Buoyancy: A Real-World Example

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.

Answer to last post’s thought question: How are submarines similar to drogues?

A surfaced submarine is positively buoyant. A diving submarine is negatively buoyant. A submarine staying still in the water column is neutrally buoyant. 

Below: Doug works on “Echo,” helping to get it back in the water after the tether was cut by the boat propeller.  The yellow parts of this device were made of dense, buoyant foam.


Doug_Post1_echo.jpgRemotely Operated Vehicles (ROVs) are devices used to explore the ocean. You might have seen them on TV a couple summers ago during the BP oil spill disaster taking video of the oil coming out of the broken pipe a mile deep in the Gulf of Mexico.  ROVs can cost several million dollars. On occasion, they get tangled in things that lie on the bottom of the ocean, lose power, or suffer cut cables. Losing ROVs in any of these ways is not a pleasant experience. 

While I was working in the Black Sea, the tow cable that connected us to our ROV was severed by our ship’s propeller.  There were a lot of nervous nellies on board that began scanning the water surface for our bright yellow device named “Echo.”  I went to the galley, got a cup of coffee, and went up to the boat bridge. This was the highest vantage point to keep an eye on the water surface. About twenty minutes later, “Echo” was spotted at the sea surface.  We retrieved her, spliced the tether back together, and re-launched her, continuing our operation.

Thought question: How would you design an ROV that had a good chance of being found if it failed completely?



Answer:
Echo was engineered to be slightly positively buoyant, meaning that it will float just above the surface of the water.  If it had been negatively buoyant, it would have sunk to the bottom and been lost forever. (In the oceanographic profession, we don’t admit to losing these items, we just say that they are in “wet storage.”) If Echo had been neutrally buoyant, it would have just sat in the water column, where it was located when the tether was severed. In this case, again, it would not have been recovered. Things that are positively buoyant float. Echo was built with floatation foam so that, in the event of a catastrophy, she would eventually float to the surface.  And thankfully, she did.

By Doug Levin

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