main frame

The main frame is the transverse structural member that defines a ship's maximum breadth. It runs athwartship (side to side) at the widest point of the hull, typically located between one-quarter and one-third of the ship's length from the bow. This frame carries the greatest bending loads during operation and serves as the primary reference point for the entire hull structure.

In steel ship construction, the main frame consists of a deep vertical web stiffened by horizontal flanges, creating an I or H-shaped cross-section. The web may be 12 to 20 millimeters thick on large vessels, while flanges extend 150 to 300 millimeters depending on the ship's size and intended service. This frame anchors the outer shell plating and inner longitudinal stiffeners, transmitting hull loads to the keel structure.

All other transverse frames are molded (shaped) smaller than the main frame, creating the ship's characteristic shape. Forward of the main frame, frames taper toward the bow; aft frames taper toward the stern. The main frame's dimensions determine the section modulus at midships, a critical parameter for calculating the ship's resistance to bending in waves. Its position and dimensions are among the first decisions made during hull design.

The main frame intersects with the ship's longitudinal stiffening system, particularly the centerline keelson and side girders. These connections are heavily reinforced through doubler plates and increased welding to handle the concentration of stress. Damage to the main frame from collision or grounding can compromise the entire hull's structural integrity.

In naval and large commercial vessels, the main frame often houses critical systems: fuel oil tanks, ballast tanks, or machinery spaces may be positioned at this section. The frame's location and strength make it the natural dividing line between forward and aft structural zones, affecting how compartmentation is planned and how loads are distributed through the ship.