I have mounted many behind the sail mast without any problems. At our shop we do not goop the transducers in. We cut the foam block smaller than the transducer on the top and wider on the bottom, goop it to the hull.
When using pour a little water into the foam block insert the transducer.
I also used this method on my Revo with good results in both fresh and salt water. Here's a link with some info on some marks on my FF. The thread is mostly on using a downrigger though.http://www.fishsniffer.com/cgi-bin/forumsyabb/YaBB.pl?num=1243374684