[PSUBS-MAILIST] Personal_Submersibles Digest, Vol 86, Issue 24

Sun Aug 16 15:08:56 EDT 2020

Jon,
It's a simple question, but there is no simple answer. For PSUBS situations
though, whatever answer you get is irrelevant.

Even 1 pound of additional displacement from air in the MBT will cause you
to rise, just not very fast. Drag is a V^2 relationship, so in order to go
twice as fast, you would need four times the buoyancy. In shallow depths,
the air is also rapidly expanding, so if you're interested in
time-to-surface you may have to account for that as well.

In the ocean there is a thermocline somewhere around 300 or 400 feet. The
density change of the water is enough to notably slow, or even stop ascent
or descent.

For extremely large, or extremely deep-diving vehicles, you begin noticing
either increasing or decreasing total net vehicle buoyancy depending on if
the hull is more or less compressible than the surrounding seawater.

In the end, the most practical thing to do is to add or vent air to obtain
and maintain the speed that you want. Like Vance says, watch the bubbles.
That answer isn't satisfying enough if you're sitting at 300ft without
enough weight to go deeper, or if you're sitting at the drawing board
trying to size a VBT or an appropriate amount of weight for an
ascent/descent dropweight system.

Thanks,

-River J. Dolfi

rdolfi7 at gmail.com

On Sun, Aug 16, 2020 at 2:31 PM via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

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> Today's Topics:
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>    1. Re: Buoyancy dynamics (River Dolfi via Personal_Submersibles)
>    2. Re: Buoyancy dynamics (via Personal_Submersibles)
>    3. Tv clip (Rick Patton via Personal_Submersibles)
>    4. Re: Buoyancy dynamics (Jon Wallace via Personal_Submersibles)
>    5. Re: Buoyancy dynamics (Jon Wallace via Personal_Submersibles)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Sun, 16 Aug 2020 12:10:40 -0400
> From: River Dolfi via Personal_Submersibles
>         <personal_submersibles at psubs.org>
> To: via Personal_Submersibles <personal_submersibles at psubs.org>
> Subject: Re: [PSUBS-MAILIST] Buoyancy dynamics
> Message-ID:
>         <CAGL1GvEmGNJQivZyA9M=
> UraBnRErO+0-zT80hs6pzOCv6nh5nQ at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
>
> 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
>
> rdolfi7 at gmail.com
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> Message: 2
> Date: Sun, 16 Aug 2020 16:29:48 +0000 (UTC)
> From: via Personal_Submersibles <personal_submersibles at psubs.org>
> To: "personal_submersibles at psubs.org"
>         <personal_submersibles at psubs.org>
> Subject: Re: [PSUBS-MAILIST] Buoyancy dynamics
> Message-ID: <336158316.2048549.1597595388304 at mail.yahoo.com>
> Content-Type: text/plain; charset="utf-8"
>
> Complicated. Just watch the bubbles. 60 feet per minute, give or take.Vance
>
> -----Original Message-----
> From: River Dolfi via Personal_Submersibles <
> personal_submersibles at psubs.org>
> To: via Personal_Submersibles <personal_submersibles at psubs.org>
> Sent: Sun, Aug 16, 2020 12:10 pm
> Subject: Re: [PSUBS-MAILIST] Buoyancy dynamics
>
> 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
> rdolfi7 at gmail.com_______________________________________________
> Personal_Submersibles mailing list
> Personal_Submersibles at psubs.org
> http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
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> ------------------------------
>
> Message: 3
> Date: Sun, 16 Aug 2020 10:03:20 -0700
> From: Rick Patton via Personal_Submersibles
>         <personal_submersibles at psubs.org>
> To: psubs room <personal_submersibles at psubs.org>
> Subject: [PSUBS-MAILIST] Tv clip
> Message-ID: <5261E030-B4CA-4161-A8C6-4C22F21072C6 at gmail.com>
> Content-Type: text/plain; charset=utf-8
>
> I posted a link where you could see the segment I did on ?What?s it,s
> worth ? on the A & E channel that showed my sub but didn?t hear any
> feedback so not sure it actually got posted. The producers edited a half
> hour down to about 5 minutes so my plug of our group unfortunately ended up
> not making the final cut.
> Rick
>
> Sent from my iPhone
>
>
> ------------------------------
>
> Message: 4
> Date: Sun, 16 Aug 2020 18:09:06 +0000 (UTC)
> From: Jon Wallace via Personal_Submersibles
>         <personal_submersibles at psubs.org>
> To: Personal Submersibles General Discussion
>         <personal_submersibles at psubs.org>
> Subject: Re: [PSUBS-MAILIST] Buoyancy dynamics
> Message-ID: <1025047738.2798248.1597601346123 at mail.yahoo.com>
> Content-Type: text/plain; charset="utf-8"
>
>  Sorry, I was not very clear, let me rephrase; you are at 300 feet depth.?
> How much water displacement (generally) is necessary in the MBT to get the
> sub heading to the surface at say, 30-60 feet per minute???
> Jon
>
>     On Sunday, August 16, 2020, 11:08:52 AM EDT, Brian Cox via
> Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>
>  Jon,? ? ? I think you're better off dealing with the hyrdrodynamics of
> the hull shape and nose shape and sufficient propulsion rather than try to
> gain any small amount of drag reduction you would gain from getting the sub
> higher in the water .? ?That said,? the ww2 subs were able to go faster on
> the surface, but was that because they had the benefit of the powerful
> diesels on the surface?? ?
> Brian
> ?
>
> --- personal_submersibles at psubs.org wrote:
>
> From: Jon Wallace via Personal_Submersibles <
> personal_submersibles at psubs.org>
> To: Personal Submersibles General Discussion <
> personal_submersibles at psubs.org>
> Subject: [PSUBS-MAILIST] Buoyancy dynamics
> Date: Sun, 16 Aug 2020 14:56:42 +0000 (UTC)
>
> Assuming perfect neutral buoyancy, how much MBT water displacement is
> needed to get a sub or particular size/weight moving topside at a
> reasonable speed?
> Jon_______________________________________________Personal_Submersibles
> mailing listPersonal_Submersibles at psubs.orghttp://
> www.psubs.org/mailman/listinfo.cgi/personal_submersibles_______________________________________________
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> ------------------------------
>
> Message: 5
> Date: Sun, 16 Aug 2020 18:30:41 +0000 (UTC)
> From: Jon Wallace via Personal_Submersibles
>         <personal_submersibles at psubs.org>
> To: via Personal_Submersibles <personal_submersibles at psubs.org>
> Subject: Re: [PSUBS-MAILIST] Buoyancy dynamics
> Message-ID: <458331127.2804282.1597602641132 at mail.yahoo.com>
> Content-Type: text/plain; charset="utf-8"
>
>  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
> rdolfi7 at gmail.com_______________________________________________
> Personal_Submersibles mailing list
> Personal_Submersibles at psubs.org
> http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>
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> End of Personal_Submersibles Digest, Vol 86, Issue 24
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