Until this year I have relied for depth sounding on my NASA Target 2 depth gauge which is a stand alone instrument.
I knew that the Garmin 50S had a sonar capability bur never bothered to use it. With some time to play around with the Garmin recently I discovered that its depth reading with the sonar gave different (wrong) figures. Moored in my marina berth where the depth below the NASA transducer was 5' the Garmin read 12'.
On investigating where the Garmin transducer was located I found it was in a plastic bag full of water in a 5" piece of drainpipe siliconed to the hull just behind the battery compartment. It then became apparent that the transducer was not a "In Hull" type but designed for transom mounting with a bracket for the purpose. I removed it from the hull and fixed it to a pole suspended over the transom and got even more puzzling readings 320' ?? There are two sonar settings on the Garmin 77 & 200 KHz but both showed the same spurious results.
I started to investigate the Garmin website for "In Hull" transducers and with the help of Active GPS website it would seem that I need a Garmin Airmar P79 transducer plus a 8 to 4 pin adaptor cable costing around £190. Whilst I feel like going for this option if it will give me accurate depths visible on the chart plotter there is one issue which is of concern. In the data about this transducer it says not suitable for hull thicknesses more than 5/8"(16mm). Does anyone know what the Mac hull thickness is in the flat section under the rear berth ?