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Quick Take: For less than $100, you can have a handy fisherman's (or fisherwoman's) companion. The Humminbird PiranhaMax 20 is a dual-beam sonar fish finder that detects fish below and around the hull of your boat and displays them as icons on a 4-inch grayscale backlit screen. The sonar transducer can be mounted on your boat's transom or as a through-hull fitting, and the unit itself is compact (4.4 by 6.1 by 3.25 inches) for easy installation anywhere in the cockpit. Increase your chances of landing a trophy with a built-in fish alarm or use the zoom to view bottom structure and locate that secret fishing hole. The PiranhaMax 20 also displays water temperature and depth, and it'll warn you if you're navigating into shallow waters. A word of advice, however: This device may find the fish, but you still have to catch them.