Mako Skiff owners-humminbird helix 5 question

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I have a 17ft Mako skiff and had a Helix 5 sonar installed. The transducer was installed on the left side of the motor. I cannot get a solid bottom reading when on plane. Works fine at slower speeds. I have heard that I should have mounted the transducer on the right side of the motor and that would have the transducer out of the wash created by the prop? However, Bass Pro Shop didn't think that would make a difference, and that it was more the tri-hull of the skiff. Anyone had any experience similar to mine with some ideas that might help? thanks