At Portsmouth, channel design is constrained by a number of elements. One is the historic harbour entrance, bounded on both sides by structures dating back to Henry VIII which, understandably, must not be touched. Similarly, the Spit Sand Fort must be avoided, as should other sites of historical interest such as that occupied by the remains of the Mary Rose.

From a navigational point of view, the present approach to the harbour entrance involves a number of course changes in or near the entrance, which puts a large ship at risk. This being the case, the elimination of these changes was seen as an important part of any new channel design.

Portsmouth, however, possesses a significant natural advantage. There is a natural channel, underlying the present approach channel, which has been formed as the tide moves water in and out of the harbour.

By making use of this channel, BMT, using its PCSS fast time manoeuvring simulation model, was able to produce a design which both eliminated the awkward course changes at the harbour entrance and made the fullest use of the available, natural, deep water.

This so called 'Blue 2' channel led to significantly reduced dredging costs compared with the alternative 'straight in' channel and avoided both the Spit Sand Fort and the Mary Rose site.

Dr Ian Dand of BMT Seatech Ltd, who is responsible for the design study, said, 'Using sophisticated computer simulation, we have been able to show that the existing, natural channel, with some dredging, can be used for the large warships proposed for the future. This will minimise environmental disruption as well as reducing marine risk.'