Someone else had issues and I had issues from a beach launch.Was working perfectly well till then.Once home and by chance noticed the transducer had tilted up a bit,tilted back down to where I thought was right and now works.Added a speed paddle to it and thought it was interference but no just angle of transducer and it wasnt up much either .

Initially I had it a little flatter and about 2mm higher at the bracket but picture had a lot of noise however still gave me a bottom reading when planing.
I don’t believe the current position is to radical but then I’m no expert. (Scuppers are always up when at sea & yes I have the latest software)

I had a lot issues reading on our old 70s hull, that when made sounders wher not in the relms of the ave person.
 In the end I had to drop, as suggested in other posts, the transducer a few mm deeper than the manufactures  and other experts instructions.
 I ended up still maintaining a slight angle... the angle between the join at the front to the bottom at the back of the transducer.

Then caused another issue, mist spray going over, and being dragged into the engine. This is very bad for the engine, slow death.
So using a 5L plastic container I cut a template that is held in place behind the transducer mounts. 8 or 9 yrs on never got around to making a stainless or alloy one.



Side note: that rope /bungy on the ground is for solo launching.
 I actually use it for all launching now.
One end goes on the winch pillar, other on the front bollard.
Boat slides off, bungy starts to stretch  as boat drops off the end roller, and stops just before meeting the end of the white rope. Boat then floats back and its on the end roller of the trailer.

That should work. Make sure that you use the seam on the left edge (looking forward) of the transducer, not the centre of the transducer. Mine didn't work until I dropped it down to use the left front seam edge. I had used the seam at the centre of transducer previously.

Losing the bottom at 17 knots means you are close to the sweet spot, mine was losing the bottom at similar speeds, another couple of mm down should do the trick, am going out to the coast today, will take a pic of the adjustment marks to show how little it takes.

Sorry, never got to the coast today, next weekend am going for the long weekend so will take pic then.

Will drop it down another 2 mm and see what happens next time I’m out which should be this week. At the moment I’m set exactly as Lowrance suggest. I’m guessing that as I go faster the boat is rising more out of the water so hence the need to lower it.
Be back in touch soon. Thanks for all the tips.

Sent a bunch of photos through to Lowrance tech support who suggested lowering it. They also mentioned that these longer T/Shot transducers are very position sensitive but that they don’t have inherent faults so that was good to know.
Just waiting for weather to clear up here.

I would imagine that will work very poorly. Every time my readings have deteriorated it has been due to the rear of the ducer raising. Happy to be wrong though. 

(The cable is bunched with the other cables so this is on the to do list)

 Modern transducer cables are no where as sensitive as on 'old days' Even so the manuals do say run separate and good practice is run up the opposite side of the boat to the main loom..

My main loom is high up under my gunnels. May transducer cable runs hanging a few inches below on long cable ties that hang from the main cable loom with smaller ties top and bottom of the large cable tie loop.
 I did not do it because of best practice or anything.. did it because bit lazy and what manual says about routing. It was easier to hang than put the new transducer cable in the old loom. or tie up to it.
 Its a long shot as what you are getting is not so much electrical interference but lack of signal on the screen... but could also be due to more interference from the high rpms created ed by the engine.

Often things like this can be a combo of issues.
Another thought.. is the head unit up on the dash in full sun behind the windscreen..? On days like that I throw a rag over the back of the head unit.

Before posting this just been back up to look at your pics..
 Couple things on those original pics
1/ how close you are to the outside chine. A lot of air is picked up at the bow and travels down the underside of the chine.
2/ The rear view.. the inside (motor side) of the  transducer is far too high.. That inside edge, where the moulding join between the top and bottom of the transducer needs to be at least or lower than the straight edge off the bottom of the hull.

So I would be looking at dropping till the top edge of the transducer on the engine side is at the straight edge or the hull.. or few mm lower..
 Then look at raising to sweet point  rather than lowering.
 
Either way the mist kicked up from the transducer is not good for the engine and may get wet backs at the helm as it is sucked back into the boat.