LADDER OR ELEVATOR DREDGE 69 



When the ladder is located on one side of the vessel, the hull is 

 constructed like any other vessel, but when the ladder is located 

 at the center and along the longitudinal axis of the vessel, the hull 

 is provided with a pit or well. The dimensions and form of the pit 

 vary with the work of the dredge. In dredges constructed to lower 

 the level of deep channels or harbors, the ladder working always at 

 given angle, has its lower end submerged. In such cases the pit 

 is located amidship and the hull will be a closed one, except for the 

 pit. The ladder can be located at the bow both in navigating and 

 dredging. When the dredge is constructed to w r ork in both shallow 

 and deep water and even to cut its own channel, the ladder must 

 be arranged to be raised and lowered, consequently the pit should 

 extend through the stern of the vessel. In this arrangement the 

 hull is open in the stern and a strong frame or gantry built on deck 

 is used to connect together the two separate walls of the pit, while 

 the ladder is raised or lowered by means of chains and pulleys 

 attached to the gantry. In any case the pit should be wide enough 

 to permit the ladder to work on a small radius. 



The ladder consists of a strongly built trussed beam kept in an 

 inclined position and provided with a tumbler at each end. Along 

 this beam and around the drums travel two endless chains carrying 

 the buckets. The upper end of the ladder is fixed to a tower by 

 means of turnbuckles which permit the adjusting of the chains to 

 the required tension. The lower end is suspended by chains passing 

 over pulleys fixed to the gantry. Chains attached to a drum of a 

 reversible engine regulate the raising and lowering of the ladder. 

 To facilitate the running of the chains and loaded buckets in their 

 ascent along the ladder, the upper side of the trussed beam is provided 

 with rollers. 



The tumbler or drum at the lower end of the ladder is meant to 

 guide the endless chains and buckets. These, however, are moved by 

 the driving tumbler located at the upper end of the ladder mounted 

 on a tower. Since all the strain of dredging apparatus falls upon 

 the upper tumbler, this is built very strong and it is usually made of 

 cast steel. Its cross-section is made as closely as possible a circle of 

 the smallest diameter; but in order to smoothly drive the endless 

 chain it is made polygonal, each side being equal to the links of the 

 chains and length of buckets. As a rule the driving tumbler is 

 made of pentagonal cross-section, thus wearing all the faces equally, 

 which would not be possible with a square or hexagonal cross-section, 



