1st - The 15" minimum in the installation is for outdrive boats ... Outboards need only be concerned with not mounting the xducer directly in front of the lower unit water intakes ... because the xducer can create turbulence (air) to enter the intakes ... And do not mount the xducer so close to the lower unit that the motor hits the xducer housing at full lock turn to that side ...
2nd - it's not unusual for the screen to fill with what looks like clutter as the boat speed increases... This is merely aerated water (bubbles) being created from the boat hull ... (Each area of the transom will have a different amount of aeration creation)...
While it is entirely possible to get better (cleaner) Imaging ... It is always tied to finding a spot on the transom with the least amount of turbulence being create by the boat hull ...
Sometimes this means moving the xducer up/down/over/ or even rearward ...
Be patient and make minor adjustments until the "sweet spot" for your specific hull design is obtained...
And even realize - how much weight is in the boat at any given time will change how the hull contacts the water which may also cause different results between tests ...
Rickie