Figure 10-37.--Rodmeter.

There are two types of underwater log systems.

to the ship's speed. The sensing unit (fig. 10-38) of the

rodmeter is contained in a boot at its lower end. The

One type uses a removable rodmeter and the other type

rodmeter operates on the principle of electromagnetic

uses a fixed rodmeter. The system we will discuss in

induction; that is, when a conductor is made to move in

this training manual is the type with the removable

a magnetic field so that it cuts through the lines of flux,

rodmeter. The major components of this system

an electromotive force (emf) is induced in the conduc-

(fig. 10-35) are the sea valve, rodmeter, indicator-

tor. In the case of the rodmeter, a 60-Hz current is

transmitter, and remote control unit.

supplied to a coil in the boot of the rodmeter. As the

Sea Valve

current flows through the coil, a magnetic field is

produced that surrounds the rodmeter. The water, the

Mounted in the hull of the ship, the sea valve and

conductor in this instance, flows around the rodmeter

packing assembly (fig. 10-36) provides a watertight

and cuts through the magnetic field. The induced

support for the rodmeter. It also functions to seal the

voltage is felt at the contacts, or buttons, on either side

hull of the ship when the rodmeter is removed. The sea

of the rodmeter. The magnitude of the induced voltage

valve has no function relating to the production,

is proportional to the speed of the water flowing through

transmission, flow, or conversion of data in the

the magnetic field. Even when the ship is stationary,

underwater log system.

a current flowing around the rodmeter will cause a

voltage to be induced.

Rodmeter

The rodmeter (fig. 10-37), or sword as it is com-

Indicator-Transmitter

monly called, provides an ac signal that is proportional

The indicator-transmitter displays the ship's speed

on a dial and the distance on a counter and transmits

Figure 10-37.--Rodmeter.

Figure 10-38.--Rodmeter sensing unit.

10-23