Depth below keel: store depth transducer to keel

[W-Geronius]

DBT values as provided by basic esp32-nmea2000 hold 'depth below transducer' values. In everyday use this is not a common reading - 'depth below keel' is much more popular. To calculate this, the distance from transducer to the keel's bottom should be stored in the OPB60 settings. From boat draft and transducer to keel even 'depth below water surface' could be provided.

[norbert-walter]

I think it is a good idea. But any instruments have a own offset setting and give out on the bus the adjusted value.