Standard transducer that comes in the box with the display unit, fitted inside the hull, operating through the hull material with the oil acting as a sonic conductor.
Hmmmm i tried placing the transducer on the hull and it didnt show anything on the display screen . Then again i wasnt using anything as a sonic conductor .
It needs to be sat in a pot of water or oil with no air gaps between the tranduces and the water. i'e' it will not work if you have a dual skin hull. Cooking oil is good.
stick inside your hull with a blob of silicone,find the best spot with the transducer in a bag of water
I,ve fitted 3 or 4 transom mount transducers in this manner
While sticking it to the hull with silicone works, it is just stuck there waiting to get damaged by whatever else you may want to stow in the bilges. That's why in oil in a bit of waste pipe stuck to the hull offers it some physical protection.