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Sites : Breakwater Fort

Location - Plymouth Sound, Devon, UK

Coordinates : 50 20.075N  004 08.938W (WGS84)    Depth : 12m    

Conditions : Sheltered

Type : Fort   Built : 1867
 

History

The Breakwater across Plymouth Sound was completed in 1844 to provide a sheltered anchorage for the Fleet. To improve the defences for the Sound, an 1859 Royal Commission report recommended that a fort be built on Shovel Rock in the middle of the Breakwater. The original plan was for a casemated work containing 100 guns and 600 men to close the gap in the defences between Fort Bovisand to the East and Fort Picklecombe to the West, and to protect any ships at anchor in the Sound.

The Fort was originally designed as a masonry work on four levels encased in granite but experiments showed that only an iron structure would resist heavy shell fire from rifled guns. The design was was trimmed down to a two-level iron fort with 18 rifled muzzle loading (RML) guns on the top and the magazines and stores underneath.

Work was begun in 1867 with the foundations being constructed 10m below mean sea level by divers. The breakwater Fort is 144 feet long and 114 feet wide, standing 100m North of the Breakwater. The iron framing at the front of the casemates is filled with concrete and is about 2ft thick while the iron shields of the gun ports are between 1.5 and 2 ft thick. By the late 19th century the exterior was painted in a black and yellow chequer pattern to obscure the positions of the gun ports, some of this pattern still remains. In 1875 the armament consisted of fourteen 12.5 inch 38 ton RML guns to cover the sea to the South and the channels to the sides, plus four lighter 18 ton RML guns to cover the anchorage to the North.

By WW1 the fort had been disarmed as the guns had become obsolete and it spent the rest of its life as a Naval signal station. In 1936 the fort was used as an anti-aircraft training school with AA gun positions and additional buildings constructed on the roof. The Breakwater Fort went out of military use in 1976 and spent some time as a base for offshore diver training operations by POP Fort Bovisand Ltd.

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3D Computer model of the structures

The image below shows the general arrangement of the structures around the North of the Fort.

Underwater Around the Fort

The water behind the Breakwater is sheltered from all but the roughest seas so the Breakwater Fort sits on a rocky seabed covered with soft silt. Visibility ranges between nil and 10m but is usually a few metres. The seabed is also littered with the debris of many years of use, this makes for an interesting dive and also provides homes for the local marine life.

Around the North and East sides of the fort there are a number of large structures on the seabed. Most of the structures were placed there as test exercises for the trainee divers by Fort Bovisand. The largest of these is the steel CSWIP structure, this was used for testing divers on weld inspections. Two hollow concrete blocks on plinths lie to the North-East, these were again used on inspection courses. A triangular steel structure lies to the North, this was used as a construction task for trainee divers. 

Next to the CSWIP structure is a steel cylinder on legs, this is the habitat called Glaucus.  Two divers from the Bournemouth Branch of the BSAC, Colin Irwin and John Heath, lived inside Glaucus for 7 days at 35 ft in September 1965.  Glaucus is 12 ft long and 7ft in diameter with a capacity of 480 cubic feet giving 13 tons of buoyancy, the 'house' was supported on legs above a ballast tray leaving 3ft clearance for the divers.  A simple semi-closed circuit air purification system was used with food and supplies brought down by divers.

Eight metres to the North of the CSWIP structure is an artificial reef made from old tyres and 60m further out lies a large cylindrical lifting Camel (50 20.1032 N, 004 8.9609 W), this is a pontoon once used by the Navy to salvage ships.

The Site Recorder screen image below shows a multibeam echo sounder (MBES) image of the structures to the North of the Fort (University of St Andrews).


Mapping the Site

In 1996 a team of divers and archaeologists led by Peter Holt used the Breakwater Fort as a testing ground to evaluate the accuracy of underwater mapping techniques. The area around the two concrete blocks was chosen to be mapped in detail using trilateration with tape measures, acoustic distance measuring equipment and an underwater acoustic positioning system. Many years later the site was also mapped using a multibeam echo sounder (MBES) by Martin Dean and the University of St Andrews.

Sinking the Tavy

To enhance the area to be mapped it was decided to sink a small vessel between the two blocks, this vessel would be used for tests on high-resolution mapping and photogrammetry. The Trinity House pilot vessel Tavy was donated to the project by Nigel Boston so the team stripped the vessel of its wheelhouse to allow safe access inside then removed all the remaining fittings. Nigel Boston took Tavy out to the Breakwater Fort and with the help of the project team put the vessel in position between the two blocks, the process recorded for posterity by the local TV station. Alongside Tavy the team also placed one of the cannons recovered from the Coronation (1691) site, this gun had been lying inside the harbour at Fort Bovisand and had been sued by divers for lifting exercises. 

Control points were added to the concrete blocks, Tavy and the wall of the Fort. Sets of direct tape measurements between the points were recorded by many divers over a period of 12 months. The divers were a mixture of trained archaeologists, NAS trainees, experienced divers and trainee divers. By collecting and processing the measurements it was possible to determine the accuracy of tape trilateration used underwater and to put metrics on the effectiveness of training divers to do this work.

The site was also mapped using a Sonardyne Homer Pro underwater acoustic distance meter. Beacons were placed on a selection of the control points then acoustic distance measurements made to the beacons from other points. The distances measured were then processed in the usual way. The team also used a Sonardyne DiveTrak acoustic positioning system (APS) to map the control points. This system would position a transducer carried by a diver that was placed on any point, the positions were calculated in real time by computer then shown on a chart display.

The survey research resulted in a paper published in the International Journal of Nautical Archaeology, described in the Survey Research pages. The need to process tape measurements spurred on the development of the Site Surveyor computer program, something that would eventually grow into the Site Recorder 4 GIS.

The tests with the acoustic positioning systems provided essential information about mapping shipwrecks using APS. This led to the use of Sonardyne systems for mapping the Mary Rose in 2003, the U166 and many others.

The Breakwater Fort Site Today

Commercial diver training at the Fort has finished but the site can still be dived. The site is much as it was in 1996 when it was mapped apart from some severe damage to the stern of Tavy where the aft deck section has been ripped off.

The image below shows the control point network used for the tape trilateration accuracy tests.


Bibliography
  • Bovisand - The Book, McDonald K.,1981, [ABE]
  • Fort Bovisand, McDonald K.,1997, [ABE]
  • The Historic Defences of Plymouth, Pye A. & Woodward F.,1996, [ABE]
  • Glaucus Project Report, Irwin C. & Heath J., 1965, Bournemouth BSAC
 

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