Lowrance ST-TU Transom Speed & Temp Paddlewheel Sensor

The Lowrance ST-TU does two simple jobs, and it does them well. It reads your boat speed off a small paddlewheel, and it reads water temperature, then feeds both numbers back to a compatible Lowrance sonar or GPS unit. One transom-mount sensor, two readings you actually use every single time you head out on the water.

This one is aimed at boaters running older Lowrance electronics. It's the genuine service sensor for a big spread of units. That includes the Elite-5 and Elite-5x, the FishEasy and FishElite lines, the FishMark and FishStrike sounders, the SeaChamp, SeaCharter and SeaFinder DF series, the Mark-5x and Mark-5x Pro, the M56 and M68C S/MAP heads, and the X67C and X96. If your speedo or temperature readout has quit and you've still got years of good use left in the head unit, this is the part that brings those numbers back. One quick note before you order: match the sensor to your unit's connector and the readouts it supports, since the older lineup used a few different plugs over the years.

Water temp is a lot more than a nice-to-have. It's how you find the thermocline, track a temperature break where bait stacks up, and read the water when a bite window starts to open. Speed over water tells you when you're in the right trolling range and keeps your lures running the way they're meant to. Get both on the same screen and you're fishing with real information instead of a guess.

For trollers this matters even more. When you're pulling spoons or crankbaits, a half-knot swing in speed changes how a lure runs, and a lot of the time the fish tell you which side of that line they want. Watching real speed over water, not just GPS speed over ground, lets you account for current and tune your presentation until you start marking hookups. The temperature reading rounds it out, because pattern the temp and you often pattern the fish.

Install is transom-mount and about as simple as marine electronics get. The bracket goes on the transom, the sensor rides just below the hull bottom, and the cable runs up to the unit. A couple of things matter here. Get the paddlewheel alignment right, roughly an eighth of an inch below the hull on a planing boat, or your speed can read high or low once you climb up on plane. And go easy on the bracket screws. The housing is plastic, and overtightening is the quickest way to crack it on the first hard chop.

A paddlewheel is a moving part, so it wants the occasional look. In brackish or dirty water it'll pick up growth and start to drag, and a fouled wheel reads low or drops to zero. A quick clean gets it spinning free again. The temperature side is accurate to about half a degree, though it lags a touch, thirty seconds to a minute behind a fast change, which is completely normal for any in-water temp probe. None of that is a knock on the sensor. It's just how paddlewheels work, and knowing it up front saves you a needless service call.

It's a direct-connect sensor, not a networked one, so it wires straight to your compatible unit with no extra hub or module in the middle. On bigger boats where the run to the helm is long, plenty of owners add a short extension to the cable and it handles that without trouble. One tip once you're wired in: if the screen shows dashes instead of numbers, check your menus before you blame the sensor. A blank speed or temp readout is very often just a source that needs turning on in the unit's setup.

Lowrance has discontinued the ST-TU, so we carry it as a replacement and service part for the boaters still running these units. If that's you, it makes sense to grab a spare while they're around. It's a genuine Lowrance sensor, the same 000-0099-75 that shipped with these systems, and it's the honest way to keep an older setup reading true on speed and temperature for a good few seasons yet.