During the state of transition the first Cunard liner to cross the atlantic was the Britannia, powered by steam, paddles and sail.

In wooden ships the keel forms the base of the whole structure; from it rise on either side a large number of ribs, consisting of strong timbers usually built up of several pieces, and having the requisite curvature according to the shape of the vessel; to the ribs are attached by bolts or wooden pins the planks that form the outer skin or covering, the interstices between the planks being made water-tight by caulking; internally beams extend from side to side to support the deck or decks.

An iron vessel is lighter than a wooden one of the same size, and with iron the same strength may be obtained with less weight. Iron is also far more manageable than wood, as it can be bent with ease into any required shape.

In steel or iron ships the keel is of far less importance than in wooden ships, and does not as in them hold the position of foundation or 'back-bone' to the whole structure, since an iron vessel ought to be mutually supporting throughout. The keel is constructed of plates riveted together, sometimes hollow, and riveted to it on either side, rise the ribs, which are girders built up of plates, and to the ribs on the outside is fastened the plating. The plating consists of sheets of iron-plate overlapping each other at the edges, where they are riveted together. There may be an inner skin of plating as well as an outer. The ribs are tied together and at the same time held apart by beams of iron, which support the deck or decks. The decks consist of wooden planking with thin metal plates below. In the finer class of ships there are watertight partitions or bulkheads of iron stretching across the vessel from side to side and from keel to deck, with water-tight doors in them, so that if in case of accident water get. into one of them the rest may keep the ship afloat. Sometimes there is also a bulkhead extending lengthwise.