Finally got back to your post... The big problems with using an external transducer is how to mount it, how to keep it from stretching the cable, and how to keep the cable from being damaged when stowing. I have included some pictures to show how I solved these issues. First, I recommend you watch some YouTube videos on how to run and silicone the transducer cable down the shaft.
1) I use the high speed transducers because it does not produce as much drag, in my opinion. The transducer is mounted to the trolling motor head via a Transducer Shield bracket, which give it wonderful protection.
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2) My biggest problem has been damaging the cable. I solved this problem by cutting the end off the threaded end of a sprinkler head, cutting an elongated slot in the side, and then silicone it in place. This prevents the head from coming back too far and damaging the cable. the reason for the sprinkler head is the large neck at the base. I am sure PVC pipe or other things could be used but I wanted the extra big base for protection.
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3) I finish with two zip ties just below the head. Which prevents the height adjustment from slipping up to far and damaging the cable.
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I sure hope this helps you from buying as many transducers as I have had to buy.