RE: Trolling Motors

["Morrisson, Richard D" <Richard.Morrisson@PSS.Boeing.com>]

Ken,

I recently purchased a 30 pound thrust Minn-Kota trolling motor for $109
with the intention of using it in a homemade diver propulsion unit.  When
reading through the literature after getting the unit home I discovered they
don't warrent this unit for salt water.  Do you know why that might be?  The
possibilities I can think of are: shaft material (although it appears to be
stainless), seal design, paint used (it looks pretty good too), and possibly
the speed controls (exposure to salt sea air).  I would be interested in
knowing who to talk to at Minn-Kota also (might save me a part of the $20
you spent).

By the way, I agree that the Johnson trolling motor would be highly suspect
for use un-changed at 700 fsw.  Even the best pressure activated teflon
seals from Shamban will have some leakage after a while and it would only
take a small amount of sea water to ruin a motor.  Perhaps the referenced
user only ran for short periods of time and checked the motors after each
use.  Anyway, I will be looking at pressure compensation to a couple of psi
above ambient (similar to your system) with a relief set a few psi above
that (a deadband to keep the air consumption to a minimum).

I enjoy reading your inputs, thanks.

Dick Morrisson

> ----------
> From: 	Subscuba@aol.com[SMTP:Subscuba@aol.com]
> Sent: 	Thursday, October 22, 1998 8:43 AM
> To: 	personal_submersibles@psubs.org
> Subject: 	Trolling Motors
> 
> I use on my Sub a Minnkota 70 pound thrust 24 volt Trolling Motor ($169
> each).
> I examined the shaft seal and found it is a simple "Cup" seal in which the
> shaft rotates in a rubber seal. Thus the shaft does abrade the rubber.
> This
> degradation is a function of differential pressure across the seal and for
> low
> pressures is long lasting.
> 
> When I was talking to Minnkota, they stated the seal wasn't good for much
> more
> than about 30 feet of fresh water before it could leak. I believe his
> response
> was conservative but their seal certainly would not work at 700 feet and
> if it
> did it wouldn't last long. I ended up pressure compensating my motor with
> a
> somewhat simple SCUBA regulator such that the differential pressure across
> the
> shaft seal was less than 2 feet of seawater and was positive. This means
> that
> any leaks would leak air out and not corrosive seawater in. At least I
> hope
> so, I still have not made sea trials yet although the my Wet Sub is ready.
> 
> The point of this is that since the Evinrude Trolling Motor is in the same
> competitive market I personally doubt that their shaft seal will reliably
> operate with 700 feet of sea water differential pressure. Pressure
> compensation, I believe, will be required.
> 
> If someone is planning to use the Evinrude Trolling Motor, it would be
> critical to call the vendor and get real data on their shaft seals. I know
> I
> did on the Minnkota Trolling Motor. I spent about $20 on the phone calls.
> I
> also got design information on the Motor Controller I designed. High surge
> currents can demagnetize the Trolling motors permanent magnet stator.
> 
> Good Luck,
> 
> Ken Martindale
>