Alternate Transducer Installation
Lots of people have problems installing large side-scan transducers on the transom without getting interference from the outboard motors. Here's an alternate installation method that has served us well on our Glacier Bay catamaran. 

A drawing is worth a thousand words...

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Photo album of the install on our Glacier Bay

Using this method, you can avoid any interference from the outboards and low points on the hull. As a bonus, you can get very clean water flow and almost zero noise on the downscan channel when running at speed.

Some notes:

1. You might have to cut out a section on the rear of your trailer bunks for clearance.

2. You must swap the left and right channels in the HDS configuration settings when the transducer is mounted in this configuration (wire to the back).

3. The spacer is cut from a small piece of aluminum tube. (Nylon spacers from the hardware store would also work.) The front of the bracket is held in place by a single screw that goes through the bracket, the spacer, and into the hull.

4. We fabricated a fairing out of PVC pipe to protect the transducer from seaweed, logs, and idiot dolphins, but the sonar performs well without it.

Lowrance will happily sell you a second transducer and a Y-cable, but this method works just as well with only the stock hardware and a $2 spacer.