Between 1937 and 1942 Swan, Hunter & Wigham Richardson's Wallsend shipyard built four twin-screw motor ships for Port Line.

The first was the refrigerated cargo ship Port Jackson, launched in November 1936 and completed in January 1937.

[5] She had a pair of five-cylinder, single-acting, two-stroke diesel engines,[5] built by William Doxford & Sons of Sunderland,[2] which between them developed a total of 7,500 bhp[6] and gave her a speed of 16 knots (30 km/h).

Port Napier's holds were adapted to carry 600 mines,[7] and chutes were installed in her stern to lay them.

A pair of narrow-gauge railways was laid inside the ship, one on each side, to carry each mines on a trolley from her holds to her stern chutes.

By mid-August she had joined the 1st Minelaying Squadron at Kyle of Lochalsh (Port ZA), along with four other auxiliary minesweepers, and an escort of Royal Navy destroyers.

[6] On 17 August 1940 the Squadron including Port Napier left Kyle of Lochalsh on minelaying mission SN 12.

On 11 September 1940 the Squadron including Port Napier left Kyle of Lochalsh on minelaying mission SN 41.

[9] On the evening of 26 November a gale blew, causing Port Napier to drag her anchor.

Her anchor chain fouled that of a collier, and the two ships drifted until they ran aground on the shore of the Isle of Skye.

For safety, all ships, including the collier, left port, and local residents were evacuated.

[10] The remains of Port Napier's hull are on the south side of Loch Alsh at position 57°15′57.24″N 5°41′15.03″W / 57.2659000°N 5.6875083°W / 57.2659000; -5.6875083.