(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. 2. Approximate size and type of vessel. 3. Nature and extent of its superstructure. 4. Would the removal or destruction of the superstructure leave enough water between the hull and the surface for large vessels to use the channel? [Illustration: _Figure 53. Two half-sunken ships and the bomb crater in harbor’s seawall and quay show the results of well-planned destruction._] 5. Nature of bottom and effect of currents. 6. Is ship filled with rock or concrete? 7. Indications of mines and booby traps. (_c_) This information determines whether the superstructure can be cut through or flattened with explosives or if the hull has to be broken up before the channel is cleared. Destroying the hull is a long, difficult task and should not be attempted if the removal of the superstructure affords enough clearance. (3) _Harbor._ (_a_) _General._ When the port repair ship enters the harbor, her master reports to the port commander for instructions. These instructions include— 1. What jobs the ship is to do and what priorities have been set on them. 2. What materials, equipment, power, tools, and personnel the ship is to furnish the port construction and repair group. 3. Whether temporary construction or repairs are to be made or permanent repairs made. 4. Surveys to be made of underwater damage and obstructions in harbor area.