When I first got my Blackdog Cat I install a Lowrance Elite-9 Ti with a Navionics Chart.
When traveling above 8 knots it would not pickup fish sign.
I have recently I have install a DB-9 paired with Airmar TM260 1kw along side my Lowrance. I only install the standard NZ chart on the DB-9 mainly to keep the price down and because I already had Navionics on my Lowrance.
At the higher speed I run Lowrance on the chart and the DB-9 on ACCU-FISH it will automatically mark the fish on the DB-9 chart show you can go back to them.
At lower speeds I can run both at the same time with the lowrance set to chirp.
The DB-9 marks fish will shallow water and with the TM260 you can pretty much guarantee the fish will be under the boat with the lowrance in Chirp mode the fish can be off to the side due to beam width