I had an email request to find this project from 2010… I updated the code too!
This project takes the pulse from a paddlewheel boat speed sensor and makes an NMEA output on the serial port. It should work with most through-hull speed transducers, and also with windspeed transducers too. The NMEA stream comes from the arduino serial port, so if you have a USB arduino you can hook it up to a laptop easily. If you want to hook it up to another NMEA instrument you’ll need something like a MAX232 chip to take the TTL logic from the serial port pins and convert them to RS-232 (or convert to RS-422 to meet the NMEA specification).