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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