The Humminbird units sends the co-ordinates to the I-pilot ......The I-pilot has it's own GPS and uses it to navigate to the waypoint or track sent to it.

If you put an 1199 at the console, you would probably want to add an external GPS mounted as close to it's transducer as possible. This will give you the most accurate waypoint mark.....it would be good to make that one a Heading Sensor GPS.

The 959 at the stern wouldn't necessarily have to have a transducer if you had the 1199 transducer mounted there since once your network is complete you can share the transducers......You would even be able to view SI on the 959 thru the 1199.....Read more about the networking and you will see there is an extreme amount of flexibility with sharing data, transducer, GPS, Waypoints, etc.

It would probably also be a good idea to make the bow 959 a DI unit..... That way, you will also have a dedicated DI transducer for better DI clarity, and you would be able to share that on either of the other units.....The one on the stern could just be a 959 ci to save some money.