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CM2 Sidescan Sonar User Manual
7. Notes for the Helmsman
7. NOTES FOR THE HELMSMAN
Navigation
Use the sonar's plotter window to show waypoints, the track and the swept ground.
Steering
The quality of the sonar images depends on the skill of the helmsman. This applies
especially to the HF and EF operation.
The helmsman is so important because the towfish can only collect good images if it flies
straight. Because of this, steering a sidescan sonar survey is not the same as steering a
conventional sounding line.
A turning towfish stretches the image on one side and compresses the other, wasting the
high resolution of which the system is capable. (A high power telescope can only be
effective if it is held steady!) As well as corrupting the outer areas of the image, a turning
towfish also upsets the matching between the gain profile and the beam shape, causing
alternate light and dark patches in the inner areas of the image.
Use small, slow wheel movements even if this means that the vessel temporarily leaves
the planned survey line. If an autopilot is available it will, under most conditions, steer a
better sonar course than a human helmsman, even though the heading may need to be
trimmed occasionally.
Remember that the towfish follows the stern, particularly when the tow cable is short, and
is therefore affected by rudder movements as well as by course deviations.
At the end of the survey line, the helmsman must warn the sonar operator before turning
and must not turn sharply, or the towfish could strike the bottom.
Emergency Actions
Emergency actions should be discussed in advance of the survey, between the sonar
operator and the helmsman. The planned actions will depend on the water depth and
depth variation, bottom type and the danger from obstacles.
If sharply rising ground or an obstacle is detected on the ship's echo sounder it may be too
late to raise the towfish by hauling in the tow cable, particularly if the cable is several
hundred metres long. Here the quickest way to raise the towfish may be to increase the
ship's speed as quickly as possible. Of course if, despite this manoeuvre, the towfish still
strikes the obstacle or bottom the impact will be at higher speed.
Slowing to avoid an impact with an obstacle will almost certainly drop the towfish on to the
bottom, but may reduce the risk of losing the towfish.
If the towfish gets entangled, and the ship is manoeuvring above, beware of catching the
tow cable in the propeller.
Version 3.6 (2008)
17
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