The Humminbird XFM 9 20 Plastic Flush-Mount Thru-Hull Transducer is an economical alternative to bronze transducers, and works with thinner hulls such as aluminum. Constructed with an internal, steel sleeve that provides extra reinforcement, this thru-hull transducer is not recommended for trailered boats, and Humminbird recommends you have it professionally installed due to the fact that it requires a hole be drilled in the hull. The XFM 9 20 is a dual-beam, flush-mount transducer with 20-, and 60-degree beam angles and operating frequencies of 200 kiloHertz and 83 kiloHertz.