Kevin,
This is what is in my boat
1197C SI at the consoule
The HDSI transducer at the back of the boat in the center
XP-9-20 inside-the-hull DualBeam transducer
AS-Si-DB-Y transducer splitter cable
This is on an 18'6" Triton with a 150 HP Mercury with an 8" jack plate. The nice thing about the jack plate is it allows me to trim the motor all the way down and still have plenty of space between the motor and the transducer. You can look at the installation pics of mine if you are interested.
With all of this said I can keep the bottom to speeds around 40 mph with no problems, between 40 and 50 it is stetchy but seems to pick up the bottom from time to time. 50 to 60+ there is nothing. Having said all of that it also will pay to set the max depth on your unit to something around 200 feet as it will help you pick the bottom back up quicker as you come down off plain.
Hope this helps,
Ben