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In the installation of a sonar transducer, there is a primary decision that needs to be made, generally: will it be installed as a through-hull transducer, or as an in-hull transducer?
As for where to install it on the hull, that's a broader question.
Let me ask, before I go any further with advising you:
1. is the transducer a Helix 5 Series sonar transducer?
2. did you receive an installation manual with it, or not?
Additional: having looked it up, and assuming that your sonar transducer is a Humminbird model, there is a slightly different set of choices for those, as to mounting type:
1. transom mount
2. trolling motor mount
3. plastic in-hull mount.
And it seems that the first option, transom mounting, is the standard one for Humminbird, since the adapters for the other mountings are 'sold separately'. At least, so says this page, which shows examples of those mountings and goes into detail: