Hey Ben, on my G3 Angler, if I drop my transducer where you have yours I get a rooster tail spray on my engine cowling but I get depth readings up to 45-49mph. When I raise the transducer up a 1/8" the rooster tail is gone but so is my depth reading over 8mph. I to have the poly plate where I can make adjustments to the transducer. I would recommend you experiment with moving the transducer around.
Being we have an aluminum boat and the traditional in-hull transducer will not work, I simply have my electronics on full view chart and watch the depth contour to keep me safe when traveling across the water at high speeds.
I have been looking at the following (link) transducer but am skeptical... http://www.radarsonics.com/AlumaDucer.htm
I may order one and try it on my boat as an alternative for our aluminum boat customers wanting an in-hull transducer if it is proven to be worthy.
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