Ship Structure Fundamentals and Drawing Interpretation

What a Ship's Hull Must Do

A steel hull is a hollow box that must carry cargo loads, withstand wave-bending forces, resist twisting, and keep water out — all at once. Naval architects divide that job among a set of structural members, each named and positioned precisely. Reading and interpreting structural drawings is the first skill a shipfitter develops, because every cut, fit, and weld you make follows those drawings. This lesson walks through the major structural elements and shows you how to find them on the three standard drawing views.

The Three Standard Drawing Views

Shipyard drawings present the hull in three complementary views:

• Lines plan (body plan, profile, and half-breadth): Shows the curved shape of the hull. The profile view looks at the ship from the side. The half-breadth view looks down from above. The body plan shows cross-sections of the hull at specific frame stations, with the forward half on one side and the aft half on the other.
• Structural General Arrangement (GA) drawing: Overlays the structural members — frames, girders, bulkheads, and so on — onto the hull form so you can see their positions and spacing.
• Midship section drawing: A detailed cross-section at the widest, most heavily loaded part of the hull; it is the primary reference for scantlings (the sizes of structural members).

Frame stations are numbered from the forward perpendicular (FP), which is the vertical reference line at the bow, toward the stern. Each station number multiplied by the frame spacing gives the longitudinal distance from the FP.

Major Structural Elements

Keel

The keel runs along the centerline at the bottom of the hull from bow to stern. It is the ship's backbone. The flat plate keel (a steel plate on the centerline of the bottom shell) is the most common form in modern steel ships. The keel resists the bending moment that tries to "hog" or "sag" the hull in a seaway.

Frames

Frames are the transverse ribs of the hull. They run from the keel up each side to the deck and give the hull its cross-sectional shape. Frames are spaced at regular intervals called frame spacing, typically stated in millimeters or inches on the frame-spacing schedule.

Floors

Floors are transverse structural members that span the bottom of the hull between the keel and the bilge. They tie the bottom shell plating to the inner bottom and resist the upward water pressure acting on the bottom. Solid floors are full-depth plates; open (lightened) floors have cutouts to reduce weight and allow bilge water to drain.

Girders

Girders are longitudinal members running parallel to the keel, usually in the bottom structure. The center girder (also called the vertical keel) sits on the centerline. Side girders run outboard of it. Girders stiffen the bottom against bending and support the floors.

Stringers

Stringers are longitudinal members running along the sides of the hull at various heights. They tie the frames together lengthwise, distribute local loads, and stiffen the side shell against racking and buckling.

Longitudinals

In a longitudinal framing system (common in large ocean-going vessels), closely spaced longitudinal stiffeners replace some of the transverse frames along the bottom and deck. They run fore-and-aft and are very effective at resisting the hull's overall bending moment. Heavy transverse frames called web frames or transverses support the longitudinals at wider intervals.

Deck Beams

Deck beams are transverse members that support the deck plating. They carry the deck load down to the frames and longitudinal girders. Camber — the slight athwartship curve of the deck — is built into the deck beams to shed water.

Bulkheads

Bulkheads are vertical dividing walls inside the hull. Transverse bulkheads run athwartships; they divide the hull into watertight compartments, which limit flooding if the shell is breached. Longitudinal bulkheads run fore-and-aft and are used in wide-beam ships and tankers. Watertight bulkheads must be built and tested to the standards of the classification society (ABS for most US commercial vessels).

Shell Plating

Shell plating is the skin of the hull — the steel plates welded together to form the watertight envelope. Plates are identified by their position: keel plate, bottom plating, bilge strake, side shell, and sheer strake (the top course of side plating at deck level). Plate thickness is heavier in high-stress areas such as the keel and at the waterline.

Reading Structural Drawings Safely

Drawing interpretation is a safety-critical skill. Acting on a misread drawing can result in a structural member being omitted or mislocated, which can cause hull failure. Before marking or cutting any material:

• Confirm you have the current revision of the drawing (check the revision block in the title block).
• Verify the scale before using a scale rule; never scale from a photocopy that may have been reduced.
• Identify every weld symbol on a detail before fitting parts — missing a required weld is a structural deficiency.
• In the shop, apply lockout/tagout (LOTO) procedures on all cutting and material-handling equipment before adjusting or clearing jams.
• Wear required PPE (safety glasses, steel-toed boots, hard hat in the construction area) when moving between the drawing office and the building way.