Furuno DS-30 Operator's Manual page 37

Interpreting the display
No
1 Transverse speed at bow
2 Longitudinal speed
3 Reading change as follows whenever the MODE key is pressed
Transverse speed at stern (over-the-ground or through-the water)
Rate-of-turn gyro or gyrocompass connection required*
Rate-of-turn speed
Rate-of-turn gyro or gyrocompass connection required*
Depth (Under-keel Clearance)
Heading
0° is displayed in the case of no gyrocompass connection.
Course (over-the ground or through-the-water)
Look at the display window of the longitudinal speed at bow to
judge whether the course over the-ground or through-the-water is
displayed.
With no gyrocompass connection, drift angle is displayed.
*: When rate-of-turn gyro or gyrocompass is not connected, these display read "---".
Display Item
8-4
Indication/Remarks
Speed over the ground or
through-the water is
displayed depending in the
tracking mode.