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

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

----- Original Message -----
From: <Subscuba@aol.com>
> "I am using the Minn Kota without any modifications to the motor or seals.
I
> use a small tank (30 Ft^3) with a Scuba regulator adjusted to give a
slight
> positive pressure under normal conditions. This means the seals have to
> withstand about 1 to 2 psi positive which also implies if I get a leak air
> will leak out not water in. Pat (Capt. Nemo) probably knows more about
this
> than me and I suspect more experience. The concept is simple as is the
> implementation."

Oh gosh, oh shucks, I'm really in the hotseat now!  ;-)

Ken, it sounds to me like you're using a (dial adjusted?) second-stage SCUBA
regulator; this would make sense to me since your application is in a
wetsub, and you're in the water where you can manually control the flow.  Am
I right?  If so, I was doing it a bit differently: manually valving off the
ballast air system, which is fed by the low pressure port of a first stage
regulator providing about 120 PSI.   I had to watch two gauges (system
pressure and depth pressure) to know when to add air, and how much.  Also
had to vent the system on the way up.  Not really hard to do, but one more
task for the pilot.

Later I started thinking of installing a one-way flow valve (like an
anti-siphon valve) as a vent on the housing system: it wouldn't let water
in, but would let excess air out.  Going down, I can give the housing a shot
of air every twenty feet or so, and the valve will let out any excess beyond
depth pressure.  On the way up, when depth pressure decreases, the excess
air will escape the one-way valve and release housing pressure
automatically, saving my seal.  Looks like it should work; and it will be a
simple addition; but I haven't actually tried it in the water yet.  Maybe
next month...

"PS Pat, 4 horsepower (746 watts)? We went through this before, I would like
a 4 HP motor."

Heh!  Yeah, I see what you're getting at.  I think there's a big difference
in what Minn Kota calls a 4HP and the actual rating of what a 4 horsepower
electric motor would be.

The 24VDC model 4HP was a stock unit MK sold in the late 1980's: their
biggest at the time, I think.  They had problems burning out lower end
(motor) units, and discontinued it, I was told.  I got mine as a
factory-rebuilt from the company; and I don't think they make this one any
more; but I believe parts are still available, maybe even complete lower
ends.

I've always wondered if it put out an actual 4 ponies worth of thrust.  I
say I wonder because I really don't know.  The potentiometer has several
settings, from a creeping low speed, to maximum thrust.  When I first got
the motor, I clamped it to the deck alongside my pool and ran it for a
while: seemed to spit pretty good.

But I encountered a power loss when I put it in the Nautilus.  I mounted the
motor externally in an aft free-flooding section; the battery box was inside
the pressure hull tailcone; and the potentiometer was up front with me.
That meant a LOT of wire running back and forth between components, whereas
the stock unit only had about three feet or so.  The techs and I think this
caused a power loss when I opened the max-thrust gate.  I can hear it kick
in like it wants to go, but the prop slows way, way down, and almost stops.

Anyway, I plan to relocate the potentiometer aft in the tailcone with the
battery box, shorten the leads to near-stock length; and run a simple
mechanical linkage rod from the control handle up front to the pilot's
station: hopefully this will solve the problem, and the next time we dive
she'll be developing full thrust.

The MK that looks good to me now is the 36VDC, 101#-thrust "Monster" (that's
what I heard they call it): it's available for sale now, and they had some
pictures of it on their website when I checked it a few months ago.  If and
when I replace the E-motor in the Nautilus, that's what I'm going to use;
until they come out with a bigger one, that is!   ;-)

VBR,

Pat