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the SI transducer must be mounted on the transom with a clear path to both the left and right side. At the side imaging seminar this spring they were recommending using two transducers for your SI unit, mount the SI transducer high(it still must be in the water when at slow speed and using the SI) and the second transducer mounted in the normal position, I would guess this one could be mounted in the floor of the boat, there is a cable available that connects the two transducers and they switch back and forth automatically so when your at high speed the 2D transducer operates to give you normal sonar views and when you slow down the SI transducer kicks in so you can get your SI view ( you also get the regular sonar views with transducer)
The reason for this type of set up is to protect the SI transducer, if the SI transducer hits a object in the water (and since you have the have it mounted slightly below the bottom of the boat like any other transducer) it is a $480.00 fix–OUCH ! by mounting it in a higher position you are protecting it from this type of damage.
The skeeter boat center has a expert on the SI and I’m sure you can get more info on this from them. You can also call the Huminbird Customer Resource Center for information on this, 1 800 633 1468, hope this helps, good luck