Furuno FMD-3100 Manual page 21

Scenario two: The safe draft of the vessel is not equal with any of the available depth contours for example
the safe draft of the vessel is 13 metres and the depth contours available in the ENC are 5.4-9.1-10.9-18.26
metres.
For scenario one the situation is clear (Fig 8, p22). The mariner will set the safety contour and safety depth
equal to the safe draft. The safety contour will become the boundary that distinguishes between 'safe' and
'unsafe' waters and the depiction of this boundary will be clear to the OOW.
Fig: 9 Safe draft not equal with one of the available depth contours
For scenario two the situation becomes unclear (Fig 9 above). ECDIS systems are designed in such a way that
when the selected safety contour does not coincide with an available depth contour they default to the next
deeper depth contour. In the above example the safety contour will not perform its function namely, to be
the depth boundary between 'safe' and 'unsafe' waters. This of course results in an ENC image that does not
reflect the reality and herein lays the problem.
3) Possible Workarounds for Scenario #2
Scenario two can be addressed/worked around in a variety of ways. Each one has distinct advantages and
disadvantages but all of them include the manual drawing of No-Go Areas so that the safe area is easily
visible to the navigator. Any approach to this No-Go area will give a visible and audible alarm to the navigator.
Without drawing the No-Go Areas none of the following can be considered safe.
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