The Olympic-class ships’ aesthetic perfection–slim grace rather than mere ponderous bulk–was evenly matched by their technical sophistication, and the most remarkable and highly touted feature of their design was their watertight construction. Above the keel there was a conventional double bottom, seven feet deep, which ended at the turn of the bilge, but the hull itself was designed to incorporate a very carefully thought-out arrangement of watertight partitions. Rather than being built with the usual one or two “collision bulkheads” in the bow, the hull was divided into sixteen watertight compartments of roughly equal length, formed by fifteen watertight bulkheads built laterally across the ship.
The arrangement of these bulkheads was far from arbitrary: several ships had been lost in the past half century to collisions with other vessels, most recently the White Star Line’s own Republic in 1909, which had been moving slowly through a fogbank off the coast of Massechusetts when she was rammed broadside by the Italian liner Florida. The collision opened up two adjacent watertight compartments aboard the Republic, and the flooding was uncontrollable: when those compartments filled, they caused the Republic to settle deep enough that the water level rose above the top of the watertight bulkheads and began flooding the rest of the ship; the Olympic-class liners were designed to avoid a similar fate. These new ships were capable of floating with any two of their sixteen watertight compartments flooded, since a collision with another ship couldn’t do worse than open up more than two adjacent compartments. In fact, they could float with any three compartments flooded, and under certain circumstances even float with four compartments open to the sea.
After calculating how having two adjacent compartments flooded would affect the ships’ trim, the designers determined that the first two and last five bulkheads need only go as high as D Deck, while the middle eight carried up only to E Deck, which at midships was barely fifteen feet above the waterline. The designers’ research showed that even if one of the vessels were struck amidships and two compartments flooded, the weight of the seawater in the open compartments would be insufficient to pull the ship deep enough that the water would begin to overflow the top of the bulkheads into adjacent compartments.
Connecting these sixteen compartments were a series of immense watertight doors. Normally left open during the ship’s operations, they could be rapidly closed by any of three different methods. There was a master switch on the bridge that closed most of the doors automatically, including all the doors on the bottom deck, or they could be closed individually by tripping a manual switch in each compartment, and there was a float-triggered mechanism that automatically closed a door if there was six inches or more of water on the deck of the compartment.
So comprehensive were these watertight arrangements that in a commemorative issue of the prestigious British journal Shipbuilder, published on the occasion of the Olympic‘s launch, the authors of the piece labeled the ships “practically unsinkable.” Before long, and perhaps inevitably, the qualifying adjective was forgotten by the general public, and the ships–and in particular the Titanic–came to be widely accepted as being “unsinkable.”