Got an HDS 12 at the wheel with full functionality. In your opinions and your experience do you need or prefer the same (sonar, Si, Di, etc.) at the bow?

DI and GPS. No need for SI, JMO

I have a HDS-9 at the bow with DI, sonar, I did not buy the transducer for SI because it becomes distorted and basically useless when turning the TM. You could network and run SI at the bow without distortion, but I'm typically wanting to see directly below me. Also, when running DI at the bow, a lot of TM will cause interference, run a ground from your TM and I also went as far as putting a thin sheet of lead I got of Amazon between the transducer and the TM, worked great.

No Si makes sense.

Is the HDI skimmer mounted on the TM the best option for this setup?

You can mount it on the keel or the trolling motor.

I would agree that you don't need sidescan at the bow with a Lowrance unit.
But I will say that I had a hummingbird unit with side scan at the front of my old boat and I could use it almost like 360 imaging. Very handy for seeing deep structure.

I have been exploring the same line of thought for my new boat.
Just using sonar and DI. And maybe at times some GPS just to latch on to my way points.

I also saw a video on Youtube and one of the pro anglers put his
larger graph on the bow, said it was easier to see when standing up,
and thats where he spent most of his time.

I also saw a video on Youtube and one of the pro anglers put his
larger graph on the bow, said it was easier to see when standing up,
and thats where he spent most of his time.
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I've heard that as well but I'm usually up there fishing and not searching. Mostly. Conversely, when I'm at the wheel I'm doing a lot of idle motoring for research and confirming what could be under the surface after studying a topo map.

Really?

I have my 1199 at the trolling motor with 360. I rarely use side imaging anymore.