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.
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.