[PSUBS-MAILIST] Buoyancy dynamics

[Jon Wallace via Personal_Submersibles]

 No River, nothing that complicated.  Just curious if there is some formula or rule-of-thumb regarding how much displacement is required to get a sub started to the surface, from a specified depth.  Assuming perfect neutral buoyancy any change to positive would theoretically start the process but in a practical sense some forces could counteract a minor change in positive buoyancy.  Obviously it doesn't require 100% total displacement of the MBT, but will one pound do it?  two?  three?  Just looking for a reasonable estimate, if one exists.


    On Sunday, August 16, 2020, 12:12:31 PM EDT, River Dolfi via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  
 
 Jon, are you asking about how to correlate buoyancy condition with vertical speed through the water? IE "If I drain 50lb from the VBT and make myself 50lb positively buoyant, how long will it take me to reach the surface 600ft up?"
You would have to know the drag coefficient and a reference area of the sub in question in the up and down direction. Usually that kind of number is determined with fancy CFD or wind tunnel testing. I've got a couple data points about buoyancy vs vertical speed that I could crunch to get an approximate drag coefficient appropriate for small submersibles, then you could calculate the speed for any sub of a given cross sectional area.
Thanks,
-River J. Dolfi
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