Question

Define five of the following, stating how they affect the ship structure and what component parts of the ship structure help to resist the effect of;

Answer

Racking.

When a ship rolls there is a tendency for the ship to distort transversely in a similar way to that in which a picture frame may collapse. This is known as racking and is reduced or prevented by the beam knee and tank side bracket connections, together with the transverse bulkheads, the latter having the greatest effect.

Water pressure effect.

A considerable force is exerted on the bottom and side shell by the water surrounding the ship. The double bottom floors and side frames are designed to withstand these forces, while the shell plating must be thick enough to prevent buckling between the floors and frames. Since water pressure increases with the depth of immersion, the load on the bottom shell exceeds that on the side shell. It follows, therefore, that the bottom shell must be thicker than the side shell. When the ship passes through waves, these forces are of a pulsating nature and may vary considerably in high waves, while in bad weather conditions the shell plating above the waterline will receive severe hammering.

Panting

As the waves pass along the ship they cause fluctuations in water pressure which tend to create an in-and-out movement of the shell plating. The effect of this is found to be greatest at the ends of the ship, particularly at the fore end, where the shell is relatively flat. Such movements are termed panting and, if unrestricted, could eventually lead to fatigue of the material and must therefore be prevented.

The structure at the ends of the ship is stiffened to prevent any undue movement of the shell.

Pounding

When a ship meets heavy weather and commences heaving and pitching, the rise of the fore end of the ship occasionally synchronizes with the trough of a wave. The fore end then emerges from the water and re-enters with a tremendous slamming effect, known as pounding. While this does not occur with great regularity, it may nevertheless cause damage to the bottom of the ship forward. The shell plating must be stiffened to prevent buckling. Pounding also occurs aft in way of the cruiser stern but the effects are not nearly as great.

Local weights.

The deck must be designed to support the weight of accommodation, winches and cargo, while exposed decks may have to withstand a tremendous weight of water shipped in heavy weather. The deck plating is connected to beams which transmit the loads to longitudinal girders and to the side frames. In way of heavy local loads such as winches, additional stiffening is arranged. The shell plating and frames form pillars which support the weights from the decks. The tank top is required to carry the weight of the hold cargo or the up thrust exerted by the liquid in the tanks, the latter usually proving to be the most severe load.

Vibration from engine and propellers

If the frequency of the main or auxiliary machinery at any given speed coincides with the natural frequency of the hull structure, then vibration may occur

In the machinery space other factors must be taken into account. Forces of pulsating nature are transmitted through the structure due to the general out of balance forces of the machinery parts. The machinery seats must be extremely well supported to prevent any movement of the machinery. Additional girders are fitted in the double bottom and the thickness of the tank top increased under the engine in an attempt to reduce the possibility of movement which could cause severe vibration in the ship. For similar reasons the shaft and propeller must be well supported.