Chris German

Senior Scientist
Geology & Geophysics
Chief Scientist for Deep Submergence
Woods Hole Oceanographic Institution


Oceanus 444: World-beating AUV tractor-pull

I only got to sail on Oceanus once - with you as SSSG and me as Chief Scientist for cruise 444 (April '08) to demonstrate Sentry's readiness for the deep ocean.  I'll admit, Sentry didn't necessarily cover itself in glory that cruise (from my records, Sentry dives 001-008) but on the upside:-
(i) it didn't need rescuing by Alvin this cruise - so it was an improvement on my only prior Sentry work at that stage (Bermuda, April 2006) and
(ii) it did set a new record of the world's longest AUV tractor-pull across the seafloor.  A record that, as far as I know, still stands today.

One of the neat things about Sentry compared to ABE is that with its new streamlined shape it can make its way through the ocean very smoothly.  For Sentry Dive 007, on April 22, 2008, Sentry was released over the side of the ship at 14:11 local time and settled gracefully to the seafloor >2000m below in just over an hour.  As programmed, it then set off - tracked by our acoustic navigation systems - toward the first way point of its planned survey.  But wait, Dana Yoerger, expedition leader for the cruise, noted how fast this vehicle was moving and it seemed to be only about half the expected speed.  He checked again, and now it seemed to be going slower...   and slower.... and slower... and stopped!

Only 25 minutes into the mission and the vehicle was (literally) at a stand-still.  Yet everything had seemed to be going so well?

Another 45 minutes passed and then Sentry's rescue software cut in, Sentry dropped its weights and started to head back up where, another hour later (17:25 local) Sentry was spotted sitting unusually low in the water and then, as we lifted it inboard, we worked out why as large volumes of red-brown slurry washed out from under Sentry's skins.  Here is what we surmized:

Yes, Sentry had dropped to the seafloor as planned; and yes, it had moved off across the seabed as it was meant to, subsequently.  Trouble is, what it had "forgotten" to do, in between those operations, was to drop the extra descent weights that are only supposed to be retained long enough to carry Sentry to the seafloor.  So instead of gliding gracefully through the ocean, a few meters above the seabed, what Sentry had been doing instead was to set off along the seafloor, ploughing sediment out of the way until - even with its new improved streamlining and the advantage of its new power-packed Lithium ion batteries on-board - it had dug itself in and could progress no further.

Nevertheless, it was a valiant effort - more than 1000m carried along track in under 25 minutes (15:21 to 15:45 local). This remains, to my knowledge, the longest deep-ocean AUV tractor-pull on record.