In-hull, or ‘shoot-through hull’ transducers can transmit and receive sonar, or ‘sound pulses’ through the hull without needing direct contact with water.
On our PBO Project Boat Maximus we need sonar simply for measuring depth, but the advanced electronics of fish finders and forward-looking sonar can show shoals of fish and give an accurate picture of the sea bottom.
An in-hull transducer base unit is glued to the inside of the boat with silicone or epoxy, and filled with propylene glycol. The separate transducer then usually twist locks in to the base. The advantages are there’s no need to drill a hole through the hull, no drag or risk of fouling, and it can be installed and maintained while the boat is still in the water.
Raymarine’s P79 in-hull transducer adjusts to hull deadrise angles up to 22° (deadrise is the angle between the boat bottom and a horizontal plane either side of the keel.
It needs to be mounted so as to be below the waterline at all times. Typically a 600W transducer can transmit through 12-16mm of glassfibre, but can only be mounted on solid fibreglass laminate – no foam cores or plywood.