William Postlethwaite was born on 10th April 1891 at Higher Bebington, Cheshire. In the 1901 census he was living in the Wirral Union Workhouse in Clatterbridge with his two brothers.

William joined the Merchant Navy in 1903 when he was 12 years old. He sailed with the Blue Funnel Line, which ran steam ships to the Far East out of Liverpool. By 1917 he was a Master Mariner, sailing the world. His “Certificate of Competency as Master for Foreign-Going Steamships only” was issued by order of the Board of Trade on 24th February 1917.

Around 1927 William had to leave the Merchant Navy due to ill-health and was classified 'C3' disabled. He and his wife Gertrude moved to Hurst in Berkshire, and bought and ran the Davis Street Stores. When World War 2 broke out he rejoined the Blue Funnel Line on 12th September 1939 as a Radio Officer.

On May 10th 1940, he had an interview with the Ministry of Shipping and was accepted immediately as a Transport Officer in the Royal Naval Reserve (RNR), with the rank of temporary Lieutenant. He was 49 years old and due to his disability would not have been required to serve, had he not volunteered.

By 12th May William had returned to Liverpool to get release from the Blue Funnel Line, to collect his luggage, and to be measured for his uniform. He arrived home on 16th May from Liverpool, to await instructions. On 21st May he left for Southampton and was subsequently transferred to London, and then Newport. On 13th September, he went to Edinburgh for 3 weeks intensive training for minesweeping. It is believed that he volunteered for the minesweeping course so that he could become more actively engaged.

On 5th October William returned home from Edinburgh, having passed the exam for Group Commander. He was to proceed to Plymouth on 14th October, to take command of H.M.T. Rinovia on instructions from the Admiralty. Apparently trawlers engaged in minesweeping were captained by a Lieutenant who was known as 'skipper' and not 'Sir'.

On 2nd November 1940 H.M.T. Rinovia struck an enemy mine off Falmouth in Cornwall and sank in 48 seconds. Fourteen men lost their lives and 9 were saved. William was among the survivors that were picked up and taken to hospital in Falmouth. He arrived home on 6th November for 14 days survivors leave.

On 21st December he returned to Plymouth to join H.M.T. Tranquil. HMT Tranquil was a fishing trawler from Hull, originally called Good Luck. She had seen service in the First World War and was requisitioned again on 28th April 1940 as a minesweeper.

On 27th December he transferred temporarily to H.M.T. Fir while H.M.T. Tranquil was under repairs. From 1st January 1941 William was back in command of H.M.T. Tranquil for the next eighteen months. On 14th June 1942 she left Tilbury Dock after further repairs, and sank on 16th June 1942 after a collision off Deal, Kent. Position: 0.5m SE of Goodwin Fork Buoy 51.13.08N by 01.27.51E. H.M.T. Tranquil had hit another ship in the English Channel and rolled over onto its lifeboats. The few survivors were not in the lifeboats. The collision occurred at 2:35am in darkness. William Postlethwaite was reported missing and subsequently named amongst the dead.

One survivor of the Tranquil, Raymond Cobb, recalls that when he looked back from the water he could see "the old man" (William Postlewaite) standing in the wheel-house when she sank.

The wreck was known as the "Balloon Wreck" because she was towing a barrage balloon when she sank and all that could be seen rising from the 17m of water in which she sank was the cable with the balloon still attached.

William Postlethwaite is also commemorated on a memorial at Lowestoft which reads:

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