I just purchased an airmar transom mount transducer and the installation instructions states.

The best place to mount any TM Transducer is on the starboard side as close to the center line as permitted by prop diameter. Meaning if the prop diameter extends X inches from the center line then the trans ducer should be mounted at least X inches+ 3 inches from the center line but not in line with any ribs or chines that would cause water turbulence to pass under the transducer.

I hope this helps you.