[PSUBS-MAILIST] Dive report: Snoopy at Seneca

[Alec Smyth via Personal_Submersibles]

OK, let me try to reason this through again. Lets say the standard trolling
motor shaft seal is watertight to 50 feet, and that initially I had 1 cubic
inch of bubble in the system. Dive to 250 feet and what I would have
thought would happen is that the 1 cubic inch of air would be reduced to
1/8th in3 due to 8 atmospheres of pressure increase. I would have expected
that the compensation bladder would have just been squeezed by that volume,
and that upon surfacing the bubble would go back to 1 in3 and the pressure
in the motor to 1 atm. However, what we found was that the motor upon
surfacing was significantly pressurized. The compensation bladder (i.e.
hose) was visibly distended by internal pressure upon surfacing. There was
no leak at the hose clamps, but oil was leaking from the shaft seal.

Since there was over-pressure surfaced, clearly water had to have entered
the system during the dive, and as the hose clamps were leak free and the
seal was not, I suspect the seal. Now the question is why water would get
in. Bubble greater than the compression range of the hose? Seal offering
less resistance than the hose to compression? Shaft pumping water under the
seal during operation due to abrasions? Thermal contraction? I'm actually
not sure - any theories are welcome. One interesting detail - the stern
thruster, which was not working due to the issue with the speed controller,
did not have any oil leaks. Both side thrusters, operating, did.

The side thrusters being feeble can be explained by pressure on their
seals, or just by their age and related wear on the commutators. I will be
changing them, or at least their internals.


Best,

Alec

On Mon, Jun 8, 2015 at 2:54 PM, Jon Wallace via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

>
> It sounds like at some point water pressure at the seal was able to
> compress the bubble of air in the tube, the opposite of what was expected.
>
> I'm kind of stumped by the description of the motors performance being
> "feeble" however.  This couldn't have been primarily due to pressure I
> don't think.  Also, isn't an easy fix just to "clamp" the tube any place
> where liquid appears assuming the tube is held in a vertical position and
> the bubble is at the top (ie vise grip the tube).
>
> Jon
>
>
> On 6/8/2015 12:01 PM, Brian Cox via Personal_Submersibles wrote:
>
>  Hmm, guess I'm not getting what is happening
>
> Brian
>
>
>
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