Recap...Garmin 551S and associated transducer doesn't show water depth less than 15/20ft. It's not intermittent, it's a permanent thing. Once over 20ft, perfect. Coming back into shallow water, once under 15 ft, nothing. So there's a little hysteresis involved.
The 551 is running the latest (v5.6) software. All other plotter functions are fine.
Spoke to Garmin Technical who were perplexed...they didn't think it was the plotter but could be the transducer but they weren't sure as they'd no experience of this problem. Spoke to a respected Marine Electrician who was very clear...it's not the transducer, it's the plotter...the transducer is digital in that it either works or it doesn't
Can anyone explain to me the process by which depth readings are produced? I'm just trying to get my head around the process to try and understand where the problem might be.
* There are two frequencies...why?
* Is one frequency better in shallow water and the other in deeper water?
* If so, is the plotter/transducer capable of failing just one frequency?
* Does the transducer provide the separate frequencies or are they produced in the plotter and the transducer merely transmits and receives them through the water?
* Is there any 'intelligence' in the transducer or is it just a transmitting/receiving device that merely passes a data stream to the plotter?
Next planned step is for a Marine electrician to connect a new transducer to the current plotter, hang it over the side of the boat and see what happens. If that doesn't work, go back to the original transducer and connect a new plotter to it. If that doesn't work, open the sea cocks