Once in a while you may read or hear of a ship “breaking her back” while laboring in a heavy sea, or through suffering structural damage in running aground, or perhaps in a collision.
Ship structure pressures
The sea exerts immense pressures upon a ship’s structure – especially if the ship is long and narrow. As the length of ships has extended more and more during the past few decades (particularly in the case of crude carriers, the largest of which is 1500 feet, or 458 meters, long) – so the danger of structural stress has increased quite considerably.
The three most common stress factors are hogging, sagging and shearing. If one or more of these factors is exerted on a ship with inadequate strength in her structure, her back will be broken.
Similarly, a collision or grounding followed by pummelling from heavy seas could fracture the keel of a ship.
The keel is the lowest, continuous line of steel plates extending the whole length of a ship.
Fixed to the keel are the stem, the stern post, and the ribs or frames. Together they form the ship’s strongest single member.
In most ships, two additional keels – known as bilge keels – are fitted to the hull on either side of the central keel to assist the ship’s launch.
They also help to minimize rolling in a heavy sea.
Why ship rolls?
A vessel rolls because the water level is higher on one side than on the other: if these levels vary greatly and coincide with the ship’s natural roll, the result can be quite violent.
Rolling can be partially arrested by fitting fin stabilizers. Other special roll dampers or anti-rolling devices are fitted to ships, especially to those that carry passengers.
Hogging Lengthwise, or longitudinal, strength is vital in order to withstand the stress that occurs when the midsection of a ship is supported by the crest of a wave.
Sagging When the bows and stem of a ship are supported by wave crests, the center section is left unsupported. It can lead to a vessel breaking up amidships and sinking.
Shearing As the lengths of ships increase, so the designer’s problems multiply. Very long tankers and bulk carriers are especially susceptible to ocean and wave conditions that can produce the effect of being supported by three wave crests (as arrowed).