Keel, Girder, and Longitudinal Installation

Why Erection Sequence Matters

A ship hull is not assembled randomly. Every piece goes in a specific order so that each member can be aligned, checked, and welded without being blocked by the next piece. For a typical bottom structure the sequence is: keel first, center girder second, transverse frames third, longitudinals last. Follow this order and every subsequent member has a fixed reference to measure from. Skip a step and errors compound until the hull is out of tolerance and expensive to correct.

The Keel: Ship's Backbone

The keel runs along the exact centerline of the hull from bow to stern. In small and medium vessels you will encounter two common forms. A flat plate keel is a wide, heavy steel plate that forms the bottom centerline strake. A bar keel is a solid rectangular or half-round bar welded to the outside of the hull bottom. Either way, the keel is the first structural member erected because everything else references it.

Before the keel plate touches the erection blocks, the shop floor or building berth must be leveled and a chalk baseline must be snapped along the intended centerline. Supplement the chalk line with a laser level or transit level to project the baseline in three dimensions. Check that all erection blocks are at the same height—use a level and shim as needed. A keel that is twisted or bowed from the start cannot be corrected by welding later without introducing residual stress.

Set the keel plate on the blocks, align its centerline exactly over the chalk line, and clamp it down with dogs and strongbacks. Before any tack welds are placed, measure the offset from the chalk baseline at three or more points along the plate length. All readings must fall within 1/8 inch of zero offset. When the instructor verifies the alignment, place tack welds at intervals per the approved procedure to hold the plate while the center girder is installed.

Center Girder: Vertical Web Along the Centerline

The center girder (also called the center vertical keel in some classification standards) is a continuous vertical web plate running along the top of the keel plate, on the centerline. It ties the keel to the floor plate structure above and resists vertical bending loads. In a double-bottom hull it becomes the inner keel, but even in a single-bottom mock-up it is the primary vertical reference for everything outboard of it.

Fit the center girder web to the keel plate with a tight, gap-free fit-up before tacking. Use a square to verify it stands perpendicular to the keel surface. Plumb lines dropped from an overhead reference or readings from the laser level confirm the web is truly vertical and on centerline. Tack weld the web to the keel at intervals short enough to prevent movement during subsequent fitting.

Longitudinal Members: Spreading Strength Along the Length

Longitudinals (longitudinal stiffeners or longitudinal frames) run parallel to the keel, outboard of the center girder, at spacings shown on the construction drawing. In a longitudinally framed hull they carry a large share of the hull girder bending stress. In a transversely framed hull they stabilize the plating between frames.

Transfer the frame spacing dimensions from the drawing to the keel plate using a steel tape, scribing marks or center-punch marks at each longitudinal position. Set each longitudinal bar on the marks, clamp it, and check two things:

1. Transverse position: measure from the centerline to confirm the longitudinal sits at the specified offset within 1/8 inch.

2. Fairness: lay a wooden or aluminum batten (at least 8 feet long) along the top edge of the longitudinal from one end to the other. A fair member shows no humps or hollows; the batten contacts the member uniformly. Any gap between batten and member edge greater than 1/8 inch in a 10-foot run is out of tolerance and must be corrected by adjusting clamps or reshimming before tacking.

Tack weld each longitudinal after the instructor approves its position.

Reading the Construction Drawing

Before you pick up a clamp, study the drawing. Locate the baseline symbol and the centerline symbol. Identify the scantling (size) of each member and the spacing dimensions. Note any camber requirement—some hull bottoms are not perfectly flat but have a slight rise toward the centerline. If camber is shown, the erection blocks must be cut or shimmed to that profile before any steel is set.

Safety During Erection

PPE required throughout this lab: safety glasses or face shield, hearing protection when grinding or hammering, leather gloves when handling steel, steel-toed boots, and a welding helmet rated for the amperage used when tacking.

Struck-by and pinch points: steel members shifted with a pry bar or slid over blocks can move suddenly. Keep hands clear of pinch zones between the member and the block. Never stand directly in line with a clamp under tension.

Hot metal: tack welds remain hot enough to burn skin for several minutes. Use a chipping hammer and wire brush with eye protection when cleaning tacks, and let the area cool or use heat-resistant gloves before handling.

Tripping hazards: chalk lines, cable leads, and clamp screws on the shop floor are all trip hazards. Keep the work area clear of loose material between tasks.

Laser level: if a Class 2 or Class 3R laser level is used, never look directly into the beam. Follow the manufacturer's warning labels.

Erection Inspection Sheet

At the end of the lab your instructor will use an erection inspection sheet to record measured offsets at each check point. The sheet becomes a quality record tied to your NCCER Shipfitting portfolio. Maximum allowable deviation: 1/8 inch from baseline for keel position; 1/8 inch in any 10-foot run for longitudinal fairness. Both tolerances must be met before the inspector's signature is given.