I put my second transducer thru hull in the mirror spot to the factory thru hull on the opposite side of the keel beam, as
@Bruce mentioned. It is just about the only solution I could come up with. The transom wall is out, the rigging for the pumps on the later model boats just took it out of play. On the 2015, with the keel beam being even more pronounced, placement and tilt is going to be even more critical. On my 2012 SX240, Yamaha installed the same brand transducer that I put in for the Garmin 70S chart plotter. The 70S does have temperature and sonar, and both work perfect in this location. Of course, you need to remove the clean out tray and get down there to verify that you don't have any interference with anything else. Once verified, I taped off, marked, and drilled the thru hole for the transducer. Then clean the area thoroughly as recommended so that your sealant permanently prevents leaks. I used LifeCaulk, which is designed for underwater use. It can be cleaned off and removed if you ever needed to replace the transducer. Don't use 3M5200, it is permanent and more cement like than caulk or sealant like, and can not be removed and cleaned off if needed. It has to be scraped and ground off, so it isn't for anything that may need to ever be changed. Be advised also, that as
@Bruce mentions, that you want to make sure that you don't run the same frequency of the factory transducer to prevent interference. Also, for whatever reason, Yamaha was installing the transducers with the same tilt as the unit I used with my Garmin, which was 20 degrees, with the keel arrow pointing to the bow! That confused the hell out of me. Apparently, they felt that it would work successfully for their purpose. I called both Garmin and Airmar (transducer manufacturer), and both said that some boat manufacturers over thought installation and did that for bow rise. My factory depth readings don't read as reliably as my Garmin so I think maybe they should have followed Airmars intent. At any rate, I followed recommendations and pointed their arrow at the keel beam as intended with my new install and did not alter or change the factory transducer. I also asked about interference between the two due to close frequency. I don't remember them now, but Airmar told me that the limited spread was two different frequencies and they would work fine co-located. I also used a c-clamp to prevent turning while tightening the transducer. Here are my install pics...
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