20° Angle Gave Best Results
A primary tow-point position 3 in. up from the bottom of the dredge appeared to present the best probability for an optimum cut into the sediment when acting in combination with a 6-in. cutting blade installed at a 20° angle down from horizontal.
As originally deployed, the forward assembly of the dredge measured 3 ft in its vertical dimension and a constant cross-section was maintained from front-to-rear (fig. 17).
Fig. 17. Nodule dredge configuration shown was chosen after a series of increasingly complex dredge designs
The height of the opening was insufficient to allow the dement nodule mass to flow to the rear; the failure three hold of the mass occurring at close to 3 ft. An addition of 6 in. to the vertical dimension of the forward assembly eliminated the stoppage. The increase forward created an inclined plane along the upper side of the dredge. With the ballast chain shifted to a low point in the rear of the dredge, the inclined plane combined to provide the desired hydro-dynamic characteristics and the dredge invariably landed in correct position.
Sediment was massing to a height of about 3 ft before flowing into the bag. Nodult-3 alone, washed of sediment during ascent filled 30% of the bag. The maximum return was 72%, but this efficiency was rarely approached.
Larger Dredge Lines Cause Problems
The main dredging line is a 7/8 in. diam. 3 x 19 to balanced rope. It is constructed of plow steel with 19 wires in each of three strands, laid in a right, regular Seale pattern. The rope resists rotation, twisting and kinking and offers the highest strength-to-weight ratio of n rope suitable for oceanographic service. The relatively large diameter wires in each strand provide increased resistance to abrasion.
The rope is rated at 78,000-lb breaking strength with an limit of about 58,5000 lb and weighs 1,1 lb per ft in This size wire provides a working load of 17,600 lb with 22,000 ft of wire deployed. The available working load was determined by our desire to limit the maximum static load at the winch to 44,000 lb. In practice, maximum loadings rarely exceeded 30,000 lb and never reached the elastic limit.
The 3 x 19 construction will rotate about 1° per 100 ft out, a significant factor in view of the long lengths deployed. A pressure-sealed swivel is used between the rope end-fitting and the bottom gear. A closed-end shelter socket, providing close to 100% of the breaking strength of the rope is an end-fitting. This socket is shackled to a chain pendant, rather than directly to the dredge, to provide maximum flexibility.
A conventional safety chain is rigged from the forward shackle, aft to the dredge. The connecting shackles are sized to provide the weakest points in the system. If excessive strain is placed on the forward shackle, it will part, thus transferring the strain along the chain to the rear of the dredge. This will upend the unit and possibly free it from the abstraction.
The winch system consists of a main traction engine which tonslons the rope through two in. bull wheels, and a take up reel. Both tractions engine and reel are driven hydraulically through a system of individual motors and pumps, powered by a Detrolt Diesel I2V7I.
The tension at the reel and at each of the bull wheels can individually adjusted.
The machine will pull in excess of 45,000 lb and is capable of holding twice that amount with all brakes set. Average line speed is 350 fpm. An advantage of the system is its constant tension feature, the ability to hold the load at any pre-set tension and pay out when that tension is exceeded such as when the dredge lodges against an immovable object.
A separate reel and traction system was selected so that relatively light loads could be placed on the reel. The winch rope can be spooled onto the reel at about 6,000 lbs pressure, which obviates the need for pressure ling, a very difficult task with the 3 x 19 wire.
A separate electro-hydraulic system powers the winch, unloading rack, screw conveyor and two small auxiliary tuggers. As the dredge is snaked aboard, it is restrained from excessive lateral movement by two heavy pipe ails. Once the rack, the unit's movement is restricted by the sides the rack and the dredge bag can safely be secured in place. Two hydraulic pistons elevate the rack to an attitude of 50° above horizontal, 45° being the approximate flow threshold for most wet nodules. In most cases, the material will flow rally into the receiver bin. Throughout this sequence the main wire is slacked to allow forward movement of the dredge, but is never unshackled. After dumping, the unit can be immediately returned to the water, accomplished through standard flights of a 12-in. diam. screw conveyor. They are manually fed into the conveyor through an entry port in the bottom of the receiver. Upon exiting the conveyor, they are diverted into the storage bin via a common coal chute.
Storage bins were fabricated of 2 in. angle-iron framing. Compartment provided sufficient rigidity and prevented any free-surface effect and cargo shifting. Designed capacity was a nominal 100 tons, but the settling effect of this quantity of modules increase the capacity by about 20 percent.