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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