There are four of these: Spitbank (closest to Portsmouth), St Helens (close to the Isle of Wight), Horse Sands and No Mans Land (the latter two are in the middle of the Solent and clearly visible from the Club, Horse Sands being the nearest to the Club).
All the forts were built between 1861 and 1880, (for comparison, HMS Warrior was launched in 1860).
The eastern part of the Solent is very wide, however the majority of it is very shallow with sand banks protruding from Portsmouth and the Isle of Wight resulting in a very narrow deepwater channel that the forts were positioned to command.
Be wary about approaching any of the forts closely because they affect the wind significantly on all sides and it is easy to get becalmed and pushed onto them by the tide.
You can sail all the way around Spitbank and No Mans Land Forts, and around St Helens at high water. However the Submarine Barrier goes all the way to Horse Sands Fort (see section 5.4).