Re: [PSUBS-MAILIST] Seals again

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

----- Original Message -----
From: "Steve Lindblom" <s_lindblom@conknet.com>
To: <personal_submersibles@psubs.org>
Sent: Tuesday, October 03, 2000 10:33 PM
Subject: Re: [PSUBS-MAILIST] Seals again


"Cave divers are using scooters to making 3 mile+ cave penetrations at 300'
depth. Were you planning on doing anything more critical than that?"

Hi Steve!  Nope!  I always operate my submarine within an ample margin for
safety.  ;-)



"I think anyone building a sub where motor failure could be life threatening
ought  to be looking at something a lot better than troller motors."

That's why deep submergence vehicles don't use Minn Kota's.  My point was, a
failed DPV can be ditched (or brought to the surface) easily; whereas a
motor failure in a submarine might force the occupants to abandon and lose
the boat.  But now that you mention it, I agree that anyone involved in
unusually life threatening underwater operations would be wise to bet their
survival on something better than a trolling motor: including your cave
diver;  who, according to your description of the propulsion systems used in
DPV's, might be using a Minn Kota or the like at depths far beyond what the
factory intended.




"Seals fail because they wear out (or are damaged in installation) 99% of
the time. The solution? replace them routinely before they wear out"

Just hope the seal doesn't have a hidden defect, or that you don't
inadverdently damage it during installation; because then (without pressure
compensation) your first indication of trouble will be symptoms resulting
from a flooded motor housing.



"And according to Minn Kota these motors will run flooded so a seal leak
doesn' mean the sub taking an instant plunge to the bottom."

The Techs at Minn Kota and I are old friends, and I believe I was the first
to post the fact that M-K motors can run while flooded on this forum.  Minn
Kota motors are factory warranted to withstand one additional atmosphere: no
more.  Yes, they may run while flooded, and one result will be a more-rapid
deterioration of the brushes.  But any time we let water into an electric
motor, there are many reasons why it may fail to operate, so we should do
all we can to avoid this situation.  Again, pressure compensation makes
sense.



" Anyhow, if a seal can fail without warning, what about regulators, dynamic
O-rings, gauges etc?. Don't know how anyone can seriously argue that a
pressure tank, which must be regularly refilled, one or two regulator
stages, some hose and gauges and opv could be more reliable than a couple
properly engineered seals. A freeflow, for example, or a creeping 1st, could
pressurize the seals in reverse - just the way they are not intended to take
pressure - and easily blow them right out."

Who needs all that equipment?  A simple gate valve can provide air from the
ballast supply; and a one-way valve venting the housing (two in series if
you want  safe reduncancy) will automatically and safely release pressure
exceeding ambient, thereby protecting your seals from blowout.  That's less
complex and more reliable than a reserve parachute!  When insurance for the
motor is this simple, safe, economical, and reliable, it doesn't make sense
to be without it.



"The pressure on the armature/shaft at depth is going to be equal to the
area of the shaft where it comes through the seal, times the pressure."

Wrong.  It's at least going to equal the pressure exerted on the area of the
prop hub.


"Given a 1/2" shaft at 330', that would only be 27 lbs."

Given a prop hub diameter of 4", the pressure exerted internally on the
thrust bearing will be a lot more than the figure you've come up with.  I
have to go to work in a few minutes, I don't have my calculator handy, and
don't have time to do the math for you right now; but run the numbers
yourself using a 4" diameter prop hub surface in place of your 1/2" diameter
shaft, and you'll see the additional force exerted on the thrust bearing is
a lot more than what you've come up with.



"On a motor designed for 100 lbs of thrust, that's not a heck of a lot
extra. With a 3/8" shaft at 100' its only 5 lbs."

Again, you need to re-calculate the force exerted on the shaft AND
everything attached to it: i.e., the prop.  It's a lot more than you think
it is.



"remember, these motors aredesigned to handle thrust."

The thrust bearing is designed to absorb forces imparted to it under normal
operating conditions; excessive force can exceed design specifications and
adversely effect the motor.  Pressure compensation is proven to avoid seal
leaks and bearing problems safely, reliably, and economically; that's why a
lot of people use it.  Personally, I wouldn't submerge without it.

Very best regards,

Pat