What Is the Frame Spacing on Ships?

A ship's framework is like a rib cage and the hull is like the skin. The ribs hold the hull in place, not allowing it to flex. Because the hull curves to produce the classic boat shape, every rib has a unique shape. Frames are spaced unequally along the length of the ship. Marine architects and engineers vary the number and the spacing of the frames based on the requirements of the hull.

The frames, also called ribs or transverses, look almost like a human rib cage after they are assembled. The backbone, called the keel, runs along the very bottom of the ship. The frames are attached to the keel and support the hull on their exterior surface. The keel must run straight, because the proper alignment of the frames depends on it. This method of shipbuilding is thousands of years old; the Vikings used a rib and keel design for their ships.

How the frames are spaced along the keel is partly art but mostly science. Marine architects or marine engineers look at the overall ship design and determine where the most stress occurs. Engineers place more ribs in high-stress areas. They use computer animations and even build model ships, which they test in a tank. In the University of Iowa's tank, for example, ship models can be tested in both calm conditions and simulated storms.

The hull of a ship or boat has very few straight surfaces. The hull bends smoothly from bow to stern, to slice cleanly through the water. Where the hull has severe curves, such as the bow, more frames are placed to support the curvature.

Some ships, called icebreakers, are specially designed to undergo severe stress on the hull. Icebreakers pound through a frozen body of water, breaking up the ice to form a clear travel channel for conventional ships. The hulls of icebreakers are specially designed to take the stress and have extra framing. Engineers use complex equations to determine frame spacing on high-stress icebreaker ships.