I recently bought a Garmin 93sv and mounted my transducer on the bow.  About 12” to the right of the motor. Followed the pattern to set the depth    When I get on plane it will not read depth  the depth just blinks what it was before I got on plane.   I have a javelin fiberglass boat.   I put it where there where no ridges on the boat in front of the transducer.  Any suggestions on what I need to do 

Surely you mean stern instead of bow.  A pic would be helpful.

A transducer on stern (back) will not be in the water when on plane so you will not have any depth/sonar reading. Only way to have depth reading while on plane is by having an in-hull transducer. Typically you can use a transducer y cable for you brand to connect 2 transducer to the unit and use you in-hull transducer for depth.

 

Some of the older boats came with a in-hull transducer already glassed into the boat. Look for it and its cable in your battery compartment floor in front of the motor. If you have one you may also need an adaptor to connect it to your y cable. If not you will have to add one if you want depth reading on plane.