The training ship Boscawen first arrived in Portland Roads in February 1862. She was a three decked sailing ship of the line, launched in 1844 but reduced to a hulk in 1862 and converted into a training ship for boy seamen. HMS Britannia had been the first training ship at Portland but the conditions in the anchorage had not suited her and she moved to Dartmouth where she became the forerunner of the Royal Naval College.

The original Boscawen at Portland left in 1873 and was replaced by HMS Trafalgar, which took on the name Boscawen and remained at Portland until she was sold in 1906. As the Royal Navy grew in size towards the end of the 19th century, so additional accommodation was required for boys' training. To meet this demand, two old broadside ironclad warships were brought into service, first Minotaur in 1898 to be followed by Agincourt in 1904. They were known as Boscawen II and Boscawen III respectively.

The design of warships changed dramatically in the last half of the nineteenth century with iron and steel taking the place of the 'wooden walls' and armour plate and with heavier breech loading guns in turrets taking the place of cannons. Much of the training of the boys, however, still reflected life under sail and activities ashore and on board prepared them for life at sea. They were required to set up topmast rigging and cross upper yards, and they learned to scrub and wash hammocks and to make and mend clothes. They also cleaned boats, took lead line instruction, engaged in physical drill, including dumb bell exercises and gymnastics, landed field artillery, learned rifle drill and, at the turn of the century, went ashore to receive machine gun and ammunition instruction. They coaled and painted ship, and formed a fire brigade which could be called ashore in an emergency. For this the 520 boys in 1872 received 3d per week pocket money. Supplies of food, regularly taken aboard, included fresh vegetables, beef, 'mouton', suet, corned pork and bread. Fresh water was also supplied. On Sundays the boys attended divine service. During the summer they were given leave and this provided an opportunity for dockyard men to join the ship to make good defects.

On some occasions the boys were prevented from going ashore by bad weather, and there were times when the ship was quarantined due to scarlet fever and other infectious diseases. Conditions were harsh and punishment could be severe. In 1866, for example, two boys each received 24 ‘cuts’ of the birch.

The Boscawen training ships left Portland in 1905 and the name lapsed until 1932, when the naval base at Portland was commissioned. This shore base, or 'stone frigate' was called HMS Boscawen later to become, with the advent of the helicopter, HMS Osprey. Fittingly the name Boscawen reverted to an organisation training young sailors, The Sea Cadets.