This invention relates to a multi-blade ditching machine which is used for burying submarine cables or the like in the sea floor and/or for pulling them above the water surface, and more particularly to an improvement in blades of multi-blade ditching machine.
To protect submarine cables or the like from fishing tools, it has been the practise in many countries of the world to bury the cables or the like in the sea floor and to pull out such buried cables or the like by ditching a ditch for repair in case of faults. For this purpose, ditching machines which ditch the soil or sediment of the sea floor to a predetermined depth have been used, and such ditching machines are called cable-buriers or cable-searchers depending on the purposes thereof.
Although the structure of the cable-buriers is somewhat different from that of the cable-searchers due to the difference in their purposes, the essential ditching portions of both the cable-buriers and cable-searchers are similar to each other, and such ditching portions use water jets or plows for ditching ditches on the sea floor. Conventionally, two types of ditching portions with plows have been used, i.e., single-blade type ditching portions and multi-blade type ditching portions.
With a conventional multi-blade ditching machine, the soil ditched by any one blade is placed in a space defined by the difference of the effective widths between that blade and the immediately preceding blade. In order to restrict the height of the excavated soil below a certain limit (for instance, below twice the excavating depth, which limit can be set as high as about four times the excavating depth) the effective widths of the adjustment blades should be increased by a factor of 1.5 as the position of the blade is forwarded. If the number of blades in one ditching machine is increased in excess of four, the widths of the leading blades become large, resulting in large ditching areas which mean large ditching resistances. Accordingly, the overall ditching resistance of the ditching machine cannot be reduced. Therefore conventionally, the four-blade excavator is generally accepted as providing the minimum ditching resistance.
As explained above, each blade of the conventional multi-blade ditching machine has a portion which pushes away the ditched soil over the same width as the ditching width, so as to provide a space for ditching and soil push-away by the next succeeding blade. Thus, the conventional multi-blade ditching machine has a shortcoming in that each blade thereof is required to have an unnecessarily broad effective width in order to restrict the height of the soil ditched by the next following blade thereof, and each blade thereof requires the immediately preceding blade thereof to have a still broader effective width for the same reason.