Number 1 Ship Tank, Haslar Marine Technology Park (former Admiralty Experiment Works)

A covered ship model testing tank, now known as Building 23, constructed in 1886 and extended in 1957 based on designs by the engineer R E Froude. The building is located at Haslar Marine Technology Park on the site of the former Admiralty Experiment Works.

The structure is built of red stock brick laid in English bond with rubbed brick and concrete window dressings. The pitched roofs are covered with Welsh slate laid over wooden boards, with timber roof trusses spanning the original waterway and steel trusses in the 1957 extension.

Number 1 Ship Tank has a linear plan form oriented on a north-west to south-east axis. The long south-west elevation borders the south-west boundary of the site, oriented perpendicular to Haslar Lake to the north and Haslar Road to the south. This elevation was built off the pre-existing boundary wall to the Gunboat Yard (Grade II*), which was raised and modified to support the roof. The principal original part of the building is the long single range housing the waterway. A double-gabled range adjoining to the south-east, also constructed in the original phase, previously housed the model-making workshop, boiler house, and offices. These ancillary rooms are oriented parallel to the waterway with direct access through to the waterway range and between each of the offices. The 1957 northern extension increased the length of the waterway from 122 metres to 168 metres. A drawing office adjoining to the north-east was also added in 1957.

All of the ranges are of a single storey. The double gabled roofs over the south-east part of the building rise slightly higher than that of the waterway and are hipped at their south-east end. 18 pairs of skylights punctuate the pitched roof either side of the ridgeline of the waterway arranged in three groups: a northern group of nine pairs, a central group of three pairs, and a southern group of six closely-spaced pairs. A further row of seven single roof lights occur on the eastern half of the roof. A tall, tapered, rectangular chimney stack with a machicolated brick corbel course at its top rises from the north-east elevation of the boiler house.

The long north-east and south-west elevations of the waterway are divided into regular bays of recessed brick panels with projecting brick piers topped with brick corbel courses. The piers correspond with brick buttresses on the interior walls. These elevations were largely blind originally, with a small number of square single-pane windows that had blinds to control internal lighting levels. Some of these original window openings survive with flat arches of rubbed brick, but most of the blind bays now have modern two-pane windows inserted during the 1990s office conversion. Most of the fenestration appears to be uPVC with concrete sills. Three new entrances were added to the north-east elevation in the 1990s to provide access to the subdivided offices. The entrances are raised above ground level reflecting the inserted office floor and are housed in projecting glazed porches. The main entrance to the building is on the south-west elevation, with a new opening formed beneath a steel lintel in the 1990s. This entrance has glazed double-doors and is also raised above ground level and accessed by steps.

The double-gabled range to the south-east has the same pattern of brickwork as the waterway to all its elevations. The north-east elevation has tall, two-pane windows with flat arches of rubbed brick in each of the recessed panels. Some of these openings may be original while others were added in the 1990s, and the windows themselves are largely uPVC. Single-pane windows have also been inserted into two bays on the south-west elevation. The south-east elevation is partly obscured by several modern lean-to structures built between the principal building and the adjacent Gunboat Yard boundary wall, but it retains the pattern of recessed panels and brick piers that continues from the other elevations. The north-west elevation of this range where it projects out from the waterway is blind with a stepped pattern of brick corbels following the roofline. The 1957 drawing office extension to the north-east has stretcher bond brickwork and a flat roof behind a low parapet with concrete coping stones. The north-west elevation has a pair of three-light windows to the single range and three two-light windows to the drawing office extension.

The main entrance opens into a reception area which provides access to the various office spaces. The double-gabled range has been subdivided into offices and meeting rooms. The largest room in this part of the building is a conference room which retains original king post roof trusses of timber supported by cast iron columns. These trusses continue along the length of the original waterway, and they have stencilled numbers which were primarily used by the carriage operators to judge when to apply the brakes during experiments. Buttresses occupy the spaces between each truss and correspond with the brick piers on the external walls. The originally continuous space of the waterway has been subdivided to provide office accommodation. The tank survives underneath and now forms the basement to the offices, although it too has been subdivided. The tank as originally built was 122 metres long and was extended to 168 metres in 1957. It is 6 metres wide and 2.7 metres deep and has vertical sides and a flat bottom. A sand and gravel beach added to the north-west end for one of the final experiments also survives beneath the modern floor. The 1957 north-west extension has steel roof trusses of a similar design to the timber originals, and this part of the building has also been subdivided into offices.

A square storage building, originally the Fire Engine House, stands close to the double-gabled range and is now joined to the north-east elevation of the waterway by a semi-glazed link. The building is constructed from red stock brick laid in English bond, with a recessed brick panel to each face and corner buttresses echoing the appearance of the principal building. The roof is covered with Welsh slate. The building has modern double doors to its north-west elevation and a wide window to the north-east, both with cambered arches of rubbed brick. The south-east elevation has a small lean-to extension of brick with a hipped roof.