The NASA Electromagnetic Log and Data Box measures boat speed using no moving parts. It is robust and accurate, generating an alternating magnetic field below the boat and measuring the resulting electric field produced as the boats moves through the water.

With a resolution of 0.1 Knots across a range of 0.2 Knots to 40 Knots, the control box will convert sensor signal to boat speed as well as trip distance and total distance, and is able to send it to any compatible NMEA-0183 display.

Specifications:
Speed Range: 0.2 -40 Knots.
NMEA-0183 output sentences VLW+VHW.

Transducer Dimensions:
Hull Skin Fitting Outer Diameter: 50mm
Hull Skin Fitting Inner Diameter: 40mm
Hull Skin Fitting Height: 70mm 
Complete Transducer Overall Height: 115mm (Allowing 30mm minimum height clearance for cable)
Cable Length: 5.0m