posted 07-17-2015 04:35 PM ET (US)
quote:
I tried to re-calibrate the GPS...
I don't think you can perform any calibration of the Global Positioning System. The Air Force of the USA takes care of that. It is also unlikely you can perform any calibration of your individual GPS receiver. The manufacturer embedded the algorithms used to calculate position in the receiver, and you cannot change them.
quote:
...and also move the position of the [SONAR transducer] on the back of the boat, but to no avail.
The inference I am making from your description is your SONAR transducer is a transom mounted type.
If your depth sounder only gets a bottom echo and a valid reading when the boat is not moving, the most likely cause is the position of the transom-mount SONAR transducer. Many mounts have a tilt break-away, and the transducer could have been moved out of the normal position. This can easily be determined by visually inspecting the transducer and its orientation to the hull bottom.
It is also possible that the transducer has begun to fail and has lost either transmitted signal level or lost received signal sensitivity. The bottom echo signals are weaker when the boat is moving, and might be lost if either condition has occurred. There is no way to easily verify this, other than comparison of the bottom echoes you see from your boat's transducer compared to a new one.
If you have any doubt about the integrity of the SONAR transducer, and the boat is located at some distance in a foreign land, perhaps you can buy a new transducer, carry it with you on your next visit to the boat, and have it available for testing or replacement of the existing transducer. I suspect it will be much simpler to bring a spare transducer with your than to have one sent in, but I don't know the procedures for clearing customs when entering Croatia.