[Illustration: The Dhow is the trading ship of East Africa.]
[Illustration: Figure 14.]
A single gyro-wheel would constitute a satisfactory stationary, or
“land compass.” On shipboard the roll, yaw and pitch of the ship would
impose additional duty on a single wheel. It would have to point not
only True North, but also offset the effect of the sea. One of the two
wheels is arranged to always point True North, while its twin wheel
opposes and neutralizes all influences other than the force of the
earth’s rotation. The force of both wheels is utilized in seeking the
meridian.
The Master Gyro-Compass is a marvel of mechanical perfection and
ruggedness. Every rotating or revolving part moves upon special
bearings to reduce friction. It should be noted also that the
gyro-wheels do not directly operate the compass card. The compass card
is turned by a small electric motor (Azimuth Motor), Figure 17. The
slightest change in position between the wheels and card operates the
“trolley” or electrical contact, which controls the Azimuth Motor. The
card is made to “shadow” the wheels. The follow-up is so close that the
card frame has been called the “phantom.”
An electrical transmitter, Figure 17, is operated by the movement of
the card. This transmitter is the means by which the repeaters are
kept in unison with the movements of the Master Gyro-Compass, and made
to show the exact reading at any instant. Again the Azimuth Motor
furnishes the very slight amount of power required to operate this
device.
[Illustration: Figure 13.]
The Master Compass is placed near the center of the ship at the water
line. At this point the effect of rolling is at a minimum. It is,
however, not necessary to place it exactly at this position. Figure
13 shows the approximate location of the various pieces of equipment
aboard ship.