Figure 52. Temporary bridge built on a keeled-over freighter. Trucks are using it to unload a Liberty ship.
(9) Install navigation aids and moorings.
b. Following are some of the jobs the port repair ship might be required to do:
(1) Install navigation aids in entrance channel. After the harbor area has been swept of mines by the Navy, the port repair ship marks the entrance channel with buoys. This is done by soundings and by using hydrographic charts of the harbor. Soundings are made by the ship’s fathometer, by lead lines, and by a portable depth finder. The portable depth finder works on the same principle as the ship’s fathometer (par. 8) and is installed on one of the two powerboats. The powerboat operates ahead of the port repair ship, takes continual soundings, and marks the boundaries of the channel. The crew of the powerboat is constantly on the alert for underwater obstructions and signals the ship if any are found and whether the ship must change her course or stop.
(2) Blockships.
(a) A blockship is a prepared underwater obstacle sunk to prevent ships from following navigable channels. One or more blockships may be found in an entrance channel. These ships are often filled with rock or concrete to make them more difficult to remove.
(b) When soundings, intelligence reports, or information received from the minesweepers indicate the location of a blockship, a diver is sent down for reconnaissance. The information required to determine the course of action is—
1. Position blockship is in. That is, is it on its side, at an angle, or resting on its keel.