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

Sun Aug 16 16:11:13 EDT 2020

 River,
Exactly, which is why I was looking for a "reasonable" vertical rate but it may have been misleading to mention that.  The specific rate is not as important as just ensuring enough buoyancy to overcome things like density changes or other forces as you've identified.  What I am actually trying to do is ensure I have enough HP air at a specific depth to ensure enough displacement occurs to ensure surfacing.  This seems to be much more critical for deep diving subs than shallow diving ones.
Using K350 specs as a guide, Kittredge required 128 pounds of lead for the emergency drop weight system.  I assume then, that 128 pounds of positive buoyancy is "enough" to initiate surfacing from 350 feet in a 3000 pound vessel and overcoming any external forces such as drag, thermocline, etc.  For the moment, until someone tells me differently, I think it would be reasonable to suggest that half that amount or 64 pounds displacement (1 cu ft seawater) is even enough to ensure surfacing.
Jon

    On Sunday, August 16, 2020, 03:10:52 PM EDT, River Dolfi via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 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:

   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)

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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_______________________________________________
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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

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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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