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
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.
The rodmeter (fig. 10-37), or sword as it is com-
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 10-38.--Rodmeter sensing unit.