[PSUBS-MAILIST] FW: VBT pump

Wed Aug 16 11:20:44 EDT 2023

 I'll chime in here...I removed the VBT from the K600 because it was fairly well rusted inside and needed to be replaced, however I am planning on using the ballast/float method instead and forgoing the VBT altogether for many of the reasons Alec and Cliff cited.  I will say that it can be challenging to change ballast on-station since the weight/floatation has to be transferred somewhere and by somebody, however I see the K600 being trimmed for specific missions prior to launch and not being trimmed dynamically on-station to embark/disembark passengers or equipment.
Jon

    On Wednesday, August 16, 2023 at 09:43:35 AM EDT, Alec Smyth via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 I second that (getting rid of VBT). I got rid of the VBT in my K250 years ago to make space and turn it into a 2-person and to try and to simplify, and it worked so well I never thought of using any other method on the current sub. The only difference with Cliff's method is that I add or remove floats, while he uses ballast. But either way, once you have a reference point for how much ballast/buoyancy you need for a given crew weight, you just make incremental changes and it never fails. You may be off neutral by just a few pounds, but if you have vertical thrusters they're usually just ticking over at slow RPMs to compensate.

Best,Alec
On Wed, Aug 16, 2023 at 9:30 AM Cliff Redus via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

 Hugh, on my original design for the R300 I had a hard VBT just aft of the pilot. At the time I had a small HPU in the boat.  My plan was to use a Oberdorfer Bronze gear pump driven by a small hydraulic motor to move water in and out of the VBT.  I don't remember the specs on the pump.  I had motorized ball valves on the vent and flood side of the VBT.  Prior to using this set up, I did some testing of this gear pump which I still have.  What I found was an unacceptable amount of flow slippage at my max design depth.  There was so much slippage that the pump could not blow the VBT at max depth.  At that point I abandoned using a pump and vent to blowing the VBT with air.  I eventually abandoned the VBT altogether for operability reasons.  When I used this hard VBT to get neutral on the surface, the pressure in the tank was low.  When at depth if I ever tried to use the VBT, water would rush in until the pressure equalized with ambient water pressure.  This made the boat negatively buoyant until air was added.  The other thing that bugged me was I had a float style level sensor in the VBT.  The water level in the VBT was always moving around so you never trust the reading.  I eventually abandoned the VBT and now just add ballast at the boat CG where the VBT was located.  I find this works great.  The other issue is the VBT restricted access to equipment aft of the VBT so to work on any of this equipment, I had to remove the VBT.   All in all, I found the VBT to be a pain in butt.
Cliff
Oberdorfer 944 Bronze

    On Sunday, August 13, 2023 at 04:57:24 PM CDT, via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

Hi Guys

What make of pump is best for VBT water transfer in/out for 250 psi 1.5 kw.

I am looking at a gear pump 1.5kw Oberdorfer 944 Bronze.  These are only rated for 10 bar.  I tried an hydraulic gear pump 30cc where I had a stainless and bronze gear set made up but the aluminium body corroded form the trials and I just found that they had used steel backed sleeve bearings.

Regards,

Hugh 
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