The 15" distance is to keep the xducer from causing turbulence in front of the prop...which could cause your prop to "spin out" ...which could cause potential damage to your motor and outdrive...
If you are unable to mount the HDSI low enough to eliminate left SI blockage...you have the option of using a 2nd HDSI xducer on the opposite side of the outdrive...
You will need the AS SILR Y cable for a dual HDSI setup...which disconnects the left SI piezo in the starboard HDSI xducer and instead wires in the left SI piezo of the port HDSI xducer...(the unit will still "see" this setup as a "single HDSI xducer" ...because the AS SILR Y cable is merely a cable spitter...)
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If you are also having difficulty getting high speed 2d depth readings from the the HDSI xducer ...you also have the option of installing a XNT 9 20 Dual Beam xducer epoxied in the bilge...
You would also need the AS SIDB Y cable for this setup...which disconnects the 2d piezo in the HDSI xducer and instead wires in the 2d piezo of the bilge installed XNT 9 20....(the unit will still "see" this setup as a "single HDSI xducer"...because the AS SIDB Y cable is merely a cable splitter)...
So to set this all up you would ....:
*Plug the starboard HDSI xducer into the "starboard leg" of the AS SILR Y cable ....
*Plug the port HDSI xducer into the "port leg" of the AS SILR Y cable...
*Plug the AS SILR Y cable into the "HDSI leg" of the AS SIDB Y cable ...
*Plug the bilge mounted XNT 9 20 Dual Beam xducer into the "Dual Beam leg" of the AS SIDB Y cable....
*Plug the AS SIDB Y cable into the back of the head unit....
*Set the "Transducer Select" menu to "Hi Def SideScan"....
The unit will "see" this entire setup as 1 single HDSI xducer without any settings changes....
Follow the instructions at the link below to test and install a shoot thru hull XNT 9 20 Dual Beam xducer for high speed depth readings...(start at post#2 in the thread at that link)...
http://www.bbcboards.net/humminbird-sonar-gps/172972-installation-networking-information-resource.htmlRickie