Posted by B. Suttie on
January 11, 2004 at 14:34:48:
Hello, you have a great site, thanks for the time you obviously put
into it. I am in the process of a redesign of an ROV that I built and
used last summer. The ROV worked great, but in this redesign stage I am
looking to increase the depth range of the trolling motors. I understand
that tranformer oil can be used to fill the motor housing and equalize
the pressure on the motor. However, I have also heard of pressurizing
the motor housings with compressed air to achieve a similar effect. Have
you tried this technique, if so how did you go about pressurizing the
housing, and have you been happy with the results.
Any help you could
provide me on this would be greatly appreciated.
Posted by Pat Regan
on January 11, 2004 at 15:48:47:
In Reply to: Pressurizing
Trolling Motors posted by B. Suttie on January 11, 2004 at 14:34:48:
Thanks Brent, I'm glad you like the site.
On pressure compensation of trolling motors:
MINN KOTA rates their motors for one additional atmosphere over
surface pressure, or 33 feet depth. Techs admit that people have been
using unmodified motors on submersible projects down to three additional
atmospheres, or 99 feet, with an approximate 5% failure rate.
The O ring seal can be replaced with a matched ceramic conical seal,
but this usually involves machining the shaft to a close tolerance of a
few microns. That involves disassembling the motor; and when taken
apart, it's important to time the brushes correctly and torque the
housing properly during reassembly or she won't work right. This kind of
modification should be done by an experienced E-Motor tech. And then,
the depth increase isn't as good as what's achieved with compensation.
Actually, a MINN KOTA will run for a while when flooded with water;
it will run slower, however. The danger comes from the brush springs
rusting and breaking, which will result in an unexpected total failure.
Though MINN KOTA doesn't officially recommend it, saying "gums up the
brushes", motor lower ends can be pressure compensated by filling them
with transformer oil. The trick is to impart a slight vaccuum to the
case when filling to eliminate any voids that would defeat compensation.
The case must be totally filled with oil. This will result in some
reduction in armature rotation speed, but it will serve to compensate
the motor.
People also compensate with air pressure from a tank. I've done it by
plumbing air lines from a pressurized source to the housing, and also
equipping the housing with a one-way check valve as an overpressure
release. In practice, I open the air supply valve occasionally, the air
pressurizes the housing, and the overpressure goes out the overflow
valve. The right amount goes out because the external water pressure is
all that's holding the valve shut. When the pressure inside the housing
is greater than the ambient water pressure, the one-way valve opens to
release the excess. Thus, we don't overpressurize the housing and blow
the seal. On the way back to the surface, the reduced external pressure
on the exhaust valve allows the greater pressure inside the housing to
escape, again avoiding overpressurization that will blow the seals.
A second stage SCUBA regulator can also be fitted to the housing as
another means of pressure compensation. If it's at the same depth as the
housing, or maybe a couple inches deeper, depth pressure acting on the
diaphragm will admit air to the housing in the correct amount.
Overpressure will be exhausted through the regulator when surfacing.
Despite what you might have heard, doubling the O-rings on the shaft
will not double the depth capabilities. If the depth pressure is
sufficient to defeat the outer seal, it will then bring its entire
pressure to bear on the inner seal which will also fail because it's no
stronger than it's external counterpart. All double O-rings do is
provide a backup if the outer seal fails from wear at depths the O-rings
are designed to withstand.
Hope this helps. Thanks for posting your question here where others
can benefit from it.
VBR,
Pat
Posted by Pat Regan
on January 28, 2004 at 13:23:27:
In Reply to: Re:
Pressurizing Trolling Motors posted by Richard on January 27, 2004
at 18:51:29:
: Hi
: Iown a sportsub sambiaent pressure submarine designed and sold by
international venture craft. The Minkota motors on all sportsub are
pressurised from the inside. I have had no problems wih my sport sub
when taking her down to 130 feet.
Yep. Ron Leonard and friends at IVC do a great job on the SPORTSUB.
Send me some pictures of yours. Maybe I can set up a SPORTSUB page in
the SUBMARINES department.
Air pressurization is a good way to go; I prefer it, in fact. As long
as the pressure inside the motor lower end housing is approximately
equal to the ambient depth pressure outside the sub, it's like she's
running near the surface and watertight integrity is as good as it ever
was.
I've had some guys say they think air compensation is too complex.
(Of course, these same guys don't build or operate subs, but there ya
go.) Anyway, it's really not any more complex than the other things one
needs to do to drive a submarine, and not nearly as complex as some
folks might imagine.
My advice to anyone running an externally mounted E-motor on a sub,
DPV, or ROV: PRESSURE COMPENSATE!
See you underwater! ;-)
Pat
Posted by john on June 03, 2004 at 03:51:29:
In Reply to: Re:
Trolling motor modifications posted by Pat Regan on June 03, 2004 at
02:56:59:
Hi Pat
Thanks for responding. what I am trying to do is pressure compensate
my trolling motor so that water does not enter. I would like to use
these motors for a small wet sub design. I was looking at the trolling
motors on the INVC sport subs which I was told were pressurised to 140
psi which makes them water tight to 300 feet. I tried pressurising my
motor by adding air down the trolling motor support arm were all the
wires run through, but when pressurised air blows out around the shaft
were the prop is attached. These motors I have are brand new. I'm pretty
sure the boys at INVC modify there motors some how to allow for 140 psi
pressurisation as the sport sub uses a scuba regulators low pressure
port as the air feed into the subs air system. I am realy baffled about
this topic and there isn't much out there in the ways of help after
searching the net. Maybe INVC change the seal I don't know and Ron isn't
to quick to release secrets which I respect. If you can help me figure
out how to make these motors air tight it would be greatly apreciated. I
only want my sub so dive to 100 feet max. Maybe trolling motor
modifications would be a good topic for your site.
Thanks
John
Posted by Pat Regan on
June 03, 2004 at 15:01:13:
In Reply to: Re:
Trolling motor modifications posted by john on June 03, 2004 at
03:51:29:
Sure John, no problem. Actually, there's a fair thread on this
subject here on the Board already. Scroll down and read PRESSURIZING
TROLLING MOTORS by B. Suttie, and my answer to him.
A section of the forthcoming MANUAL also deals with Pressure
Compensation.
In response to your recent post (above), the following comments; (and
please don't think my use of CAPS is SHOUTING; there's no way to
underline or italicize words on this forum, so I use caps):
We really don't want to PRESSURIZE trolling motors beyond ambient
pressure; we want to PRESSURE COMPENSATE them so the pressure inside the
case is about the same as it is outside at any given depth.
This means adding pressure to the housing, but ALSO either: (1)
adapting the lower end housing with an overpressure release valve; or
(2) using a SCUBA regulator which has it's own exhaust system built
right in; so the overpressure can escape. That way we don't put too much
pressure into the housing and blow the shaft seal from the inside out.
In reading your description of how you attempted to pressurize your
motor through the structural shaft, and now have air escaping out the
prop shaft O-ring, I'm afraid you might have blown your shaft seal
already. I'd recommend having it checked before putting it in the water.
If you're not absolutely certain of your abilities to disassemble the
housing, check the seal, repair it if necessary, and reassemble the unit
before putting it into service, I'd strongly recommend having an
experienced motor tech do the work for you.
Regarding your other concerns: I think the thread on PRESSURIZING
TROLLING MOTORS will answer most of them. Scroll down the list here at
the Message Board and you'll find it. Beyond that, here's a few more
thoughts:
For a wetsub, I'd pressurize with a second stage SCUBA regulator: (1)
seal the structural shaft or housing wire inlet with something
watertight like epoxy (being careful not to get it into the inside of
the housing where it might gum up the armature); and (2) connect the
regulator mouthpiece to the housing via an air line. You'll have to do
little "adaptive plumbing" here: replacing the regulator mouthpiece with
an air line adapter, and finding a way to route the airline securely to
the structural shaft or directly to the motor unit.
The concept is that increased water pressure acting on the second
stage regulator diaphragm admits only the right amount of air pressure
into the motor housing; and any over pressure in the housing relative to
decreased depth pressure (like when we're ascending to surface) will
automatically exit through the regulator exhaust, not the shaft
O-ring.
Hope this helps.
Pat
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