This method has been used successfully when removing large vessels, but the practicability of rendering them airtight chiefly depends on the damage which they have sustained. The pumping-out method, comparatively speaking, is the most simple one to adopt, provided that the damage to the hull is small. The damaged portion must be repaired by divers unless the more elaborate method of building a coffer-dam--i.e., a sort of dock--around the ship, is pursued. Divers cannot work in a strong tidal current or in rough weather. The repair of holes under water is rendered extremely difficult, if not actually impossible, when the bottom of the ship is badly holed with the ship resting on the damaged portion. The ship must be made watertight, or nearly so, below the surface of the sea before she can be lifted. The word "watertight" is qualified here because, as a matter of accuracy, the ship can be pumped out and lifted, provided that the pumps can eject water at a greater rate than the latter is flowing in. Before passing on to consider the next method it may be as well to remark that special difficulty is experienced when moving sand--i.e., silt--has access to the holes in the ship. Dispersion by means of explosive charges may, {47} under certain circumstances, be a simple operation, but, on the other hand, there are certain conditions which put this method outside the pale of choice. For instance, in the case of a ship sunk in a narrow channel where much silt is experienced, the explosive method is almost worse than useless. For every explosion in a given section of a vessel will tend to shatter that portion into several pieces. Each piece falls to the bottom and forms a new obstruction. Silt then enormously aggravates the situation, for the sand will collect against the obstruction until it becomes a miniature sandbank. Such shoals are then difficult to remove. A bucket-dredger--i.e., a vessel fitted with an endless chain of buckets for scooping up the bottom--will break her buckets as soon as they encounter the steel kernel of the shoal. On the other hand, a suction dredger--i.e., a vessel designed to suck up sand off the sea-bottom--cannot raise solid material. Neither type of dredger can remove the _cause_ of the shoal; any removal of sand under such conditions is merely temporary; the sand will recommence building up the shoal as soon as the dredger ceases work. Dredging against such obstacles is of little more use than dredging against rocks.