Modify a NMEA183 sentence in Signal K (Solved without needing to modify)

[Jodel]

I have worked around the problem.  I manually added direct connections to Opencpn in its connections tab and it reads the DBK sentence correctly. So now I have nothing going through Signal k, but my next step will be to route all the NMEA from  Opencpn to signal K .
This seems to be the reverse of the thinking in Openplotter but so far seems to work better for me.  Any disadvantages to this approach?
Jodel

If you plan to always be running opencpn when the Pi is on then should be OK. Might be worth an ask on the signalk slack channel> http://slack-invite.signalk.org/
They quite like the feedback. I thought every nmea sentence coming in went straight back out again on port 10110 but apparently not, my wind sensor sends some none standard ones which don't make it. Thanks for posting, it helps bringing obscure things like this to light 

Thanks for that.  As I will be using the set-up purely in the context of navigation as opposed to monitoring sensors etc. I should be OK.  As I have the boat in commission on the water  about 15 mins. from my house I will be able to play around with the various options over the winter months. 
I was surprised that Opencpn dealt with the non standard NMEA DBK sentence and ignored the extra first field.
The Stowe instruments  that I have are probably considered obsolete now but as they are working well and put out all the data in NMEA 183 they are proving easy to integrate in Openplotter.
My next task is to input the route data from Opencpn to the Stowe Databox and see if it sends it out  with the other NMEA stuff.
JOdel