# Fitting a Fish Finder or Chartplotter: Transducers, Power and Mounting **By Sawley's Auto and Marine** · 2026-09-19 A chartplotter or fish finder is the single upgrade that changes how a boat gets used. It is also the one most often let down by its installation: a good unit, wired to the wrong place, with the transducer bolted where it cannot see. Half of the "it doesn't work at speed" calls we get are a transducer problem, not a sounder problem. ## What are you actually buying? Three things in one box, and it pays to know which ones you care about. **The display.** Screen size drives price more than anything, and the right size is the one you can read from the helm in full sun with polarised glasses on. A seven-inch screen is fine on a small tinnie where you are a metre from it; on a cabin boat with the screen an arm's length away you will want nine or twelve inches, and it is much easier to live with a screen that is slightly too big than slightly too small. Touch is convenient at rest and frustrating in a chop; the better units give you buttons as well. **The sonar.** Conventional **CHIRP** sonar is the general-purpose fish finder: a cone under the boat, good depth capability, clear returns on fish and bottom. **Down-imaging and side-imaging** give a photograph-like picture of structure — reefs, weed edges, drop-offs — which is what most gulf fishermen actually want when they are looking for whiting ground or a snapper reef. The **3D sonar** on the top units draws the bottom in three dimensions and is a genuine tool on structure, if you fish that way. The Raymarine [Axiom range](/collections/raymarine-axiom-chartplotters-marine-displays) offers each of these, and the [Element series](/collections/raymarine-element-s-hv-gps-fishfinders-sonar-displays) puts the down and side imaging into a simpler, keypad-driven unit at a lower price. **The charting.** The plotter half. Check that the chart card covers the gulfs in detail, because the value of a plotter on the Yorke Peninsula is knowing where the reefs and the shallows are before you find them. ### The transducer is the job The display will work anywhere. The transducer will only work in clean water, and finding that spot on the transom is most of the skill in the installation. - **Away from the propeller.** Mount it on the side of the transom opposite the propeller's rotation, clear of the wash. - **Clear of anything ahead of it.** Strakes, rivets, planing plates, a transducer for another unit, a live-well intake — anything upstream throws turbulence and bubbles into the transducer's path, and bubbles are what the sounder cannot see through. This is the usual cause of a screen that goes blank above a certain speed. - **Level, and at the right depth.** The face should be parallel to the water surface with the boat on the plane, and just below the hull. Too high and it comes out of the water at speed; too low and it acts as a scoop and throws spray. - **Sealed.** Every screw hole in a transom is a leak in waiting. Fill the holes with marine sealant before the screws go in, not around them afterwards. Through-hull and in-hull transducers avoid the transom entirely and give a cleaner signal on larger boats, at the cost of a hole through the bottom or a fibreglass-only hull for the in-hull type. The [Raymarine transducer range](/collections/raymarine-transducers-sonar-transducers-sensors) covers all three styles; matching the transducer to the sonar you have bought is the first question to settle. ### Power and mounting Wire the unit to the switch panel through its own fuse, on a circuit that does not drop out when the engine is cranked, because a plotter that reboots every time you start the motor is a plotter that loses its track. Voltage drop over a long run makes electronics misbehave in ways that look like faults in the unit, so use cable sized for the distance. On a boat with more than one device, the NMEA 2000 backbone ties them together — engine data on the plotter screen, position on the radio — and it is much easier to fit at the start than to retrofit. Mounts, cables and backbone parts are in [chartplotter accessories](/collections/raymarine-chartplotter-accessories-mounts-cables-modules). Mount the display where you can see it without leaning and reach it without letting go of the wheel, out of the direct line of the sun if the helm layout allows. A bracket mount suits an open boat where the unit comes home with you; a flush mount looks better on a cabin boat and is harder to steal. ### Then go and tune it The first trip after fitting is a sea trial for the sounder. Run at each speed and watch for the point where the bottom drops out, which tells you whether the transducer is in clean water. Adjust its angle a few degrees at a time. Set the depth range and the gain by hand once rather than trusting auto forever; auto is a good starting point and a mediocre finishing one. We fit and wire sounders, plotters, transducers and the NMEA networks that join them as part of [device and accessory fitting](/pages/device-accessory-fitting) at Kadina, and we test them on the water before the boat goes home. If you would rather buy the unit and have it fitted right first time, ring [0412 532 984](tel:+61412532984). **Tags:** Electrical, Guides, Raymarine --- > Source: [Sawley's Auto and Marine](https://www.sawleysautoandmarine.com.au/blogs/guides/fitting-a-fish-finder-or-chartplotter)