I have a 998ci si hd at the console, transom mount transducer, everything displays and reads fine, except for the depth reading which comes in and out. It almost always goes away, or reads 2.9' when you get on plane.
Any recommendations for what may be wrong? Its driving me nuts.... Thanks

I had the same prob with my 999 I lowered the transducer and works 99% of the time on plane now. The installation instructions says you can try to lower/angle the transducer further down. It is probably picking up to much air/turbulence. Now I am no expert and am new to this, so you may want to further research your problem. Check out this web site for further great info http://forums.sideimagingsoft.com/
Also Wayne P may pop in and give his expert advice. Good Luck

You have several options, you can try and find the sweet spot which would require a lot of trial and error of moving the ducer around, and to and from lake, unless you do it at the ramp. another option is you can use through hull transducer or is what I did and probably the easiest was mount another transom mount 2d transducer and raise the si di ducer higher to prevent damaging the high end hd ducer.

Your transom mount transducer will not work properly while on pad running, it is out of the water in most applications but even at a slow on plane speed, if the transducer was in the water you would have way too much interference from the water running from under hull to get a true reading. What I have done on my units is run an in-hull 2d sonar transducer and also run my transom-mount Si transducer. The in-hull will generally work pretty well up to about 40mph. Hope this helps.

Properly installed, a transducer will give you depth readings at 70+ mph whether it is transom mounted or shoot-thru.

The best installation for a Side Imaging unit on a suitable fiberglass hull is the Side Imaging transducer on the transom above the hull bottom and a shoot-thru dual beam XP 9 20. Those two connected to the display with the AS SIDB Y cable. Some prefer to attach the SI transducer to the jack plate if they have one.

Properly installing the shoot-thru seems to elude many. On the water testing before attaching it to the hull is required to locate the best place for it so it can give you depth at speed. When that is done, as long as the hull under the transducer has solid water contact it will work.

My transducer works fine from zero to 30 mph which is top speed for me using my 899, mounted on my transom. I run it at higher speeds all the time looking for structure.

Just did this video on understanding how to mount a transducer. Hopefully it helps