1 INTRODUCTION A. GENERAL DESCRIPTION OF THE LOG SYSTEM 1A1. General. The submarine underwater log system is a device for indicating, in knots, the speed of the ship as it travels through the water, and for recording in nautical miles the distance traveled. The principal components of the system are located in the forward torpedo room, below the light draft water line of the ship.
 Figure 1-1. Underwater log in position in submarine extended down below the forward torpedo room.
 1 B. PRINCIPLES OF OPERATION 1B1. Principles. The underwater log system operates on the principle of hydraulic pressure actuating electrical and mechanical units. These units are so calibrated that hydraulic pressure is translated into terms of speed and distance. The hydraulic pressure acts through the rodmeter. This part of the underwater log system extends through the hull of the ship into the water. There are two passages in the rodmeter. When the ship is at rest, the hydraulic pressure is equal in both passages, and is due only to the weight of the water above the system. This pressure is known as static pressure. As the ship moves forward, the movement creates additional pressure in the forward passage of the rodmeter. This added pressure is known as dynamic pressure, and is the actuating force which operates the system. The dynamic pressure developed in the forward passage in the rodmeter can be determined by creating a force of known value and using it to equalize the dynamic pressure. The amount of force required to equalize the dynamic pressure is converted into units of speed. These units are registered on indicators calibrated to read in knots, and in distance traveled.
 Figure 1-2. Elementary diagram showing fundamental principle of operation. Top: When submarine is stationary, static pressure only is present in the underwater log system. Middle: As submarine moves forward, the movement creates dynamic pressure within the log system. Bottom: Application of known force (A) measures dynamic pressure in terms of speed. 2