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Nab Tower
The Solent, off the Isle of Wight, UK
Nab Tower
associated engineer
Richard St.George Moore
Sir Alexander Gibb
Guy Anson Maunsell
date  June 1918 - September 1920
UK era  Modern  |  category  Lighthouse  |  reference  SZ740860
ICE reference number  HEW 1313
photo  PHEW
A steel and reinforced concrete tower originally intended as the first of a series of World War I anti-submarine towers across the English Channel. Nab Tower was still under construction when war ended, so it was used as a replacement for the lightship that had marked Nab Rock at the eastern entrance to the Solent. It is still in use today, though automated and unmanned.
Nab Tower was one of the five 'mystery towers' that were under construction for the Admiralty during 1918, though more were planned. It and one other were built at Shoreham-by-Sea in West Sussex, near John ver Mehr’s yard (TQ222048) where Guy Maunsell's 'Creteships' were constructed. Both Alexander Gibb and Maunsell were involved in the design of the towers, which were intended to form a linked chain between Dungeness, Kent, and Cap Gris Nez, northern France, to intercept German U-boats.
The two towers at Shoreham Harbour were the only ones near completion when the Armistice (11th November 1918) stopped the project. Trinity House decided to use one as a lighthouse; the other was later broken up for scrap — perhaps just as well, as it was reportedly too big to fit through the harbour entrance. Apparently as many as 3,000 civilian labourers and up to 5,000 Royal Engineers were working on the towers, round the clock.
Nab Tower consists of a 24.9m high four-tier reinforced concrete pyramid, to which are fixed three 23.1m high concentric steel cylinders with an outer layer of latticed steel bracing. The bottom tier of the pyramid is octagonal and the upper three tiers are hexagonal. Each tier is made of interlocking hollow hexagonal cellular caissons 1.8m wide reinforced with 52mm steel mesh.
The cylinders are 2.4m, 9.1m and 16.8m in diameter, the central shaft providing ventilation for the seven storeys of crew's quarters and other rooms between the two inner shafts. For defence purposes, the space between the outer and middle cylinder was to be filled entirely with concrete, as is the second storey of the inner section.
Two tugs pulled the tower 66km from Shoreham to the Solent on 12-13th September 1920, where it was floated into place. The bottom tier — 59.1m long and 49.4m wide — acted as a raft. Buoyancy was provided by the honeycomb construction of the base, which has 18 watertight compartments that were flooded once the tower was in position.
The tower developed a tilt of 1 in 60 soon after erection but has not shifted further. It measures 52.9m from the sea bed to the helicopter pad at the top, which was originally the gun deck. During World War II (1939-45), it was equipped with two 40mm Bofors guns and shot down several enemy aircraft.
Nab Tower was manned by three keepers until it was automated in 1983. It was converted to operate on solar power in 1995, when a new 11,739 Candela lantern was installed.
On 7th November 1999, the freighter Dole America hit the tower and damaged 16 of the concrete cells. The cells were repaired by underwater grouting between April and August 2001, at a cost of some £300,000.
Construction: Royal Engineers
Research: ECPK
"Alexander Gibb: The Story of an Engineer" by Godfrey Harrison, Geoffrey Bles, London, 1950, second edition 1966
"Subsea Repair of Nab Tower" by James R. Dale and Andy Goddard, Posford Duvivier Ltd, Haywards Heath, 2001
"Maunsell: The Firm and its Founder" by Nigel Watson and Frank Turner, AECOM Technology Corporation, 2005
reference sources   CEH South

Nab Tower