Transducer suspended in vegie oil. square to the bottom
Rather than using vegetable oil you should use mineral oil. Other than that, what you have outlined is pretty much the way it works. It does have to be aimed through an area that is solid fiberglass, though, and will not work on cored hull boats (boats that have balsa wood in the hull) since that would aim the transducer signal through air and that won''t work. Before you invest be sure that the hull is solid fiberglass.