I'd follow the install instructions from airmar which are pretty much what I posted above. the p79 is not a normal puck transducer. the CHIRP Models are: P75M, P95M
http://www.airmartechnology.com/uplo.../17-217-01.pdf
6. When you are satisfied that the location of the transducer is
optimal and the orientation of the base corresponds to the
deadrise angle of the boat, apply a generous bead of silicone
sealant to the underside of the flange of the base. (Follow the
sealant manufacturer’s instructions for its use.) Press the flange
firmly in place to form a liquid-tight seal. Allow the sealant to cure