Horse Sand Fort

Horse Sand Fort is one of the larger Royal Commission sea forts in the Solent off Portsmouth, Hampshire, England. It is 240 ft across, built between 1865 and 1880, with two floors and a basement and armour-plated all round.

The original armament was to have been 45 10-inch and 44 12.5-inch rifled muzzle-loading (RML) guns on the gun floors and 10 12" RMLs on the roof in five turrets. In fact the turrets were never built and the limited space meant the 12.5-inch guns had to be operated with less than full charges of powder. In 1882, 12-inch rifled breech-loading guns were placed in alternate bays.

Horse Sand Fort was built on a ring of masonry consisting of large concrete blocks with an outer skin of granite blocks, the interior being filled with clay and shingle and covered with a thick layer of concrete. The lower foundation walls of fort are 59 ft thick. The fort is split into 3 levels with top measuring 204 ft in diameter. The floors would have originally provided storage of armoury and guns and the things needed to sustain the men that were stationed on site. The top of the fort consisted of a lighthouse and various chimneys and ventilators. The fort has its own Artesian well (aquifer) which provided fresh water. The seaward side of the fort was covered in a heavy iron armoured plating to protect it from sea borne attack. Access to the fort was via a wooden decked landing stage supported on cast iron piles.

Other sea forts include Spitbank Fort, St Helens Fort and No Mans Land Fort. During the second world war extensive submarine defences were built in the form of large concrete blocks running about 6 ft below sea level from the fort to the shore at Southsea. With only a single narrow gap to allow small craft to pass through, this barrier (and a much shorter one running south from No Man's Land Fort towards Ryde Sands) remains as the cost of demolition is deemed too high.

In March 2012, the fort was purchased by Clarenco (owners of No Man's Land Fort and Spitbank Fort) and is to be converted into a museum.