Garmin 160 Blue Problems

I have recently bought a Garmin 160 Blue fish finder.

The unit loses lock on the botton and will not show depth once you start moving much greater than about 5 kts. Once you stop it takes upto 60 seconds to give you a updated bottom sounding.

The main unit has been back to Garmin and I am on my THIRD replacement, and on my second transducer.

The transducer is mounted about 2-3" outside the prop arc and about 0.25" below the transon, measured to the bottom of the transducer. and mounted level with the water (at a angle to the hull, being a V.) I have tried diferent transducer depths with no success.

Boat is a Ribcraft 4.8

Has anyone else had there or similar problems?.
Is it just my setup or does Garmin have a problem>?.

Any comment welcome as I am stuck.:(

I had a Garmin 235 (with fishfinder). The unit worked very well EXCEPT the fishfinder. I got the unit replaced 5 times and had my installation checked professionally by two diferent companies.
I guess Garmin have a problem in this area. Their new for old replacement policy is fast and efficient and unquestioning. They admit though they NEVER analyse failed returns so they never find out whats wrong.
Seems daft to me too, but thats company policy for you.

The fact that it works at up to 5 knots indicates a problem with the positioning of the transducer. Turbulence or prop noise could be the cause. Check that the hull area in front of the transducer is clear of any protrusions. Also, 3in outside the prop arc still sounds much too close to me. Most transducers I see are mounted about midway between the keel line and the point where the transom meets the waterline.

Mike