The offshore breakwater across Plymouth Sound is John Rennie’s most southerly British work, and strategically one of the most important. Rennie’s structures were built to last and this one, considered to be among his finest works, has been protecting the city’s harbour for two centuries. At 1.55km — almost a mile long — it’s one of Britain’s largest freestanding marine structures.

But why was it built at all?

Plymouth was an ideal location on the south coast for a major military and naval base. However, although the junction of the Rivers Plym and Tamar formed a natural harbour, it was exposed to the full force of gales from the south. Without substantial structural intervention to mitigate the effects of wind and waves, it didn’t provide a safe haven for shipping during southerly or south westerly squalls. Even so, the harbour had been a Royal Navy stronghold from December 1690 onwards — with increasing port activity accompanied by rising numbers of shipwrecks as vessels were driven ashore. On a single day in 1804, ten ships foundered in the Cattewater. Something had to be done.

Ongoing hostilities between Britain and France underlined Plymouth’s defensive importance. By 1806, when Rennie was invited to consider the feasibility of constructing a breakwater across the sound, the first and second coalition wars (against the constitutional kingdom of France and subsequent republic) were over and the Napoleonic wars underway.

Rennie had been tackling harbour construction and improvement schemes since the end of the 18th century; his civil engineering expertise, and reputation for thoroughness and attention to detail, were well established. For the Plymouth scheme, he collaborated with Joseph Whidbey (1755-1833), then Master-Attendant at Woolwich Dockyard, who was known for accurately charting complex approaches. He also consulted Samuel Hemmans (1745-1819), then Second Master-Attendant at Chatham Dockyard, formerly Second Master-Attendant at Plymouth Dockyard in 1790-1799.

Rennie and Whidbey favoured a standalone barrier, though they were not alone in pondering the problem. Other suggestions included partly enclosing Cawsand Bay with a 950m pier from Penlee Point, constructing a 2.3km breakwater from Staddon Point to Panther Rock, and building a causeway from Mount Edgecumbe to Drake’s Island with a cutting through Devil’s Point into the Hamoaze. Such schemes were likely to cause silting and were abandoned.

Rennie’s rubble cored embankment of limestone blocks is founded on the shoals of the Panther, Shovel and St Carlos Rocks, 4km south of Plymouth Hoe. Its 914m central section, running west north west to east south east, is flanked by 320m arms angled shoreward at 15 degrees to the main line. According to Whidbey, the design would “check the wild inrush of waves, and at the same time, by restricting the entrances to the Sound, increase the scouring effects of the currents and thus prevent silting.”

In April 1806, Rennie reported to the Admiralty. In June 1811, he was appointed chief engineer and Whidbey resident engineer with William Stuart (1773-1854) assisting. On 12th August 1812, work began when the first 7 tonne block was sunk at Shovel Rock. The project was so important, it was known as The Great National Undertaking.

Land purchased from the Duke of Bedford at Oreston supplied the limestone for the breakwater. The quarry’s productivity increased after March 1813, using a steam-powered rock drill invented by Richard Trevithick (1771-1833). Horses pulled wagon trains from the quarry to wharfs on the Plym Estuary and barges transported the stone to site. Blocks were tipped into place, and smaller stones and rubble rammed into the gaps.

In the original design, the top of the breakwater was to be 9.1m wide and 3m above low water. The seaward face was a 1 in 3 slope and the landward face 2 in 3. Rennie thought the seaward slope should be 1 in 5, but Whidbey and the Admiralty favoured the less expensive steeper gradient.

During March 1813, the structure reached 1.5m below low water. A lightship was moored at the west end of the works and a jetty constructed on the landward side. In 1814, the breakwater was high enough to for ships to anchor safely in the sound. Around 1816, the top height was increased to 6.1m above low water springs. Storm damage in January 1817 reduced the seaward slope to 1 in 5, subsequently reinstated to 1 in 3.

Rennie died in October 1821, leaving Whidbey in charge. In November 1824, an exceptionally high storm tide destroyed 728m of breakwater and again reduced the seaward slope to 1 in 5. The Admiralty consulted Rennie’s son, also John Rennie (1794-1874, knighted 1831), who recommended flatter slopes of 1 in 5 seaward and 1 in 2 landward. The top width was increased to 13.7m and the top, seaward face and ends of the breakwater were paved with dovetailed granite blocks to low water and the landward slope paved with rubble. The seaward face was protected with a 15.2m wide berm from above low water to the toe of the slope, to dissipate wave energy. Whidbey’s proposal to erect a near vertical wave wall on top of the breakwater was rejected after a trial portion was washed away.

Whidbey apparently resigned, possibly as early as 1826, and retired in March 1830. (Sir) John Rennie was appointed chief engineer, with Stuart as resident engineer from 1829. Between 1833 and 1836, the breakwater’s west end was modified to form a circular head of solid masonry adjacent to the deep water shipping channel. It incorporated an inverted arch to act as the foundation for a new lighthouse.

Following storm damage in February and November 1838, the berm was extended and the toe buttressed. By the end of March 1841, the breakwater was substantially complete but consolidation work continued until 1850. It contains some 4.5 million tons of stone and cost more than £1.5 million to build. Concrete wave breakers were installed in 1871 and 1928, and later 100 tonne concrete blocks of truncated pyramid shape were deployed on the seaward slope for additional protection. In 1976, voids in the structure caused by settlement and erosion were filled with concrete.

Between February 1841 and November 1843, a white Cornish granite lighthouse was constructed on the west head to replace the lightship. It was designed by James Walker (1781-1862) and Alfred Burges (1796-1886), and first lit in June 1844. The beacon at the east end, constructed in 1845, is topped by a spherical 1.8m metal cage offering a refuge to stranded sailors. A fog warning bell was installed at the lighthouse in 1867. The fort, some 30m north of the breakwater, is a separate structure dating from 1860-1880.

The breakwater is an enduring memorial to Rennie, father and son. Even Napoleon Bonaparte is said to have admired its construction, looking out from HMS Bellerophon in summer 1815, while awaiting exile to St Helena. It was a popular visitor attraction in the early 20th century, with an omnibus transporting passengers from end to end. It is no longer open to the public, but the jetty and the topside rails from the original construction tramway still exist. The underwater structure acts as an artificial reef and is home to many marine species.

For additional information, please see the Engineering Timelines entry for Plymouth Breakwater at http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=99

Sources

James D. Crawshaw, “The History of Chatham Dockyard”, published online by Isabel Garford, 1999

H.D. Jones, “Observations upon the sections of breakwaters as heretofore constructed, with suggestions as to modifications of their forms” and “Discussion”, Minutes of the Proceedings of the Institution of Civil Engineers, Vol.2, pp.124-131, London, 1842

Peter Mitchell, “The Plymouth Breakwater: The Great National Undertaking”, Submerged Productions, Kindle version, 2nd edn, 2012