Design

Cunard established a committee for the design of the future Lusitania and Mauretania. The committee would be chaired by James Bain, Cunard’s Marine Superintendent. Other members included Rear-Admiral H. J. Oram, who had been involved in designing turbine-powered ships for the British Navy, and Charles Parsons, whose company Parsons Marine produced the technology of turbine engines that was fast replacing the dominating reciprocating engines.

Parsons stated that he could design engines that could maintain constant a speed of 25 knots and meet Admiralty requests on speed. Such powerful engines, however, would require 68,000 brake horse power. These would be turbine engines of unprecedented size, as the largest turbine engines built thus far had been of 41,000 bhp for the Invincible-class battle cruisers. Such an ambitious engine design remained a gamble.

Turbines had the advantages of less vibration than reciprocating engines, greater reliability at high speeds, and more efficient fuel consumption. To test out the suitability of large turbine engines for the new Cunard sisters, Cunard agreed to install turbines to Carmania, which then under construction, as a trial. Proving Parsons correct, Carmania was more comfortable, more efficient, and 1.5 knots faster than her reciprocating-engine-powered sister Caronia.

Cunard’s naval architect, Leonard Peskett, was called up to design the new twin Cunarders. A 1902 model shows the future Lusitania as a three-funnel ship, but a fourth funnel was added in 1904 to provide exhaust for additional boilers. The Cunard twins would have 23 double-ended boilers and two single-ended boilers, containing 192 individual furnaces operating at a maximum pressure of 195 pounds per square inch (psi).

The design for Lusitania and Mauretania was also changed from three propellers to four to provide additional power, each driven by a separate turbine engine. The two inner propellers were connected to reversible turbines. To improve efficiency, the innermost two propellers rotated inwards, and the outer propellers rotated outwards.

The outer turbines operated at high pressure, with the exhaust steam recycled into the inner low-pressure turbines. Since robust gearboxes for turbines was not invented by Parsons until 1916, the turbines drove the propellers directly, and had to be run at slower than optimum efficient speeds. As such, at low speeds turbines were less efficient than conventionally-powered engines, but were more efficient at high speeds.

Hull design experiments were underway in the Admiralty experimental tank in Haslar, Portsmouth.  The original design of a 78-foot beam was deemed unstable for a ship of Lusitania and Mauretania‘s height and size, thus the final design increased their beams by 10 feet.  The ships would also use a balanced rudder, where a portion of the rudder’s surface area would be forward of the fulcrum to aid turning efficiency.

Per Admiralty stipulations, all machinery for the two new ships would be below the waterline, where conventional wisdom dictated that they would be safe from gunfire.  Engines were placed in the rear of the ship, boilers were placed amidships, and space for the cargo, which could be converted to a magazine if needed, was in the front.

Lusitania and Mauretania had double bottoms and were divided into 12 transverse watertight compartments.  Any two could be flooded without causing the ship to sink.  Thirty-five watertight doors were operated hydraulically.  The rest were to be closed by hand.

As watertight compartments cannot be too large or else become ineffective, the size of the boiler rooms meant that, with the exception of the the forwardmost coal bunker, coal bunkers had to be placed along the side of the hull.  These bunkers would then double as longitudinal watertight compartments, but had the additional complication of being difficult to close when coal scraps were in the door.

Longitudinal bulkheads were also thought to better protect machinery from shelling by enemy ships, but as history would later prove, would not be sufficient to protect against torpedo attack. Furthermore, since longitudinal bulkheads concentrate water on one side of the ship, in case the ship were to be damaged, water would concentrate on the damaged side, causing the ship to list.  With enough longitudinal compartments compromised, lifeboats on the high side would be impossible to launch, and the ship would roll over from too much concentrated weight.

Of course, as Lusitania and Mauretania were unprecedented in their size for their day, these judgments can only be made in hindsight.

The size of the twin Cunarders also meant that these ships were exceedingly tall.  They had six decks of passenger accommodation, whereas previous ships had limited themselves to four.

For the ship’s plating, high-tensile steel was used instead of conventional mild steel.  High-tensile steel reduced weight and increased hull strength.  Over 4 million rivets in triple rows held together the steel plates that made up the hull.

The ventilation system of Lusitania was separated into 49 distinct but interconnected units where parts could be switched out for maintenance.  Electric fans provided seven complete air changes per hour.  Steam powered a thermo-tank ventilation system that kept the temperatures around 65 °F (36.1 °C) and kept a constant humidity.  Galleys and bathrooms had their own systems.

At the time of her completion, Lusitania complied with Board of Trade safety regulations, which required only 16 lifeboats for ships over 10,000 gross registered tons.  Lusitania was over three times this size.

Lusitania was the largest ship in the world at her launch.  She would shortly be eclipsed by her slightly larger sister, Mauretania, a few months later.  At over 30,000 gross registered tons, the Cunard sisters were 6,000 gross registered tons larger than their closest rival in luxury, Hamburg America’s Kaiserin Auguste Victoria, and had an average speed 1.5 knots faster than the fastest German ship, Norddeutscher Lloyd’s Kaiser Wilhelm II.