PROPULSION UNITS

Self-propelled pontoon barges and tugs are powered by outboard propulsion units. These units have been specially designed for this purpose and readily installed on tugs or barges of any size. The propulsion unit shown in figure 10-20 is essentially a heavy-duty outboard motor, consisting of a propulsion mechanism and a marine diesel engine mounted on a heavy structural base. Propulsion power is carried from the engine through a right-angle housing and a vertical-drive housing to the propeller. Steering is affected by shifting the propulsion-force direction; the propeller can be turned around a vertical axis in either direction through a complete circle. Each unit has a steering wheel and an indicator that show direction of

Figure 10-20.-Model L-295 diesel outboard propulsion unit.

travel. The tail section, with the propeller, is mounted on the vertical housing assembly that can be elevated outward and backward to raise it out of the water for inspection or repairs. As new equipment and techniques for amphibious operations developed, performance requirements for all components increased accordingly. As a result, propulsion units have increased in power and thrust capability.

PONTOON STRING ASSEMBLY

After the first two assembly angles have been placed on the ways, the pontoons are placed in the angles (figs. 10-21 and 10-22). The pontoons are positioned on their sides with all deck surfaces on the same side. The first pontoon will ordinarily be placed in the center of the angles with the assembly bolt holes aligned; placement of the remaining pontoons from the center toward each end can be accomplished without difficulty.

Bolting Lower Angles

As each pontoon is placed in the assembly angles, the A6B assembly bolt holes in the pontoon nut receptacles are aligned with those in the angles, using spud wrenches or driftpins as necessary. The A6B bolts are then inserted through the assembly angles

Figure 10-21.-Pontoon positioned on assembly angle.

Figure 10-22.-Pl pontoons on assembly angles

and started by hand to thread the FN1 nut (fig. 10-23). The bolts should be snugly tightened, then backed off about one turn.

Positioning Upper Angles

The second pair of assembly angles is placed on the top of the pontoons and positioned and bolted in

Figure 10-23.-Positioning of A6B bolt and FNl nut to connect pontoon to assembly angle.

the same manner as the bottom pair of angles. Spreader jacks, come-alongs, or heavy-duty pinch bars can be used to align holes for the top angles.

Tightening Bolts

After all of the A6B assembly bolts have been installed, final tightening is accomplished with an impact wrench or 48" ratchet wrench in those locations where accessories or assembly plates are not bolted to the structure.

NOTE: The proper setting of A6B bolts requires tightening to a 2,400-foot-pound torque. (The applicable rule is to draw the bolt or nut up tight and then add another half turn.)

FINAL ASSEMBLY OF STRING

CPI chain plates, LAI launching angles, or other accessories that attach to the outer edge of the particular structure under construction can be installed on the string at this time, if desired. Strings, requiring the addition of a launching angle, should be so assembled on the way that the launching angle can be installed on the top of the string. After installation of the chain plates, the A6B assembly bolts that attach the parts are tightened, and the chain plates or other accessory items are welded, as required. KP1 keeper plates can be installed at this time in all locations for which they are specified for the one string of the structure being built. After all fittings are in place and the assembly bolts tightened, the assembly should be inspected for security of bolts and fittings. After the first string has been launched, the same assembly procedures are followed for assembly of the second and additional strings, as applicable.

LAUNCHING A STRING

If the pontoon string has been assembled along the edge of a dock, it can be tilted into the water by means of jacks or a crane. If it has been assembled on a way, the anchorage is released and the string is allowed to glide head-on into the water. Note that adequate freeboard will be required for this method of launching. End launchings can be accomplished from flat or nearly flat ways by pushing the string with a bulldozier or pulling it with a tug or M-boats. Strings also have been assembled inland and pulled to the shoreline by a bulldozer. A line, secured to the string before launching, should be made fast ashore to keep the string from drifting away in either side launching or end launching and can be used to assure that the string rights itself when launched.

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