1821 On the death of the elder Rennie, the business was divided between his two elder sons, who remained in partnership as regards the works in Holland Street with George managing the principal part of the mechanical business while (Sir) John Rennie was responsible for the completion of the engineering works.
1824 Established G. and J. Rennie with his brother John
1828 April 24th. Married Margaret Anne Jackson (1806-1881)
1829 May 19th. Birth of son John Keith Rennie
1831 Birth of son George Banks Rennie
1832 Rennie worked as a consultant in the construction of the foundations of London's Grosvenor Bridge.
1841 George Rennie, F.R.S., became a member of the Institution of Civil Engineers.
1851 George Rennie 58, civil engineer, lived in Charing Cross, London with Margeret Anne Rennie 44, John Keith Rennie 21, student at Cambridge, George Banks Rennie 20, civil engineer, Margaret Jane Rennie 17
1852 Patent to George Rennie, of Holland-street, Blackfriars, in the county of Surrey, Esquire, in respect of the invention of an improved chain cable.
c.1853 Formation of George Rennie and Sons
1855 Patent to George Rennie, of Holland-street, in the county of Surrey, Engineer, in respect of the invention of "improvements in marine steam-engines.". Patent marine engine: the disc engine.
1855 To George Rennie, of the firm of George Rennie, John Keith Rennie, and George Banks Rennie, of Holland-street, Blackfriars, Engineers, for the invention of "improvements in steam engine boilers as applied to the propulsion of vessels."
1857 of 21 Whitehall Place, Westminster.
1866 March 30th. Died. His son George Banks Rennie continued his work.
1869 Obituary 
George Rennie, the eldest son of the late John Rennie, F.R.S., &c., - so well known by his numerous great engineering works in every part of the globe, - was born in the parish of Christchurch, Surrey, on the 3rd of December, 1791, and received the rudiments of his education at the establishment of the late Dr. Greenlaw, at Isleworth, whence he was sent to St. Paul's school, then under the management of the late well-known Dr. Roberts, where he remained about two or three years.
From early life George Rennie was brought up under his father, with the intention of following the profession of a Civil Engineer, for which he exhibited considerable talent.
In 1807 he accompanied his father in his annual professional tour through England, Ireland, and Scotland, visiting numerous engineering works.
He was then placed at the University of Edinburgh, under the charge of the Rev. Dr. Robertson - a near relative of the distinguished historian - in whose house he boarded for about two years with Professor Dunbar and Dr. Henry, - subsequently the eminent chemist - of Manchester.
At the end of this period he was received into the house of Professor John Playfair, by whose tuition he profited largely. During his stay in Edinburgh he studied mathematics, mechanics, natural philosophy, chemistry, and classics under Professors Leslie, Hope, Christison, Playfair, and Dunbar.
On his return from Edinburgh in the year 1811, he commenced the study of mechanical and civil engineering under his father, whose extensive engagements afforded him excellent opportunities for acquiring a thorough knowledge of the practical part of the profession. From this time he was in his father's office, who was then designing many great works, such as the Waterloo and Southwark bridges, the Lincolnshire drainage, the improvements of Plymouth, Portsmouth, and Chatham dockyards, and the Plymouth, Kinghtown, and Holyhead breakwaters and harbours, &c.
In 1818, Mr. George Rennie was recommended, by Sir Joseph Banks and James Watt, as the successor of Mr. J. Lawson, for the situation of Inspector of Machinery and Clerk of the Irons (dies) at the Royal Mint, which post he held for nearly eight years. The duties of the department were laborious, requiring an intimate knowledge of machinery, as well as of the composition of iron and steel, and of the best mode of constructing dies for coining. Mr. Rennie was very successful, and his efforts gave great satisfaction.
During the time he held the post he acquired that experience of the art of coining, which was afterwards turned to good account, when he was called upon to furnish (in conjunction with Boulton and Watt) the machinery for the Mints of Calcutta and Bombay, as well as for those of Lisbon, Mexico, and Peru.
On the death of Mr. Rennie, in 1821, Mr. George Rennie entered into partnership with his younger brother, the present Sir John Rennie, (Past-President Inst.C.E.,) and together they proceeded to complete the works of their father: among others, London Bridge, which was erected under the superintendence of Sir John Rennie, in consequence of the Government appointment then held by Mr. George Rennie.
One of the most important works which they were called upon to complete was the dockyard at Sheerness, which was at that time regarded as the most systematic establishment of the kind in the kingdom. This was commenced after a plan made in 1813 by their father, and a more extended plan in 1821, the engineering works for the dockyard alone involving an outlay of about two and a half millions sterling. The cast-iron dock-gates which were then a novelty, have stood well to this day, the heel-post being accurately ground into the masonry to insure a tight joint. The success of these gates led many years later to the construction, under Messrs. Rennie's direction, of the ten pairs of large dock-gates at Sebastopol, which are described in a Paper read at the Institution of Civil Engineers.
One of the most remarkable features of this line was the project for carrying the railway over the highest and widest part of Chat Moss, which was considered hazardous at the time, although it had been previously proposed by the late George Stephenson, by whom it was eventually executed, and it has turned out to be the cheapest part of the line. The estimate was £796,246, and the line was completed for £739,165. The gauge proposed by Messrs. Rennie was 5 feet 6 inches; but this, against their opinion, was afterwards changed to 4 feet 8.5 inches, which probably caused all the subsequent confusion, and led to the celebrated 'battle of the gauges.'
About this time (1826) the late well-known architect, Mr. Harrison, made a design for a stone bridge of a single span of 200 feet to cross the Dee, at Chester. The boldness of the proposition induced considerable hesitation as to the undertaking, and the age and infirmities of Mr. Harrison precluding his executing the work, the Corporation of Chester consulted Mr. Rennie as to its practicability, which he confirmed, and he investigated the design thoroughly, equilibrated the arch in the most scientific manner, proposed to carry the foundations of the abutments down to the solid rock on both sides of the river, instead of - as had been originally intended - building one of the abutments upon piles, on the side where the rock was at a greater depth. The design, thus remodelled, was considered practicable; but nothing was done at the time for want of funds, the cost being estimated at £54,000. Eventually, the late Jesse Hartley, of Liverpool, was instructed to carry out the work, which he consented to do "on the condition that no alteration should be made from Mr. Harrison's external design, but that the interior and all practical points should be left entirely to him." He gladly availed himself of the scientific investigations of Mr. Rennie, and with the aid of the practical skill of the late Mr. Trubshaw, as the contractor, the present Grosvenor Bridge was built.
About the year 1836-37 Mr. Rennie, in conjunction with Messrs. Chapman and Jessop, laid out a line of railway between Birmingham and Liverpool, to cross the Mersey at Runcorn, with a magnificent viaduct. This was to have been connected with another direct line, by Sir John Rennie, between London and Birmingham, passing by the Valley of the Colne, Aylesbury, Bicester, Banbury, and Warwick, which was asserted by the promoters to be more level, easier of execution, and shorter than the present line.
Mr. Rennie also laid out several other railways, including a line to connect the east and west parts of London, the Vale of Clwyd line, the Mons and Manege line, &c.
In 1846 he was appointed Engineer-in-chief to the Namur and Liege railway, for which he designed three elegant stone bridges, the largest consisting of five arches of 50 feet span each, for crossing the Meuse, at Val St. Lambert.
During this period, and subsequently, at the mechanical engineering establishment of the Messrs. Rennie, in Holland Sheet, Blackfriars, important works were constructed, of a novel and difficult character, and at a time when the numerous appliances of tools and other arrangements for facilitating works in iron at present in use were comparatively unknown.
In the construction of the two shields for the Thames Tunnel, one of the earliest, if not the first, machine on a large scale for planing iron was used, for the purpose of fitting together the different parts, and this machine is still to be seen in Messrs. Rennie's factory.
Under the direction of Messrs. Rennie, the first machinery ever used for making biscuits was constructed for the Government establishments at Weevil, near Gosport. The idea of making biscuits by machinery is due to the late Sir Thomas Grant, and his views were carried into practical effect by Messrs. Rennie and their assistant, Gideon Scott.
The corn and chocolate mills at the Deptford Royal Victualling Yard, as well a more magnificent establishment called the Royal William Victualling Yard, at Cremil Point, near Plymouth, were also due to the skill of the Messrs. Rennie; the latter being completed about the year 1833.
Among other works may be enumerated the armoury at Constantinople, capable of making five hundred muskets a day for the Turkish Government; machinery for the arsenals of France, at Chatellerault, Toulon, and Rochefort; machinery for the Russian Government for producing biscuits and for making blocks at Nicolaieff and at Sebastopol, as also for coining at St. Petersburgh; besides engines for several large vessels of war propelled by the screw, including the "Smale" frigate, of 400 horse-power, the celebrated "Wladimir", of 400 horse-power, which was sunk at Sebastopol during the siege, the "Peterhof" and the "Alexandrina," yachts of the late Emperor Nicholas, and other steamers on the Black and Caspian seas; likewise dredging machines for Cronstadt, Odessa, and the Danube.
Messrs. Rennie also designed the great steam factory at Cronstadt, and one, on a reduced scale, at Astrakhan.
Messrs. Rennie constructed many marine engines for her Majesty’s navy, such as the Samson, Bulldog, Vulcan, Megaera, Reynard, Cruizer, Oberon, &C., and for her Majesty’s yacht the Elfin.
About the year 1836 Mr. Rennie took much interest in the propulsion of vessels by means of the screw. That propeller in various forms had been proposed and partially tried by Duguet in 1727, Painton in 1768, Littleton in 1794, Shorter in 1802, and also by Napier, Tredgold, and Brown about 1825.
In 1836 Mr. Francis Pettit Smith (Assoc. Inst. C.E.) fixed a screw propeller in the dead wood of the stern of a small open boat, and worked it by a high-pressure engine, making several successful trials with it on the Thames and to Ramsgate. Mr. Rennie and his brother, who saw it, were satisfied that it must eventually succeed on a large scale. A company was accordingly formed to carry it into effect. "The Archimedes", a wooden vessel of 232 tons, was built expressly for the purpose by Mr. Wilmshurst; and Messrs. Rennie designed and constructed the engines, of 80 horse-power, with the machinery and the propeller.
Notwithstanding the predictions of its failure by some of the best mechanical engineers of the day, who have since adopted it, the Archimedes succeeded perfectly, and attained a speed of 9 miles an hour.
Messrs. Rennie subsequently, in 1840, constructed for the Admiralty an iron vessel, called the "Dwarf", of 210 tons, and fitted it with a pair of engines of 120 horse-power, with the requisite machinery and a screw propeller. This vessel, when completed and tried, attained a speed of .. miles an hour, being 4 miles an hour in excess of the paddle steamers then in use in H.M.’s navy.
This was the first vessel propelled by a screw introduced into the British navy, and it produced a complete revolution in the military and commercial navies of the whole world.
In 1852 Mr. Rennie also took up the subject of double or twin screw propellers, which had been previously proposed by Captain Carpenter, - a system now much in favour for light draft gunboats and for war purposes generally. Thus the first Mr. Rennie introduced the paddle steamboat into the navy in the year 1819, and his sons introduced the screw propeller into the navy in 1840, twenty-one years afterwards.
Having naturally a philosophical turn of mind, Mr. Rennie devoted his spare time to scientific pursuits, in making experiments and communicating the results to various scientific bodies. He was elected a Fellow of the Royal Society in 1822, was for several years one of the Vice-President’s, and succeeded the late Sir John Lubbock as Treasurer, an office which he retained until the year 1850.
In 1825 and 1826 he made a number of experiments on the friction of metals and other substances, the results of which were embodied in three Papers communicated to the Royal Society in 1828, and were published in the Philosophical Transactions. These investigations still hold a place for accuracy and practical value, and at the time may be said to have been quite new, as they preceded by three years similar experiments made by General Morin, who, assisted by the French Government, pursued the subject further.
Mr. Rennie was afterwards appointed one of the Royal Commissioners for investigating the strength of iron. In 1834 he wrote a valuable Paper for the British Association, 'On the History, Principles, and Practice of Hydrostatics, Hydraulics, and Hydrodynamics,' which, although now rather antiquated, is still considered a standard work.
In 1840 he presented to The Institution of Civil Engineers an essay on 'The Expansion of Arches,' and in l855 a 'Description of the Bridge Aqueduct of Roque Favour, on the line of the Canal of Marseilles,' as well as a 'Description of the Pont du Gard.
In 1856 he communicated a Paper to the British Association, 'On the Quantity of Heat developed by Water when rapidly agitated,' also another 'On the Resistances of Screw Propellers when revolving in Water at different Velocities.' In the following year his Paper 'On the Employment of Rubble Beton or Concrete in Works of Engineering and Architecture' was read at the Institution, and was rewarded with a Telford Medal. He had previously written 'On London, Metropolitan, and other Bridges,' for Cooke’s 'London Bridges,' and was the Author of several articles on dredging, mechanics, waterwheels, &C., in Weale’s 'Rudimentary Treatises,' and in Weale’s new edition of Tredgold. He also brought out a new edition in 1841, of 'Buchanan’s Millwork and Machinery,' with all the latest improvements, to which an extensive supplement was added. He was likewise the Author of many miscellaneous Reports on Civil Engineering subjects.
In the year 1865, on his way home from the works in Holland Street, Mr. Rennie met with an accident from which he never recovered. He died on the 30th of March (Good Friday), 1866, in the seventy-fifth year of his age; and was buried, on the 5th of April, in the churchyard of the quiet country church of Holmwood Common, near Dorking, Surrey. He was married, in 1828, to Margaret Anne, daughter of the late Sir John Jackson, Bart., M.P., who survived him, and by whom he left issue two sons and one daughter.
Mr. Rennie was elected a Member of The Institution of Civil Engineers on the 4th of May, 1841, and served on the Council in the following year, and up to the period of his accident was a constant attendant at the meetings, taking part in the discussions and evincing great interest in the progress of the Society. He was also a Fellow of the Royal Irish Academy, as well as of the Academies of Turin and Rotterdam.
Essentially a man of science, both theoretical and practical, he would probably have attained the highest eminence had his attention been devoted exclusively to purely scientific pursuits. Combining great personal worth, simplicity of manner, and scientific attainments, it was no wonder that he possessed the regard and the esteem of all who knew him, and that, while professional intercourse alone was certain to produce lasting friendships, the advantage of a closer intimacy invariably secured unfeigned pleasure.
Sources of Information
-  Wikipedia
- Leaders of Modern Industry by G. Barnett Smith: The Rennies
- Biography, ODNB