NMEA2000 is a marine version of the CAN bus developed by Bosch in 1983 for autos. It has a bus structure that lets several devices to connect to a single cable, allowing all to listen and talk to each other. Power is also distributed over the bus which simplifies layout.
NMEA2000 has been adopted by several instrument systems, implicitly creating a sensor standard. This has increased the market size and therefore reduced costs. However, most of this market is powerboats, so one of the sensors most important for sailing (boatspeed paddles) has lagged behind (here’s why paddles are important).
Numeric displays are OK for CAN bus, but its low speed, address space and small packet size limits it to sensor data rather than e.g. video feeds and off-the-boat data.
Another problem with NMEA2000 has to do with calibration. In sailboats, wind and boatspeed need to be very accurately determined, but NMEA2000 sensors generally do not support calibration. This means that for sailboats that use NMEA2000, there should be a “dirty” bus supplying uncalibrated sensor readings to the processor, and a “clean” bus with calibrated results. The Ockam system uses NMEA2000 as a sensor input bus with output onto Ethernet/WiFi and RS232.
Maretron provides a handy design tool for laying out a NMEA bus.
These sensors will require shaft extensions to get them out of the sail upwash zone.
- Airmar ST-850-N2K paddle
(We do not recommend the Airmar DST800 (depth/speed/temperature) sensor because the paddle is quite small and therefore vulnerable to boundary layer flow variability)
GPS and Compass