Here's a recent discussion. Search function is funky but give it a go - lot of great stuff on this site. Silicone blob that doesn't block the flow of water along the keelson (don't want to block waters flow to bilge pump) anywhere forward of the motor should be fine. substance you place it in(silicone, mineral oil, ultrasound jelly,whatever) should be of consistent density and transducer carefully set in it without voids or air pockets:
http://www.classicseacraft.com/forum...rev=#Post67202