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Copyright Wet Paper Publications 2014
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Figure 8.3 How a depth sounder works.
Reflection back
Electronic pulse
Depth sounder in wheelhouse
Transducers
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Figure 8.1 Prism mounted handbearing compass
Hand bearing compass
A hand bearing compass is free and can be carried to the most
suitable spot on the boat for navigational observations. The
steering compass is usually a fixture near the helm.
For this reason, the hand bearing compass is usually much
smaller and lighter and incorporates some form of sighting
device. Because it is relatively small, this type of compass
usually has amagnifying glass to enlarge the figures on the card.
Others like the one shown in Figure 8.1 may have a built-in
prism which reflects the bearing back to the eye.
Fluxgate compass
The fluxgate compass does not use amagnetic needle to provide
direction but consists of two coils carrying electric current. One
coil is mounted horizontally and the other vertically. If the
vessel changes its heading or position in the Earth’s magnetic
field it alters the flow of current in one coil relative to the other.
Being electrical, the information can be displayed digitally. It
can be fitted with microprocessors which can automatically
compensate for both deviation and variation thus providing a
true bearing.
The fluxgate compass does require power from a small battery
for it to operate. This compass must also be held fairly steady
for accurate readings and is still subject to the errors of a
magnetic compass and needs to be treated similarly.
A fluxgate compasses is shown in Figure 8.2.
Depth sounders
Depth sounders operate on the echo principle. Triggered by an
electrical impulse at regular intervals, a transducer (ceramic or
nickel element) transmits downwards a sound wave. By
measuring the time taken for the echo to return to the transducer,
depth can be determined.
The transducers may bemounted through-hull, in-hull or on the
transom.Correct installationandpositioningarevital toaccurate
performance.
Because of signal variations, depth capabilities and resolution
definition will vary between different sounders. Four examples
are:
• power output (200 – 3000 watts)
• beam width (8
o
– 30
o
)
• pulse length
• frequency (50 – 200 kHz)
Figure 8.2 Fluxgate compasses
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