Humminbird XAP-9-20 Angled Puck Transducer Inside Hull Transducers mount to the inside of most fiberglass boat hulls and transmit the sonar signal through the hull. On hulls with a deadrise less than 15 degrees, a flat bottom transducer should be used. For angles greater than 15 degrees, an angled transducer is recommended to assure the sonar signal travels downward.