On May 1, 1915, the RMS Lusitania departed from Pier 54 in New York City on it's final voyage, amid concern by crew and passengers about sailing into a war zone.
During World War I, submarine warfare had developed to a dangerous level. German U-Boats would often raid commerce - either attacking from underwater without warning, or surfacing to sink their victim with their deck gun. The submarine was now a game-changing weapon in naval warfare. Life in these cramped, primitive sea-going submarines was very dangerous.
The luxurious Lusitania, on the other hand, was a 31,550 ton passenger liner capable of steaming at up to 28 knots (~32 MPH). Capable of carrying just over 3,000 passengers and crew, she had made over 200 transatlantic voyages. Before the final voyage, some precautions against attack were taken such as painting the funnels dark gray to help disguise the ship against the night sky. The ship was so well known, however, that no attempt was made to disguise her name or profile. Regrettably, during wartime, the Cunard company that owned the ship shut down one of the boilers, reducing speed to about 22 knots.
Shortly before setting sail on May 1st, the German Embassy in Washington, D.C. published a warning in several American newspapers warning against booking passage on the trip. In retrospect this seems particularly prescient, and even at the time it produced some consternation. The warning was intended to minimize any international incident that would certainly result if Lusitania was attacked.
On May 7th, 1915, as the Lusitania approached the southern tip of Ireland, she passed within the sights of U-20, commanded by Kapitänleutnant Walther Schwieger.
As the Lusitania changed course toward the U-20, Schweiger launched a single torpedo set to a depth of 9.8 feet. The torpedo, likely a pre-WWI design - either a G6 or G7 model, could travel between 27 and 37 knots. It would close the distance in up to 50 seconds.
The torpedo struck the Lusitania on the starboard side of this ship, just aft of the bridge. The explosion was powerful enough to knock one of the lifeboats off its davits. Moments later, a second explosion followed - fueling beliefs that a second torpedo was fired.
The second explosion caused even more damage, and resulted in the ship heeling over to starboard and filling with water rapidly.
The second explosion was likely caused by a boiler explosion; exposure of a running boiler to cold North Atlantic water will result in an implosion of the boiler, which can be quite severe. Unlike modern oil powered vessels, there was not a lot of volatile material to explode on a coal fueled ship in the early 1900s.
The unlikely involvement of a second torpedo has been considered over the many intervening years. Given that U-20 was on the verge of returning to port, being low on supplies, it would have been very wasteful to fire more than one torpedo. With only three torpedoes left, it is unlikely Kapitänleutnant Schwieger would have expended two thirds of his armament on an unarmed merchant vessel, especially a passenger liner.
Two minutes after the torpedo was fired, Lusitania Captain Thomas Turner ordered the ship turned toward land, which was ten miles away. She massive ship refused to answer her helm. Captain Turner ordered full reverse to halt the ship, and possibly help her stay afloat longer (she was down by the starboard bow), but there what little steam pressure remained was rapidly dissipating. The ship was finished.
The wireless operator sent out the SOS and location in order to get rescue organized shortly before electrical power went out throughout the ship. The wireless continued on batteries while the evacuation of the ship began. Much panic and disorder marked the abandoning of the Lusitania, and only 764 people survived of the 1,959 passengers and crew. Many perished in the chilly North Atlantic. The ship itself sank by the bow, the stern and propellers being exposed for a short time, in about 18 minutes.
After the sinking, an international outcry against Germany's submarine warfare was brought out, and Germany defended their attack claiming that war material was aboard the ship, while also contending that the Lusitania was armed with mounted guns to defend herself against attack. All this was vehemently disputed, with officials noting that the ship was carrying only standard ammunition in cargo for small arms, and had done so for years. They also objected strongly to the claim the ship was sailing with mounted guns.
Actually, some merchant vessels were arming themselves against the submarine threat. Very often the submarine would surface in order to check the manifests on the ship to determine if the ship was a valid target. The crew would be allowed to leave before the ship would be sunk, usually by the submarine's deck gun. The British Admiralty encouraged the practice by merchant ships of fighting back, and even ramming the submarine if possible. A ramming attack was only once made, successfully, by the RMS Olympic, sister ship to the Titanic. The propeller of the Olympic sliced through the pressure hull of the U-boat, which was evacuated and then sank.
The aftermath of the sinking was shrouded in a shifty business, with questionable testimony from crew that indicated there were definitely two torpedoes, etc. To this day no firm account of the cargo exists. Could there have been an explosive cargo that helped doom the ship? Could the Captain and others have done anything better to assure the survival of the passengers, or even of the ship itself? Difficult to say.