It is entirely possible to mount the SI xducer low enough on the transom to get readings at high speed ...several are using this very setup ... But ...
Most high performance boat hull owners opt for the XP 9 20 And the AS SIDB Y for high speed depth readings ...
This is because these type hulls rise out of the water when on plane ...With nothing but a small "pad" of the rearward underside boat hull touching the water ...
This makes it difficult to find an optimum mounting spot on the transom that will keep the SI xducer in the water ... While the boat is no longer down in the water ...
As long as any xducer is in clean bubble free water ...sonar (any type) will work ...
Get air between the xducer face and the water ... No sonar reading ... Any brand ... Any model ... Any housing shape ...
Sonar will not work thru air ...It's that simple ...
To get the SI xducer down into the water while the hull is up on plane would require mounting the xducer below the bottom of the hull line ... This makes the $270 SI xducer susceptible to being damaged from water borne debris or obstacles that can rip that $270 xducer right out of your pocket book ...
This is why most opt to mount the SI xducer higher on the transom for idle speed scanning ... And then use the bilge mounted XP 9 20 with the AS SIDB Y cable for on plane depth readings ...
Rickie