Bulkhead and Longitudinal Fit-Up

Bulkhead and Longitudinal Fit-Up

A ship's hull is a system of interlocking parts that must line up precisely before a single permanent weld is laid. Transverse bulkheads divide the hull into watertight compartments and carry lateral loads. Longitudinal stiffeners run fore-and-aft along the shell plating and bottom, resisting bending along the ship's length. Getting both elements plumb, fair, and correctly spaced during fit-up is the shipfitter's core responsibility — errors allowed to pass at this stage are amplified by every weld and plate that follows.

The Fit-Up Sequence

Shipyards follow a consistent sequence for a reason: each step creates the reference that the next step depends on.

1. Set the keel baseline. The keel bar is the ship's centerline reference. Every measurement of height, spacing, and plumb originates here. Confirm the jig's keel bar is level before touching anything else.

2. Establish frame stations. Frame station lines are marked on the keel bar and on the jig rails at the correct spacing shown on the construction drawing. Check that your tape reads from the correct baseline end — a reversed start causes every station to be wrong.

3. Plumb each frame. A frame must stand perpendicular to the keel plane. Use a plumb bob dropped from the frame's centerline mark, or a calibrated digital level placed on the frame web. The standard tolerance used in this program is ±1/8 inch per 10 feet of height. Tack the frame only after plumb is confirmed.

4. Fair the longitudinals. A longitudinal stiffener must lie in a smooth, continuous curve (or straight line) from frame to frame — no kinks, no hollows. Hold a flexible straight-edge batten along the stiffener's face to check fairness. A shadow gap between batten and stiffener indicates a hard spot or hollow that must be corrected before tacking.

5. Fit the bulkhead. Once frames and longitudinals are secured, drop the bulkhead plate into position. The bulkhead must be plumb athwartship (side-to-side) and stand at the correct frame station fore-and-aft. Clamp it with strong-backs so it cannot move while you verify dimensions.

Plumb, Fair, and Square — What Each Means

• Plumb means truly vertical. A plumb bob hangs on gravity's line; a digital level measures deviation from vertical in degrees and converts to linear offset.
• Fair means a smooth, continuous curve with no sudden changes in direction. Fairness is checked with a batten — a thin, flexible strip that conforms to an intended curve and reveals discontinuities.
• Square means 90° between two reference surfaces. Check square with a framing square or by measuring diagonals: if both diagonals of a rectangle are equal, the rectangle is square.

Tack Welds

Tack welds are small, temporary welds that hold parts in position until full welding is complete. In marine fabrication, tacks must be treated like real welds: wrong placement or poor fusion can crack under the stress of subsequent welding and leave unfused areas in the final joint.

• Space tacks as specified on the WPS (Welding Procedure Specification) or as directed by the supervisor — typically every 12 to 18 inches for structural fit-up, but always follow the project document.
• Tacks must be free of cracks, porosity visible to the eye, and undercut.
• Tacks at the start and end of a joint must be placed far enough back from the joint end to allow the welder to strike an arc without burning the edge.
• Use the designated MIG (GMAW) procedure for tacking; do not free-style with an unapproved process.

Strong-Backs and Clamps

A strong-back is a bar or beam temporarily welded or clamped across a joint to hold parts in alignment and resist distortion. Use C-clamps and locking pliers to apply clamping force before tacking. Never rely on the tack weld alone to pull misaligned parts together — force the parts to the correct position with clamps, then tack.

Fit-Up Inspection Checklist

Before any permanent welding begins, every critical dimension must be verified and documented. The checklist used in this lab covers:

• Plumb of bulkhead (±1/8 inch per 10 ft)
• Station location of bulkhead (fore-aft position per drawing)
• Spacing of longitudinals (±1/8 inch from marked position)
• Fairness of longitudinals (batten check — no visible gap)
• Tack-weld spacing (per specification)
• Tack-weld quality (no visible cracks or porosity)

All items must receive instructor sign-off before the team moves on. This mirrors shipyard practice, where a fit-up inspector (often a Quality Control inspector or lead fitter) must approve fit-up before the welder is authorized to begin.

Safety for This Lab

• Eye and face protection: Wear safety glasses at all times in the lab. Add a welding helmet (auto-darkening, appropriate shade) whenever tacking.
• Hand protection: Leather welding gloves when tacking or handling freshly tacked steel.
• Foot protection: Steel-toed boots — no exceptions around plate and bar stock.
• Housekeeping: Coiled tape measures, dropped battens, and loose clamps on the floor are trip hazards. Return tools to the bench when not in use.
• MIG welder setup: Inspect the gun cable, contact tip, and gas supply before use. Confirm shielding gas flow. Do not weld without ventilation.
• Lockout/Tagout awareness: If the MIG welder must be serviced (contact tip change, wire jam), follow your facility's LOTO procedure — de-energize and lock out before opening the wire-feed compartment.
• Lifting: Bulkhead plates and flat bars have sharp edges and significant weight. Use gloves, keep your back straight, and ask your partner to assist.