SHIPBUILDING: MERCANTILE. As a ship is a most complex engineering structure, the process of design requires con siderable experience and skill, allied with sound judgment. When a shipowner orders a new vessel he must in the first instance have a clear idea as to the size and type and the other properties desired in order to suit the particular trade in which it will be engaged. These requirements will include the weight and space required for cargo, the number of passengers and type of accom modation for them, the speed to be attained, and the distance to be run without replenishing the fuel supply. One important factor which must not be omitted is the permissible draught of the ship, since the depth of water available in many harbours is limited. In addition it is desirable to decide in which Classification Society, if any, the vessel is to be classed, since this will determine the details of the scantlings to be employed. It is also necessary to decide whether the method of propulsion is to be by steam engines —either reciprocating or turbine or a combination of these—or by an oil engine. The choice of machinery depends upon many considerations, chiefly of an economic character, and it is by no means easy to decide which will be the most suitable to meet given conditions.
With this information at his disposal, the shipbuilder is in a position to prepare his preliminary designs and to determine the most suitable dimensions. The settling of the most suitable dimen sions is a task calling for considerable care, since the whole suc cess of the ship will depend upon the solution of this problem. With given dimensions the weight of the ship's structure includ ing hull, equipment and machinery can be calculated, and this weight together with the weight of the cargo to be carried (known as the deadweight) must not exceed the displacement of the ship at the required draught.
In addition, the depth of the vessel must be such that the distance from the water line to the deck must not be less than that given in the Tables of Freeboard. This distance, known as the freeboard, varies with the size of the ship and the nature and extent of the erections above the deck, and has been determined as the result of many years observations and experience. When
dimensions giving a suitable deadweight and freeboard have been determined, the capacities of the various holds are calculated in order to ensure that there will be a sufficient volume to enable the cargo to be carried—for example, it will be clear that very differ ent capacities will be required to house the same total weight of iron ore or of light piece goods.
If the ship is intended to carry passengers, regard must also be paid to the requirements as to the effective subdivision of the ship into watertight compartments, the object being to obtain a vessel which will remain afloat if 1, 2 or sometimes even 3 compartments are laid open to the sea by damage through collision or otherwise. The present official British regulations on this subject were formu lated on the recommendations of a Committee appointed by the Board of Trade immediately after the sinking of the "Titanic" in April 1912 with the loss of some 1,5oo passengers and crew. (See SHIPPING : Registration, Classification and State Regulation.) The determination of a suitable breadth in relation to the depth calls for some care, since upon this depends the stability of the vessel, a vital factor, since the stability must be sufficient for safety but must not be so excessive as to cause the ship to roll uncomfortably in a seaway.
Finally, regard must be paid to the various official regulations of the countries between which the ship is intended to trade.
The designer will usually have, to guide him, the details of some successful ship or ships previously built to fulfil the same or similar conditions, and he will probably know what measure of success or popularity the respective features of such vessels have earned in service. The dimensions can in this event be readily fixed to provide the necessary speed and deadweight, stability and seaworthiness, and the cost of the vessel determined. On the other hand, if the departures from previous vessels or from the usual practice be very great, much will depend upon the designer's skill and judgment.