Consider the outboard bracket
A possible solution would be to take advantage of your outboard motor bracket. On my 240 the bracket is on the starboard side. With the motor removed, I used the outter two bolts to attach a 15" length of flat aluminum bar, about 1.25" wide. Home Depot has a whole selection of different metals and sizes. Be sure to select a sturdy thickness. Drill holes at the top of the bar to fit the outboard bracket bolts and smaller holes on the bottom end to attach the transducer. Sanded and paint the whole rig, except for the tranducer, to avoid dissimilar metal problems. The Navman 4150 fishfinder shows speed and depth. The installation was easy. The configuration leaves plenty of space away from the prop, and the aluminum can be bent slightly forward toward the hull to further increase clearance. With the motor tilted up for sailing, all is well anyhow. Total investment was less than $200 two years ago..and still working perfectly. Although your boat's setup is probably a little different, perhaps a similar approach would work. Good luck. -Ed