Bulkheads, in a ship, are the partitions between the several portions of the interior, whether to separate it into compartments to suit the exigencies of stowage or loading, or as a safeguard against foundering (see BUOYANCY). Bulkheads are either transverse—i.e. running athwart the vessel—or longitudinal—i.e. in the direction of the vessel's length, and are usually made watertight, thus dividing the interior of the vessel into three, four, or more compartments, each of which is self-contained, and watertight in reference to its neighbours. Should any one of the compartments, through damage to the skin of the vessel, be laid open to the sea, its self-contained character prevents the water penetrating the other compartments, thus lessening the risk of foundering, if not altogether preventing it. In very many of the modern high-class mail and passenger steamships, the hull is subdivided to such an extent that if any two compartments be simultaneously laid open to the sea, the vessel still retains sufficient floating power. To a very large extent this degree of subdivision has been attained through the encouragement held out to shipowners by the Admiralty, that for ships so divided there is the likelihood of employment in time of war. The great majority of merchant steamers, however, are not yet so perfectly subdivided, objections being taken to the system by shipowners on the score of expense, of extra weight of hull, and of the alleged inconvenience in the working of cargo. The Merchant Shipping Act of 1854 provided for the fitting of a certain number of bulkheads in steamers, but the growth in ship dimensions soon rendered this inelastic provision utterly useless, if not mis- chievous, and it was repealed in 1862. Lloyd's Registry for the classification of ships has virtually assumed the functions which the government and the Board of Trade then discarded, and the rules of this society provide for a degree of subdivision in steamships, which is on the whole satisfactory.
In sailing-ships there is almost an entire absence of bulkheads, the only one considered necessary being that nearest the bow, the intention being to provide against damage to the bow through collision, hence this bulkhead is generally known as the 'collision' bulkhead. In numberless instances the collision bulkhead has saved the vessel which struck another 'bow on;' but the common case is for the iron ship which is struck to founder, if her broadside is penetrated below water.
Since 1874-78 the practice of carrying water ballast (see BALLAST) in the bottom of merchant steamers, to secure the requisite stability when unloaded with cargo, has resulted in various types of double or cellular bottoms, which are of the greatest benefit in the event of damage to the vessel's outer skin. The presence of the inner and watertight bottom in such cases has not unfrequently been the salvation of the vessel after grounding on rocks.
In ironclads such as the Achilles and the Devastation, which have inner bottoms, there are compartments to the number of about 105, each of which is watertight as regards its neighbours. In the torpedo-ram Polyphemus and other recent vessels the subdivision principle has been carried out to even a more elaborate extent.