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Soup, you absolutely do NOT want to mount it where you have it taped on in the photo - that area will experience a lot of turbulence and, in any event, be out of the water at speed.

If it were me, I would mount it directly below the scupper hole shown in the picture; that will be in the water even if you are planing. It probably violates the 15 inch manufacturer's recommendation, but that's not a hard and fast rule - not like it's going to work fine at 15+ and suddenly fail at 14. If it will fit, you could even run the transducer cable through the scupper hole for a cleaner installation.