A Royal Navy submarine, launched over 100 years ago, has been revealed as the last remaining Pembroke Dock-built warship still above water.

HM Submarine J3 is today used as a breakwater off the military base of Swan Island, at Port Philip, in the Australian state of Victoria. Partly dismantled, J3 is in a prohibited area but easily recognisable.

It had been thought that every one of the 260 ships built at Pembroke Dockyard between 1816 and 1922 had either gone to breakers’ yards, been lost at sea, or sunk in battle.

But research by Pembroke Dock Heritage Centre volunteer Andy Bird uncovered a photograph of J3, as she is today, off Swan Island.

Two of the submarine’s bronze propellers are on display outside the Queenscliffe Maritime Museum, near Swan Island.

J3 and her sister submarine J4, also built at Pembroke Dockyard, were among six submarines of the same class transferred to the Royal Australian Navy in 1919, after the First World War.

Only J3 is still above water. Her sister rests on the seabed at Port Philip and is a popular historic diving site.

Maritime historian Ted Goddard, a volunteer researcher at the Heritage Centre, recalled: “J3 (1915) and J4 (1916) were the first of five submarines built at Pembroke Dockyard. The others were L10 (1918), H51(1918) and H52 (1919).

“The J class were large submarines of 1,210 to 1,820 tons designed for greater endurance than previous classes, with long-range wireless to enable them to reconnoitre in enemy waters. They also had triple-shaft diesels to drive them at 19 knots and were capable of operating with surface ships in a scouting role. They were regarded as the cream of the submarine service with elite commanding officers.”

After being commissioned, both submarines joined the 11th Submarine Flotilla at Blyth in Northumberland but saw little action.

In March 1919, they were transferred to the Royal Australian Navy, along with four other submarines of the class. The following month they left Britain, accompanied by HMAS cruisers Sydney and Brisbane, and the depot ship HMAS Platypus, sailing via Gibraltar, Malta, Suez, Aden, Colombo and Singapore.

The voyage was not without incident however, as J3 broke down and had to be towed to Aden and then Colombo for repairs.

The submarines arrived at Sydney on July 15, but apart from local exercises and a 1921 visit to Tasmania, the submarines saw little use. They were paid off into reserve in July 1922 and sold off in 1924.

J3 was partly dismantled and in 1926 took on her new role as a breakwater off Swan Island. Her batteries also found a new use, being used to power an electric trolley at the Naval Mine Depot on the island.

J4 sank at her moorings and was raised before being scuttled in the Bass Straight, off Port Philip Bay in 1927.