Competition has been the driver of much of the evolution in ship design, and this has been particularly noticeable in recent years. Because merchant ships operate in a largely free market, an owner or operator can expect to gain business by offering a better service. That could mean conveying cargo more cheaply, offering passengers greater luxury, providing exporters with more dependable or more regular sailings, or delivering perishable goods such as fruit or meat in better condition. To do this an owner will seek to improve his vessel.

For instance, when the steam tramp was threatening the business of sailing ship owners in the late nineteenth century, one way in which the latter responded was to switch to iron or steel for building the hull. This allowed them to build vessels which carried more cargo with only a small increase in crew size, so that they could offer lower freight rates. In response, the tramp steamer itself got bigger and its engines more economical, and it drove the ocean-going sailing ship into extinction. Eventually, the steam tramp was itself made extinct by the modern oil-engined bulk carrier, which offered greater efficiency. Shipping offers many examples of Darwin’s principle of survival of the fittest.

Changes like those described do not happen overnight, however. Technology develops relatively slowly, so that a ship can usually expect a life of 20 to 30 years before it is economically outclassed, and by then it is usually worn out.