If the USA entered the Great War because of the Lusitania, they certainly took some time making their minds up about it! She was torpedoed on 7 May 1915.

She was identified by the Germans as possibly an Armed Merchant Cruiser (she was on a list of possible AMCs published by the Admiralty) but she was never used as such.

She was carrying contraband war materials - rifle ammunition and unfilled shells, but these are unlikely to cause a secondary explosion. The various causes of a second explosion are well covered in Diana Preston's book 'Wilful murder - the sinking of Lusitania' an excellent book on the subject. Dives on the wreck have shown no evidence of this cargo exploding.

Back to Dreadnoughts. The name Dreadnought was one of the names used regularly by the Royal Navy for one of its major units. It became available when the previous bearer of the name was put into reserve in 1905. The next name used for a battleship was Bellerophon, and I'm sure that would have given us a lot more trouble discussing 'Bellerophon Battleships'! Ships with that name were informally known as Billy Ruffian.