After the initial success of its first liner, SS Great Western of 1838, the Great Western Steamship Company collected materials for a sister ship, tentatively named City of New York. The same engineering team that had collaborated so successfully on Great Western—Isambard Brunel, Thomas Guppy, Christopher Claxton and William Patterson—was again assembled. This time however, Brunel, whose reputation was at its height, came to assert overall control over design of the ship—a state of affairs that would have far-reaching consequences for the company. Construction was carried out in a specially adapted dry dock in Bristol, England.
Adoption of iron hull
Two chance encounters were to profoundly affect the design of Great Britain. In late 1838, John Laird's 213-foot (65 m) English Channel packet ship Rainbow—the largest iron-hulled ship then in service—made a stop at Bristol. Brunel despatched his associates Christopher Claxton and William Patterson to make a return voyage to Antwerp on Rainbow to assess the utility of the new building material. Both men returned as converts to iron-hulled technology, and Brunel scrapped his plans to build a wooden ship and persuaded the company directors to build an iron-hulled ship.
Great Britain's builders recognised a number of advantages of iron over the traditional wooden hull. Wood was becoming more expensive, while iron was getting cheaper. Iron hulls were not subject to dry rot or woodworm, and they were also lighter in weight and less bulky. The chief advantage of the iron hull was its much greater structural strength. The practical limit on the length of a wooden-hulled ship is about 300 feet, after which hogging—the flexing of the hull as waves pass beneath it—becomes too great. Iron hulls are far less subject to hogging, so that the potential size of an iron-hulled ship is much greater. The ship's designers, led by Brunel, were initially cautious in the adaptation of their plans to iron hulled-technology. With each successive draft however, the ship grew ever larger and bolder in conception. By the fifth draft, the vessel had grown to 3,400 tons, over 1,000 tons larger than any ship then in existence.
Adoption of screw propulsion
Replica of Great Britain'
s original six-bladed propeller on the museum ship. This propeller proved totally unsatisfactory in service and was quickly replaced with a four-bladed model.
In early 1840, a second chance encounter occurred, the arrival of the revolutionary SS Archimedes at Bristol, the first screw-propelled steamship, completed only a few months before by F. P. Smith's Propeller Steamship Company. Brunel had been looking into methods of improving the performance of Great Britain's paddlewheels, and took an immediate interest in the new technology. Smith, sensing a prestigious new customer for his own company, agreed to lend Archimedes to Brunel for extended tests. Over several months, Smith and Brunel tested a number of different propellers on Archimedes to find the most efficient design, a four-bladed model submitted by Smith.
Having satisfied himself as to the advantages of screw propulsion, Brunel wrote to the company directors to persuade them to embark on a second major design change, abandoning the paddlewheel engines—already half constructed—for completely new engines suitable for powering a propeller.
Brunel listed the advantages of the screw propeller over the paddlewheel as follows:
- Screw propulsion machinery was lighter in weight, thus improving fuel economy;
- Screw propulsion machinery could be kept lower in the hull, lowering the ship's centre of gravity and making it more stable in heavy seas;
- By taking up less room, propeller engines would allow more cargo to be carried;
- Elimination of bulky paddle-boxes would lessen resistance through the water, and also allow the ship to manoeuvre more easily in confined waterways;
- The depth of a paddlewheel is constantly changing, depending on the ship's cargo and the movement of waves, while a propeller stays fully submerged and at full efficiency at all times;
- Screw propulsion machinery was cheaper.
Brunel's arguments proved persuasive, and in December 1840, the company agreed to adopt the new technology. The decision became a costly one, setting the ship's completion back by nine months.
Reporting on the ship's arrival in New York, in its first issue Scientific American opined, "If there is any thing objectionable in the construction or machinery of this noble ship, it is the mode of propelling her by the screw propeller; and we should not be surprised if it should be, ere long, superseded by paddle wheels at the sides."