Cranes and gantries are not the only equipment carried to facilitate discharge.  With greater specialisation of vessels for particular commodities or trades a range of highly individual systems have been created to discharge bulk cargoes into dedicated shore installations.  These include conveyor systems, elevators based on ‘Archimedes screws’, and suction systems. An axial stern ramp can also act as a stern door where the ramp is short;  alternatively when the axial ramp is long then a separate watertight door is used, closing prior to the ramp being raised. Bow openings allow access to cargo spaces through the forward end of the ship and there are two types commonly in use, bow visors and bow doors.  A bow visor is a portion of the ship’s bow, hinged at the weather deck level and is raised upwards when entry is required.  A separate inner watertight door is provided along with axial ramp. The twin bow ‘clamshell’ doors (see next page) re located in the plating around the stem and are hinged to open outwards from the centre line.  Their operation, sealing and securing arrangements are similar to that of a bow visor and also have an internal watertight door and axial ramp. In ships having several vehicle decks, a multi-purpose internal moveable ramp is used as less space is wasted. As an alternative to ramps for transferring vehicles from one deck to another, elevators or lifts are employed. The advantages of elevators over ramps are that they require less internal space and cargo can be stowed upon them. Disadvantages of elevators over ramps are: Positioning of elevators within a ship must be carefully chosen. More complicated in way of electric and mechanical equipment and hydraulics. If too close together when serving different decks manoeuvring space is likely to become restricted. Interrupted cycle of operation unlike ramps which can be used continuously.