=e. Armament.= For AA protection, the ship carries six 20-mm AA machine guns. Mounted on the after end of the poop deck is a Navy 3″/50 (3-inch bore, 150 inches long) dual-purpose AA gun. 8. EQUIPMENT FOR SHIP OPERATION. =a. General.= This paragraph describes briefly the major items of equipment required for the operation of the ship. =b. Navigation equipment.= (1) _Compasses._ (_a_) The ship is equipped with magnetic and gyrocompasses. The master gyrocompass is located below decks in the gyro-room. The readings of this compass are transmitted to repeaters, which are similar in appearance to magnetic compasses and are placed in the pilot house and at other points where knowledge of the ship’s heading is required. Gyrocompass equipment includes the master compass and its supports, batteries, a motor-generator set for supplying power in case of failure of the ship’s supply, instrument panel, and a panel for fuses and switches for the repeaters. (_b_) The magnetic compass is in the pilot house. It is mounted in a compensating binnacle that neutralizes or compensates for errors introduced into the compass by the magnetic materials on the ship. (See figs. 8 and 9.) (2) _Pelorus._ A pelorus is mounted on each wing of the navigating bridge. The pelorus is a graduated circle mounted in gimbals with an alidade pivoted over its center. It is used to measure directions to some distant point like another ship or an object on land. It gives the bearing relative to the ship’s heading or, if set to the ship’s true course, the true bearing to the object. [Illustration: INBOARD PROFILE _Figure 6. Inboard profile of engineer port repair ship showing location of main installations._] [Illustration: INNER BOTTOM PLAN _Figure 7. Inner-bottom plan showing location of Diesel-oil, fresh-water, and salt-water tanks._] [Illustration: _1. Wheel._ _2. Compensating binnacle._ _3. Magnetic compass._ _4. Telephone._ _5. Speaking tube._ _6. Gyrocompass mounted on binnacle._ _7. Engine room control._ _Figure 8. View of pilot house._] [Illustration: _Figure 9. Drawing of navigating bridge showing location of equipment in pilot house, chart room and radio room._] [Illustration: LIGHT SHIP WITH 418 TONS BALLAST—2204 TONS _Figure 10. Dead-weight scale and load-line marks._] (3) _Radio direction finder._ Located on the navigating bridge is the ship’s radio direction finder. This instrument receives radio signals and establishes the bearing of the sending station. It consists essentially of a loop antenna for receiving the signals connected to a radio receiver that makes the signals audible. The position of the ship may be determined from radio bearings by taking cross bearings on two or more stations, by two bearings on the same station and the distance run between bearings, and by a bearing and a sounding. (4) _Fathometer._