Plymouth Sound, Devon, UK
John Rennie snr
Sir John Rennie
12th August 1812 - 1850
ICE reference number
photo Eleanor Knowles
Plymouth Breakwater remains one of the largest freestanding marine structures in Britain. It was built on rocky shoals at the entrance to Plymouth Sound, 4km from Plymouth Hoe, to protect the city's harbour from south westerly gales.
The breakwater is a rubble cored offshore embankment of limestone blocks weighing up to 7 tons each. It has an overall length of 1,554.5m and width of 13.7m and its top surface is 6.1m above low water. The crest and part of the slope above the low water mark are paved with limestone. Unusually, there is no protective wall above the crest.
There is a lighthouse of white Cornish granite at the west end, 23.8m above the top of the breakwater and first lit in June 1844, and a beacon at the east end built in 1845. The lighthouse and beacon sit on arms of equal length set at 15 degree angles from the central 914.4m long section, pointing landwards.
Engineer John Rennie (1761-1821) was asked to advise on the feasibility of constructing a breakwater across the Sound. He reported in April 1806, assisted by Joseph Whidbey (1757-1833), Master-Attendant at Woolwich Dockyard; and Samuel Hemmans (1745-1819), Master-Attendant of Plymouth Dockyard. The Admiralty instructed Rennie to prepare a detailed design in summer 1811.
Limestone was supplied from a quarry on 10ha of land at Oreston, Plymouth, purchased from the Duke of Bedford for £10,000. The first block was dropped onto the seabed from a sailing barge on 12th August 1812. By spring 1813 the tops of some stone blocks were visible at low water spring tides.
The laborious process of quarrying the limestone with hand tools was speeded up by Richard Trevithick (1771-1833), who invented a mechanical device powered by a steam engine. It used rotary bits to bore holes through the rock and was completed in March 1813. The cost of stone blocks was 2s 9d (almost 14p) per ton in 1812 and after Trevithick's machine was in use this fell to 1s (5p) per ton.
After Rennie's death, the work was taken over by his sons George Rennie (1791-1866) and Sir John Rennie (1794-1874), who added dovetailed granite blocks at low water level for strength and altered the seaward slope from 1 in 3 to 1 in 5. The landward slope is 1 in 2. The breakwater cost in excess of £1.5m to build and contains some 4.5 million tons of stone.
The breakwater has proved long lasting and effective, although additional material was required to make up for consolidation and wave action. Concrete wave breakers were installed in 1871 and 1928. More recently, concrete blocks of 100 tons have been placed on the forward slope to give additional protection.
Resident engineer: Joseph Whidbey (to 1830), William Stuart (from 1830)
Built by: Plymouth Breakwater & Screw Propeller Co
Main contractor: Fox, Williams & Co
Rock transportation by sea: Billings
Rock boring machine: Richard Trevithick
Lighthouse (22nd February 1841 - 9th November 1843): Walker and Burgess
"Richard Trevithick: Giant of Steam" by Anthony Burton
Aurum Press Ltd, London, 2000
"Richard Trevithick: the engineer and the man" by H.W. Dickinson and
Arthur Titley, Cambridge University Press, London, 1934
"Life of Richard Trevithick, with an account of his inventions"
by Francis Trevithick, E. & F.N. Spon, London, 1872
"Account of the Original Construction and Present State
of the Plymouth Breakwater" by William Stuart, in ICE Proceedings, Vol.1, pp.160-162, January 1841