Deep Glider, depth, and bouyancy

[digginestdogg]

I was fascinated by the Wired article describing such an elegant engineering solution for remote data collection deep in the ocean, Deep Glider:
http://www.wired.com/news/technology/0,72794-0.html?tw=rss.index"

But, having been out of engineering school a few decades and a bit rusty in my physics and hydrostatics, I was intrigued by a statement in the article:
"Traditional gliders consume about half a watt of energy moving at a rate of half a knot. Deepglider's power consumption is about half that because of its exceptionally stiff hull that's resistant to pressure. When pressure compresses a hull in a traditional glider, it gains buoyancy and requires more energy to control."

I always thought bouyancy was the relative upward force of water on an object equal to the weight of water displaced by the object.

Ignoring the change in water density (since water is most dense at 4º C usually the temperature at depth is I recall from a Blue Planet video), assuming the glider's mass remains constant, and ignoring the pressure difference over the vertical dimension of the glider since it is quite short,

then as the hull descends and is compressed its volume decreases. This reduced volume displaces less water reducing the bouyant force on the glider making it less bouyant, not more so. Still perhaps harder to control but for the opposite reason.

Am I wrong in my cursory analysis drawing on a long-dormant physics and engineering education?

 

[Mentz114]

As I understand this, you are right. As the displaced volume decreases, so does the bouyancy. In terms of energy consumption, the stiff hull is still an asset.

 

[russ_watters]

You are correct, the article is wrong. But hey - it's Wired!

In any case, stiffer hull is better for buoyancy/energy efficiency reasons (you'll need to pump out less water to increase the buoyancy if the hull hasn't shrunk as much), it's just that they explained it wrong.

For some reason, this is a very common misconception about buoyancy.

 

[digginestdogg]

It is more complex than we thought...

Russ,

Thanks for you reply. great forum--found it 'Googling' for an answer to my bouyancy observation.

As always, understanding a scenario in physics is like building a Taylor Series approximation, you start throwing out the smaller terms until the answer fits your various criteria perhaps including accuracy, precision, simplicty, etc.

Charlie Eriksen (UW Oceanography) and referenced in the article, replied to me after I queried him about the error. It seems the scenario calls for a few more terms whose numerators grow and denomiators shirnk under the unusual conditions of great depth.

Specifically, I was assuming, as we are taught in conventional engineering hydrostatics and hydrodynamics, water is an incompressible fluid or that any compression that occurs was negligible. Not an accurate assumption at the depths we are talking about particularly with the subtleties of the changes in bouyancy we are considering.

Here is what Charlie had to say:
" The reporter has the difficult task of communicating technical
information to a wide audience. In this case, the wording could have
been a bit clearer, but probably at the expense of increasing the
article length, something likely resisted by the editor.

One glider model, the Seaglider, has a hull that is neutrally
compressible. That is, its hull changes volume with pressure at the same
rate as seawater. In the other two upper ocean gliders now in use, Spray
& Slocum, the hull is stiffer than seawater, as with Deepglider. For
these vehicles, increased pressure provides increased buoyancy, since
seawater compresses more than the hull does. In order to maintain
negative buoyancy on the dive portion of a surface-to-depth-to-surface
cycle, the buoyancy control system of these vehicles has to reduce
volume more than it would on a neutrally compressible vehicle. These
vehicles then have to increase their volume correspondingly more to
climb back to the surface. This is why the energy consumption is greater
on the non-neutrally compressible gliders than on Seaglider.

Thanks for your interest in this technology.

regards, Charlie Eriksen"

 

[russ_watters]

Wow! That's pretty stiff. Water will compress roughly 1.5% at the depths they are talking about. I never considered the possibility that the glider would be less compressible than water.

Still, yeah, that's a badly worded sentence.