[PSUBS-MAILIST] FW: VBT pump

[Alec Smyth via Personal_Submersibles]

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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