Re: [PSUBS-MAILIST]Seals (was Pressure compensation)

["Captain Nemo" <vulcania@interpac.net>]

----- Original Message -----
From: "Steve Lindblom" <s_lindblom@conknet.com>
To: <personal_submersibles@psubs.org>
Sent: Tuesday, August 15, 2000 3:10 AM
Subject: [PSUBS-MAILIST]Seals (was Pressure compensation)


"I had gone through the archives...it never seemed to get down to
specifics... people said they were using Minn Kotas, but not how long for,
how deep or what they were doing about seals."

Hi Steve,

I'm running a Minn Kota 4HP in my sub, and I can tell you what I'm doing
with that.  Some of the other guys are doing something similar, and it seems
to be working for all of us.

First, the Techs at MK tell me their seals will hold integrity for "about
one additional atmosphere of pressure."  That says to me I should be able to
dive my lower-end (motor) unit to about 33 feet (give-or-take) before she
floods.  Now, even if she does flood, the motor will still continue to run:
but the brushes will wear down at a much faster rate; so obviously I want to
avoid that.

For me, pressure-compensating the motor housing with compressed air works
well.  It can be plumbed through pipes and valves from a SCUBA tank to
fitting threaded into a hole drilled and tapped into the motor housing.  I'm
coming off my ballast air supply, which is fed from the low-pressure port of
a first stage regulator, and gives me about 120 PSI to work with.  (That
means the system should still work at almost twice my maximum design
operating depth of 100 FSW.)

As long as I keep the gauge-indicated air pressure in the motor-compensation
system close to what's showing on the outside depth-pressure gauge, I'm OK
and she shouldn't leak.  In practice, that's not extremely difficult to do,
either.  A one-atmosphere seal-integrity range gives us a variable of about
15 PSI to play with.  By scanning the gauges occasionally on descent; and
giving it a shot of air when the gauges differ by, say, five pounds or so,
the housing shouldn't leak.  (I think it's also important to vent off the
system pressure in increments while ascending, or we may blow the seal.)


"if so many people are using these motors, I'd think it would make sense to
nail down whether seals can do the job, and if so, what seals, before moving
on to esoteric solutions."

OK.  In my explaination above, I'm talking about the stock seal that came
with my MK-4HP.  I have not experimented with the other seals people have
mentioned.  The only oil-filled motors I've seen were brushless DC; and I
suspect that filling a brushed motor with oil will degrade the brushes
rapidly and in much the same way as occurs when the housing is flooded with
water.  I haven't tried to oil-fill a MK, and I could be wrong; but that's
what I think would happen.  The techs at Minn Kota would be able to say for
certain.  I don't have their phone number in front of me, but as I recall
it's available on the Minn Kota website: typing the name into your search
engine should take you there.

Hope this helps, and good luck with your project!

Very best regards,

Pat