Answer Construction Question 1

Question 1: Explain with sketches the terms hogging and sagging. Which structural members are affected to these conditions? State the type of stresses these members are subjected to under conditions.

Answer: Hogging and Sagging-These are longitudinal bending stresses, which may occur when a ship is in a seaway, or which may be caused in loading her. When she is being loaded, too much weight in the ends may cause her to hog or if too much weight is placed amidships, she may sag. At the bottom, all longitudinal work in the double bottom gives the necessary strength.
At the top, the deck stringer and sheerstrake are thickened, so as to make a strong L-shaped girder on either side. Deck girders and longitudinal bulkheads also help to resist this stress. In large ships, it may be necessary to use special steels for the sheerstrake and bilge strakes; also further to strengthen the ship by fitting longitudinal frames and beams in the bottom and under the strength deck. The stresses are greatest amidships, so the strength of the parts mentioned is made greater amidships than at the ends.
ln long ships, the shearing stresses which occur near the neutral axis also become an important problem. In such ships, it may be necessary to strengthen the hull at about the half-depth of the ship, in the neighborhood of one-quarter of the length from each end

When a ship hogs, the deck structure is in tension while the bottom plating is in compression. If the weight amidships exceeds the buoyancy, the ship will sag, and is equivalent to a beam supported at its ends and loaded at the centre.

When a ship sags, the bottom shell is in tension while the deck is in compression. Changes in bending moment occur in a ship due to different systems of loading. This is particularly true in the case of cargoes such as iron ore which are heavy compared with the volume they occupy. If such cargo is loaded in a tramp ship, care must be taken to ensure a suitable distribution throughout the ship. Much trouble has been found in ships having machinery space and deep tank/cargo hold amidships. There is a tendency in such ships, when loading heavy cargoes, to leave the deep tank empty. This results in an excess of buoyancy in way of the deep tank. Unfortunately there is also an excess of buoyancy in way of the engine room, since the machinery is light when compared with the volume it occupies. A ship in such a loaded condition would therefore hog, creating very high stresses in the deck and bottom shell. This may be so dangerous that if owners intend the ships to be loaded in this manner, additional deck material must be provided.

  The structure resisting longitudinal bending consists of all continuous longitudinal material, the portions farthest from the axis of bending (the neutral axis) being the most important, e.g., keel, bottom shell, centre girder, side girders, tank top, tank margin, side shell, sheer-strake, stringer plate, deck plating alongside hatches, and in the case of oil tankers, longitudinal bulkheads. Danger may occur where a point in the structure is the greatest distance from the neutral axis, such as the top of a sheer-strake, where a high stress point occurs. Such points are to be avoided as far as possible, since a crack in the plate may result. In many oil tankers the structure is improved by joining the sheer-strake and stringer plate to form a rounded gunwale.