A major problem with open source navigation systems hs been connecting to NMEA and other proprietary navigation products like Seatalk.

Not only the protocol but the physical connections are difficult. To overcome this the Freeboard project has been developing simple low cost interface boards to connect typical navigation hardware to microcontrollers like Arduino and Raspberry Pi.

The early boards here are DIY affairs, and some skills with soldering etc are needed. They remain here as reference and for those who enjoy doing the hard yards :-)

For the the simplest solution see theĀ  Freeboard Interface Board . It provides a complete plug in solution to connecting to your boat. Its available as a kit of part, or complete, ready to use.