CSDs are always stationary when they are working – even if they are self-propelled. To lock the vessel into a stationary position, a CSD generally has two spud poles. One spud pole (the auxiliary spud) passes straight through the vessel, whilst the other, the working spud, is mounted on a movable spud carriage, which can be moved lengthwise along the vessel or pontoon. Steel cables are used to move the ladder or cutter head from side to side, with the spud in the spud carriage as the centre of each concentric circle that it describes.
Although the vessel is stationary, moving the spud carriage causes the dredger to move. This is known as ‘stepping’. In this way, the CSD describes an arc round a fixed point – the spud pole or working pole – and the radius of the arc is increased by ‘stepping’ ahead with the spud carrier. In many CSDs this pole is mounted on a movable carriage, the spud carriage. A second pole, the auxiliary spud, is set out of the centreline, usually on the starboard side of the stern of the pontoon. This auxiliary spud is used to keep the vessel in position, when the working spud is raised, and the spud carrier is moved back to its initial position. Since spuds are literally dropped into the soil, they have pointed ends to make sure that they penetrate the soil deeply enough to be secure. They are hoisted out of the seabed and that requires specialised hoisting wires and systems.