[PSUBS-MAILIST] Thruster bladders

[Alec Smyth via Personal_Submersibles]

Hi Rick,

You'll find people here who love oil compensation, and others who love air.
They both work, so I guess it's just a matter of what has worked for each
person.

I used oil initially. The method was to put two 90 degree nipples in the
motor can, and attach about a foot of plastic hose to them. To fill, you
disconnect the top nipple and add oil through the hose until it comes out
the top nipple. The hose wraps around the can and with that extra length is
supposed to act as a bladder. I found it worked, but only until I started
diving my K250 to 250 feet. Then, I was getting internal build-up of
pressure in the motors. In the end, I figured what was going on was that
the compression of the hose was insufficient for the thermal contraction
between the surface, where I'd filled the motors, and the very cold ambient
temperature at 250 feet. At depth, water was forcing its way past the shaft
seals, and then the seals would keep the motors pressurized as I rose. My
solution was to go to a larger bladder, namely this:

https://www.google.com/search?q=squeeze+bottles&rlz=1C1CHBF_enUS807US807&sxsrf=AOaemvLRC6ixUOHC7wIDhQcAL78ShVP-Sw:1631711091091&source=univ&tbm=shop&tbo=u&sa=X&ved=2ahUKEwig_9a_hYHzAhUgF1kFHRK2AW0Q1TV6BQgBEKkC&biw=1536&bih=664#spd=4605806885737196691

That said, the larger bladder was something I implemented just before
selling that sub, so never got to dive it. It was also always a problem
getting every last bubble out. I'd spend ages doing that, then come back
the next day and find a bubble in the hose. I was never able to solve that
issue, and I think the only way would have been to assemble the motor in a
bucket of oil. The other problem was that if there was an issue, such as
the over-pressurization one above, my sub created an oil slick. That is NOT
good, and can attract the wrong sort of attention even if you were to
use an oil that is harmless.

Now I use air compensation. So far, so good. On my first dive with the
mechanical arm, I noticed bubbles coming from a hose when I extended the
arm. The bubbles would cease when I retracted the arm. Closer examination
showed a nick in the hose, which got pinched closed when the arm was
retracted. The beauty of air is that if there's a leak, you can see and
trace it immediately. With oil, I'd have had a small slick but no idea
where it was coming from. Also, it worked well in the sense that no water
entered the arm. I have a shutoff valve, so that a leak won't drain the
sub's air supply, or another way would be to supply the air comp with a
small independent cylinder.


Best,
Alec

On Wed, Sep 15, 2021 at 12:13 AM Rick Patton via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> I am re thinking what to use for my oil expansion bladder. Would love to
> hear from everyone who chose to go with an oil filled thrusters on what
> they used and how they liked it. I remember someone saying that they used a
> squeeze ball gas tank primer which I like because it is small but I just
> spoke with someone who didn’t use a bladder and had never had a problem.
> Rick
>
> Sent from my iPhone
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