Olly Olly Oxen Free

Doug Levin is the Associate Director for the Center for
Environment and Society at Washington College in Chestertown, Maryland,
and is an expert in underwater exploration technology, as well as
designing fun programs that teach complex engineering concepts.

I’ve been peppered with questions about the safety regarding the DEEPSEA CHALLENGER sub’s return to the sea surface. My colleague and fellow blogger Jim Chiles wonders why the Mermaid Sapphire moved so far from the sub’s launch point where it waited for the sub’s return and subsequent recovery. My basic answer is I don’t know the specifics. But then I got to thinking about it. I’ll do what I do with my students and answer this with a question and experiment.  

Mr. Cameron’s sub was down 36,000ft (and change) when it started its semi-controlled ascent.

Question: How do you predict where on the ocean surface she’ll reappear, and how do you make sure that “Murphy’s Law” (what can go wrong, will go wrong) doesn’t strike and cause her to rise up directly underneath the Mermaid?

Experiment: Let’s gather fishing bobbers of different shapes, like those presented in the accompanying photograph.  Now, grab a 5-gallon bucket and roll up your sleeve (yeah, you only need one arm). In this exercise, you’ll bring each of the bobbers to the bottom of the water-filled bucket and do two things: 1) Release it and watch it as it makes its way to the surface, and 2) try to predict exactly where at the water surface it will breach.

 A Variety of fishing bobbers presented on http://www.learnhowtofish.com
Which of these shapes most resembles the sub?  I say the one all the way to the right.

Before you start, make a list of the bobbers and write next to them how you think they’ll behave when floating to the surface. Will it be a straight shot? Or, will they waffle or waggle (technical speak) on the way up? Make a “map” of the water surface and where you think each bobber will breach.

After you make your predictions, its time to test them!  Release each bobber from the bottom of the bucket.  For each one, before you release, point to the water surface in the bucket and see whether the bobber hits your mark.



Now, when you’re finished with the bucket exercise, consider that what
you’re doing is in a bucket about 70cm deep–with no horizontal currents.
How accurate were your predictions? Then, consider that Mr. Cameron’s
sub was rising through a water column of 10,000 meters.  What were the
currents pushing it horizontally? Were they flowing in different
directions at different depths?  What if they were flowing north to
south between 7,000 and 5,000m and then changed east to west for 5,000
to 3,000m? 

Then take into account the shape of the sub. Assuming it’s staying
upright, perhaps its hydrodynamic enough to come straight up. What if
something malfunctioned and the sub tipped off “plumb” (i.e. it listed
diagonally)? Would it still come straight up? Or, would the change in
the sub’s straight up/down orientation within the water column cause it
to slip to the side? 

All of these possibilities would need to be taken into account as the
Mermaid Sapphire moved away from the sub launch point. It would have
been disastrous if the impossible happened and the sub came up directly
under the Mermaid Sapphire. The bottom line…the DEEPSEA CHALLENGE game
plan worked.  This sub will rise again…and again…and, hopefully…again…
 

30639.jpg

The chance that the DEEPSEA CHALLENGER will surface underneath the Mermaid Sapphire is extremely small. Safety measures are always taken into account such as having the ship exit the launch area radius. Photograph by Mark Thiessen, National Geographic.

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