I had a similar problem finding the right spot for my tranny, the way I solved it was to fix the tranny to a 1 1/2" pole and put the pole over the transom and drag it all over the ocean. Moving the pole to various positions along the transom and to vary the depth of the tranny below the transom until I got the results I was looking for.

I admit it is a very crude way of doing things but it saved me drilling a lot of holes in the transom, I ended up with the tranny on the port side of the motor and slightly deeper than that recommended on the installation instructions, but hey it works good so who cares.

Same Hoookt.....fitted one as per instructions to last rig, sweat ass as speed.

Old gaurdian hull i have now has been a pain, two different position with a variation in heights and angle tilt all for shit readings over 10knots, think the motor and leg height just give bad water wash no matter were i set the plane.

On the weekend i fitted some marine ply using silicon between the hull and the inside of the transducer, basically filled some of the void between the transducer head and its bracket to the hull if that makes sence, im running a furuno and it actually shows this idea in the back pages of the manual.

Ill try get a photo up, didnt get a speed reading but from what i noticed with the quick trip sunday it was performing 100% better.

I installed mine so that the hull projection of the transducer is 5mm below the waterline but it is within 75mm of the swing radius of the prop due to my curved transom.

LF picture is shit but Im pretty happy with my HF readings at speed. It breaks up a bit when it gets choppy though. Excuse the terrible camera work.

Shouldve had a look at mine at the ramp yesterday!

 Iv looked at yours beau. My boat deadrise very diff but

It just skips when hitting waves but if I get 2of us to lean  port side it works alot better lol. I know tinnys will be harder but bloody bracket is so low already it's half hanging lol