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Mounting a transom transducer: where and how

A good fish finder shows a bad picture if the transducer sits in the wrong water. Garmin’s installation instructions for its GT36UHD/GT56UHD transom-mount transducers (the GT56 is our Premium Pick) set out the rules.

Where on the transom

• Clean water. The transducer must sit in clean, non-turbulent water: not behind strakes, struts, fittings, or water intake or discharge ports, or anywhere that makes bubbles or turbulence.
• Near the centerline, clear of the prop. As close to the centerline of the transom as possible, at least 15 in. (38 cm) from the propeller, and not in the propeller’s path.
• Which side. Clockwise-turning propeller: starboard side. Counter-clockwise: port side. Twin drives: between them, if you can.

How it sits

• Parallel to the water line.
• Just below the hull: about 1/8 in. (3.5 mm) below a fiberglass hull, 3/8 in. (10 mm) below an aluminum hull. Too deep can hurt the boat’s performance and raises the risk of hitting something, so adjust in small steps.

Then test it at speed

1. In open water free of obstacles, drive slowly, then speed up gradually while you watch the screen.
2. If the signal drops or the bottom fades, note that speed. Come back to it and make moderate turns both ways.
3. If the picture improves in the turns, lower the transducer about 1/8 in. (3 mm) at a time until the problem is gone. If it doesn’t improve, try a different spot on the transom.

Set the mounting bolt tight enough to hold at high speed but loose enough to let the transducer pivot if it hits something. Don’t cut the transducer cable: Garmin says that voids the warranty. Check what is behind the transom before you drill.

Source: Garmin GT36UHD/GT56UHD transom-mount transducer installation instructions. Other transducers differ: your own transducer’s instructions come first.