Use Garmin's fairing block to position your Garmin GT21, GT23, GT41, GT50, or GT51 series of thru-hull scanning transducers parallel to the waterline for increased sonar accuracy. If the deadrise angle of your mounting location exceeds 5°, you should use a fairing block to mount the device. Durable, marine-grade construction withstands extended exposure in a marine environment. Helps increase sonar accuracy Durable, marine-grade construction | Garmin Replacement Fairing Block For Thru-Hull Transducer