Flop around.. think u will find they do... bit of tolerance if hit something I think the reason is...ie no water at the ramp sitting in mud... set height and angle with at the top of the flop...
Position on the stern .. look up under the hull for any ribs, places that air can be drawn in from the bow , down under the hull out under the stern...
postion MUST BE where no air MAY go in the area of the transducer.....even 1" either side can make a huge difference

The other reason for interference is inducted electrical interferance from the transducer and its cables being routed too close to other cables.. ideally it should be on the opposite side of the boat to the other looms or at least 10" away.

Interference and air will give the symptoms u describe above.

All assuming the unit is at least set to default.

After all the messing around, moving, asking questions, research I had to do to get my transducer right.. and Im talking over 30 mph....came down to the above.
The best bit of research was looking at the same boat at ramps and trademe as to where they located on the stern.
That extender, in my opinion would not waste my time.. looking at that u are far more likely to pick up air and also prop interference.

Make sure the cables are outed secure and well away from anything else before even messing with mounting.

"close" to centreline of hull as possible
interesting.. the manual etc says starboard side of the motor and x distance from prop...which means away from the keel....if one has a keel
Mine has ended up closer than the factory recommended into the prop thu not to close to the center  approx 15".. the though did cross my mind to put moreto the center behind the prop.. but that could mess up very slow speeds maybe...???

I have set up two of these on two different boats and can tell you that the only problem is it is way too low. And there is a screw that tightens it up. As a basic guide if you look at the front of the bulb there is a mold line that goes around it. That needs to sit level with the highest point of the hull. I travel at 24 knots and get perfect reception in all depths. the water from the hull needs to brush over the bottom of the transducer. Also mine sits level, not according to the instructions but tested it and it worked so never changed it.

Surtees have a special plate to fit transducer, fitted the structure scan as close to center of boat as possible.
Pleasuretime.. that bottom photo seems like a good trans setup mine might look low in the pic above but it is only just below hull line

Tristram Grandurer 9m.

 Planing strakes.

Talking to lusties last time I had an issue with these transducers - they are apparantly designed to sit entirely under water "Torpedo" like.

.

I can see why yours is setup like that with the stern leg Steps, what does your readings on the Port side look like? Not  very good i would assume

Mines on the starboard side, and an outboard...
If too deep one ends up with spray going all over the motor.. not recommended and a wet back.....
And designed to have the bottom 1/2 of the transducer only skimming the water, as per manufactures instructions....which if wrong Im sure that world wide there would be far more controversy than there is.

2 things cause issues
1/ electrical interference.. as mentioned above
2/Air between the bottom of the transducer and water... flowing underneath

Get 1/ 100% right 1st.. if thats not right 2 is just banging your head against a brick wall, and running around in circles.
Then set up exactly was the manufacture recommends using a straight edge gong at least 1/2 m up under the  hull surface to get the height (the seam as described above) right

And study, look at the hull from the bow back, and think it through how any air that gets trapped by chines , ribs, strakes... including those that go from bow to part way down the boat.....1" either way can make a huge difference....
And u may actually end up placing on a strake, because the air trapped by it at the bow may flow from it to the flat areas of the hull... reducing friction...
Which is why I had so much trouble getting the location right on mine
If u need to go too low, I believe the reason why is that u have to get below the trapped air and location is slighly wrong

I was fortunate that at some time in our boats life it had trim tabs...and was able to use these old screw holes till finding the right position... without making new holes.


Couple pics I have posted before, from one of the instruction books I have here.. which are basically the same regardless of brand

CoastalStan wrote: Bay Marine Electronics wrote: Tristram Grandurer 9m.  Planing strakes. Talking to lusties last time I had an issue with these transducers - they are apparantly designed to sit entirely under water "Torpedo" like. They put out a great rooster tail if you do that!

I must admit it did sound strange when they told me