I'm getting ready to upgrade my console unit and I need some ideas on how to mount the transducer on the transom without putting any more holes in it. It'll be a Humminbird HD SI transducer.

The following bracket will mount to your jackplate.

http://www.transducershieldandsaver.com/product/tm-hdsi/

You will need the L Bracket. Which jackplate do you have?

Ummmmm the invisible kind. I don't have a jackplate.

I am going to mount mine to my transom. Take the screws out and fill the holes up with some 3M marine adhesive.

My bad, miss read your post,

Try stern mate. They are local.

http://www.sternmate.com/index.html

Next question is placement. That is kind of a big transducer so where is best place in transom to keep it in the "sweet spot"

Number one issue is it is Side Imaging and needs side clearance to "see" to the sides. Humminbird's Side Imaging covers from the water's surface to past vertical under the transducer on both sides.

Number two issue is of you want depth/2D sonar when on plane and just want to use the SI transducer for that, it has to have water contact when on plane. When the hull is on the surface, the transducer has to be just below the surface.

For a suitable fiberglass hull, most use a shoot-thru dual beam transducer (XP 9 20) in the bilge and attach the SI transducer above the hull bottom for protection from possible impact damage. MSRP of the HDSI transducer is $270.
The two transducers are connected to the display with the AS SIDB Y cable.

I was going to ask you Wayne if I could use my old shoot thru hull transducer when on plane. I have NO idea what model number it is, I use it on a Matrix 25 finder. Will it work with a 998 SI with the Y-cable? And the external GPS antenna that I use on the Matrix, will it work as well? Just kinda curious what has to be upgraded and what will still work.

Yes, that is a dual beam 83/200 kHz transducer. It can be the XP 9 20, XP 9 20 T, or a transom transducer used for shoot-thru. Basically all the same for sonar.

BUT you still may want to change it since transducers do wear out over time.

The GPS antenna will work also, just the one that comes with the 998's have more channels.

There are two 998's.

998c SI Combo has CCFL backlighting
998c HD SI Combo has LED backlighting and is brighter (easier to see in bright sunlight)

Everything else is exactly the same.

Ok that is good info. So if I use the SI transducer for trolling speed only, it needs to be level just inside the water line or sitting on top of the water or lower even. I'm thinking that the engine may block it on one side?

It has to be below the engine mount so it can "see" past the motor.

Send me a PM with a picture of your transom and the motor tilted all the way up and I will mark it with a suggested attachment location. Take the picture at the level of the bottom of the engine mount so I can see how the transom is formed. I don't have a transom shot of that hull in my picture library.

Do it like this: