moulded breadth and extreme breadth can therefore produce different values.
Depth, or moulded depth, is a vertical measurement. The IMO Intact Stability Code defines it from the top of the keel to the upper edge of the deck beam at the side of the freeboard deck. Draft, by contrast, is the distance between the waterline and a lower reference point on the hull. Both measurements are vertical, but they do not describe the same distance. Draft and related measurementsdiffer according to their reference lines.
Interpreting dimensions in ship data
Ship profiles show how length and breadth values are recorded for specific cruise ships, for example the Mein Schiff 4 and the AIDAprima. Such figures are directly comparable only when they represent the same measurement under the same definition. Technical standards use terms for specific purposes, so the definition underlying a dimension is part of the measurement itself—not merely a footnote.
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