Side Imaging technology generally works best in speeds up to 10 MPH. This will vary depending on the boat hull and the location of the installation.  Compact Side Imaging transducers like those that come with the 899ci HD SI are more conducive to faster speeds due to the smaller size.  

While the standard 2D sonar built into the Side Imaging transducer will work to much higher speeds, high speed readings with the transom mounted HDSI  and MEGA SI transducers on faster boats are often limited by the turbulence that occurs around the back of the boat. The HD and MEGA Side Imaging transducers are especially sensitive to this, due to their size, so we suggest the following solution.  

The MS SI DB Y connects to the XP 9 20  (for all Side Imaging units, except ONIX, ION and SOLIX) and the AS 14 SI DB Y  which connects to the XP 14 20  (for use with ONIX, ION and SOLIX).  Both of these are Y cables that connect a  dual beam transducer (for inside the hull mounting),and the Side Imaging transducer (mounted on the transom) to the unit.

This solution will only work when shooting through the hull of a single layer fiberglass hull boat.