The PRAWLER, a vertical profiler powered by wave energy

We describe a new, wave-powered profiling instrument designed as one part of a low cost, easily deployed, open ocean mooring system. The PRAWLER (PRofiling crAWLER) is a small, 15 kg instrument that uses the motion from an ordinary surface buoy to traverse the upper 500 meters of mooring line by rectifying the vertical motion with a pair of cam cleats. After climbing up to a commanded depth, it free falls and obtains a continuous ocean data profile at a terminal speed of ~30 cm/s. In Pacific and Atlantic Ocean deployments we easily obtain 20 to 30 profiles per day. Two sets of opposing cam cleats permit two additional modes: fixed and climbing down. When fixed to the line, the instrument can obtain a continuous Eulerian time series identical to a discrete instrument, or can park at depth to avoid unnecessary profiles to save power and protect from bio-fouling. It is also able to climb down the wire, needed when equatorial currents pull a mooring over so far such that the drag vector overcomes the instrument net weight. These four positions of the cam cleats are controlled by an ultra-low power microcontroller and a motor that uses about 5 joules per profile. The PRAWLER is presently equipped with a Seabird pumped CTD and an Aanderaa Dissolved Oxygen Optode. PRAWLER data and commands are transmitted via an inductive modem to the surface buoy, and sent along with buoy meteorological data by Iridium/RUDICS to shore. Powered by lithium batteries the endurance with 8 profiles per day is about one year. Deployment results and engineering observations are presented.