wrapping the cable in metal sheilding/braiding is only the first step to protecting the transducer signal from EMI. step 2 is to connect one end (and only one end) to a grounding source, preferably at the helm
rather than at the transducer.
but if you're running in your engine compartment, then it's likely the EMI is coming from your alternator
. in order to generate electricity, the alternator
creates a very strong electromagnetic field to induce current
in the secondary windings. and that field dissipates the further you get away from the alternator.
another source of EMI can be high-power lines that are running parallel to your transducer line over a continuous length in close proximity. thats why in home install wiring
, LAN cables
only cross 120VAC lines at right angles and are never run in the same raceway. if you ran them together, the 120VAC would induce noise
in the LAN line.
that said, i would try first to relocate the transducer line to another part of the engine compartment and, if possible, not in a bundle with other wires.