The Fish Hawk X4D (fishhawkelectronics.com) uses a water-pressure sensor and wireless digital data transmission to provide the precise depth of your downrigger weight. It also provides water temperature at the downrigger ball and tells you the actual speed of your lures through the water using a paddle wheel. The data reads out on a bright onboard display and can also display via Bluetooth to a phone or tablet. This technology takes much of the guesswork out of downrigger trolling.