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John Rennie, naval architect
c.1843 Born in Stranraer
1851 Living in Stranraer: Thomas Rennie 36, sawyer, Jane Rennie 40, John Rennie 8, Ann Rennie 7, William Rennie 5, Thomas Rennie 2, James Rennie 1
1867 Married Mary Thomlinson in Govan, Lanarkshire
1871 Ship' draughtsman, living in Dumbarton with Mary Rennie 23, Thomas Rennie 2, Isabella Rennie 10 Mo
1875 Presumably this was the John Rennie, naval architect, that John Bourne and Co sent to China.
1875 His son William was born in China
1881 Mary Rennie 33, naval architect's wife, lived in Helensbrugh with Thomas Rennie 12, Isabella Rennie 10, William Rennie 6, Jane Rennie 4, John Rennie 2
1891 John Rennie 48, naval architect, living in Ardrossan with Mary Rennie 43, William Rennie 16, Jane Rennie 14, John Rennie 12
1901 Ship's draughtsman, living in Paisley with Mary Rennie 53, Isabella Rennie 29, Jane Rennie 24, John Rennie 22
1918 Died in Leith
Information posted on Ancestry 
"John Rennie was born at Stranraer on 26 June 1842. He began work as a boy in the Clyde shipyards before entering an apprenticeship as a shipwright with Robert Napier and Sons in their New Yard at Govan. Determined to better himself, he studied naval architecture in the evenings at home. He worked in a succession of yards on Clydeside, becoming Chief Draughtsman for Scott and Linton at Dumbarton in 1869.
John Rennie was given the design for Cutty Sark by Hercules Linton and was responsible for all the calculations connected with her sail area and stability. The sail plan he designed gave her 32,000 square feet of sail, capable of attaining a speed of over 17 knots, equivalent to a 3,000 hp engine.
Following the closure of Scott & Linton’s yard, Rennie worked briefly for Harland and Wolff in Belfast and was then appointed Naval Constructor and Instructor for the Chinese Government at Shanghai. During the eight years he spent there in the 1870s he would frequently have seen Cutty Sark in the harbour.
Returning to Scotland, he worked for the Ardrossan Dry Dock and Shipbuilding Company (sic) and then, for 19 years, with John Fullarton and Company (sic) at Paisley. During his career Rennie also designed slide rules for use in ship-building.
John Rennie married Mary Thomlinson in Partick on 23 August 1867 and they had six children, born between 1868 and 1879. He died in Leith on 20 September 1918 whilst on a visit to his sister.
John maintained his interest in Cutty Sark to the end of his days, writing a long letter to the newspapers about her only a few years before his death."