I have a 17.5 ft seanymph and is layed out like a great lakes walleye boat rather that a bass boat, deep and more room behind console and smaller bow deck vs large bow deck and little area behind console.  I currently have a 899 hd si on my console and have been turning it around to us it while at the bow. I want a unit for my bow for jigging and dropshotting.  I am looking at either the helix 7 sonar only(non GPS), helix 7 sonar/di only(non GPS) or the 859 hd.  Not really considering the helix 7 GPS/sonar or helix 7 GPS/DI because they are priced the sames as the 859 and I really like the preset soft buttons of the 800 series but if the masses can make a argument then I am open to changing my mind.   I have no intention at this time to upgrade my trolling motor to an ipilot link.  The helix is cheaper/brighter but I would still have to use my 899 for GPS and mapping.  The 859 would be able to network with my 899, has GPS, would be easier to use ipilot link if my current trolling motor goes **** up, and would allow me to move the 899 to stay pointing towards the rear of the boat to allow other people to use it for jigging/dropshotting, or would there be interference between the two unit running at the same time?  So the way I see my choices are cheaper and no GPS or more money, GPS and networking (which I don't fully understand the functionally of other than sharing waypoints.)

 

Anybody have a similar situation and can share their insight.

The 899 and either version of the 859 (859ci HD Combo and 859ci HD DI Combo) will share GPS data, sonar technologies, transducers, temp sensors, and Ethernet accessories.

Sonar technology sharing includes viewing Side Imaging on the 859 from the 899. The Helix 9, 10, and 12 series, 9X9 and 11X9 models will do the same.

Or since would a second transducer mounted on the trolling motor be a more cost effective solution. 

Explain what that means

I would have my si tranducer on the transom like a typical setup. I would buy a second transducer and mount to the TM. When I start fishing I can just power down the unit, unhook the power and rear tranducer, mount the unit on a second ram ball,plug in new power cable and transducer. Now i have a unit reading off the TM, if we go to drive somewhere else the just reverse the process.

That would work well. I did that with my first Side Imaging unit. It wasn't much of a hassle since I spend a lot more time fishing than riding around.

Moving it would work, but if you're moving a lot it could be a nuisance. If it were me I'd get the 859 and network it with your other unit. You'll have mapping at both positions, shared waypoints, and other data. 

Not sure the cable connections will hold up for very long with constant plugging and unplugging!!