Oceanographers have started to use an underwater vehicle that can dive for days or even weeks before having to be recovered. While it does rely on batteries for propulsion, it is miserly in its power usage because most of the time it “glides” through the water. This sea glider does not have an engine.  It accomplish forward motion by submerged ascending and descending.  It ascends by changing its buoyancy and glides forward on the descent using a pair of stubby wings.    

The sea glider uses miniscule amounts of electrical power to operate pumps and valves to transfer oil back and forth between a bladder that is contained inside its torpedo-shaped pressure hull and another bladder that is outside.  

To dive, oil is transferred from the external bladder to the internal one. This does not change the craft's mass but decreases its volume, causing it to displace less water (lowers its buoyancy) and to slowly sink.

To surface, the oil is pumped back outside, increasing the volume of water displaced by the vessel (without increasing its mass). 

The sea glider uses mechanical energy instead of electrical energy to move oil.  It harvests that energy by exploiting the difference in temperature between the warm surface of the sea and its cold depths. The glider contains two wax-filled tubes which operate like pistons. When it rises towards the warmer surface the wax melts and expands, which compresses a tank of air. This compressed air is used as an energy store to push oil between the bladders.  

To move forward, the pitch of the craft is changed by pumping fluid towards the bow, which pitches it down for a dive, or by sending it towards the stern to point the nose up when ascending. The angle helps the craft's wings to provide a lifting force and translate some of the vertical motion into forward motion.    

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Sea Gliders   

Feb 28th 2008
From The Economist print edition