A frame is a transverse structural member — running side to side across the hull, from keel to deck — that supports the shell plating and gives the hull its characteristic cross-sectional shape.
The “Ribs” Analogy
Frames function similarly to ribs in a human body — a series of transverse members spaced along the ship’s length, each supporting the outer “skin” (shell plating) at that point, working together to give the entire hull its shape and structural rigidity.
Frame Spacing
Frames are spaced at regular intervals along a vessel’s length (frame spacing), and this spacing itself is used as a reference for locating positions along the ship — “frame 42” or similar references are commonly used in ship drawings and damage reports to specify an exact longitudinal position.
Transverse vs Longitudinal Framing Systems
- Transverse framing — closely spaced frames (ribs) with fewer longitudinal members; structurally simpler, common on smaller vessels
- Longitudinal framing — closely spaced longitudinal members (girders, stringers) with fewer, more widely spaced but heavier transverse frames; better resistance to the fore-and-aft bending stresses (hogging/sagging) large vessels experience in waves, which is why most modern large tankers and bulk carriers use this system, at least in deck and bottom structure
Web Frames
At intervals, reinforced, heavier frames — web frames — are positioned to provide additional structural strength at key points, working alongside the standard-spacing frames rather than replacing them.
Why Frame Condition Matters
Frames, like all structural steel, are subject to corrosion over a vessel’s service life — frame condition (particularly in ballast tanks and other areas prone to corrosion) is a genuine structural integrity concern checked during class surveys, since significantly corroded or damaged frames compromise the hull’s overall structural strength.
Exam Relevance
Frame function, spacing, and the transverse vs longitudinal framing system distinction are core Ship Construction topics in deck and engineering officer competency exams.