[PSUBS-MAILIST] Minn Kota 101 - thread spec

Sat Jul 26 16:27:49 EDT 2014

Hank, What is the story behind your magnetic coupler ? You were talking about them on the list and then you had one. Did you buy it ? If so where? Did you build it ? If so from what plans.

Thanks Pete

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On Mon, 7/21/14, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:

 Subject: Re: [PSUBS-MAILIST] Minn Kota 101 - thread spec
 To: "Personal Submersibles General Discussion" <personal_submersibles at psubs.org>
 Date: Monday, July 21, 2014, 6:20 PM

 there is a very nice
 solution here.  Magnetic couplers, it wouldn't take
 much to convert a minkota  to a magnetic coupled
 thruster.Hank

      On Monday, July 21, 2014 8:56:11 PM,
 Jon Wallace
  via Personal_Submersibles
 <personal_submersibles at psubs.org> wrote:

       Dissipation of heat to the water environment via
 conduction with
       the motor housing would help temper overall thermal
 expansion. 
       The motors are essentially surrounded by an infinite
 sized water
       jacket and not typically operated at full speed for
 long periods
       of time.  I suspect actual thermal expansion of the
 oil in the
       motor housing during typical underwater operations is
 negligible.

       In regards to viscosity effect on the motors,
 JimK's GUPPY is oil
       compensated and three 101's in a K-boat
 configuration and had no
       problem moving that heavy (7 ton?) sub.  Check out
       http://www.youtube.com/embed/u0b7NjxttL4?rel=0&vq=hd720
 at
       0:20-0:28 where he just about stops the sub on a dime
 with two
       forward 101's.  Also in
       http://www.youtube.com/embed/9Vaq4JK9wVs?rel=0&vq=hd720
 at
       3:39 he pulls out of the boat lift with just one 101
 aft, and at
       4:02 he pulls out with two forward 101's.  Alec
 has smaller motors
       on SNOOPY but they still have enough power to move it
 around.  I'm
       sure viscosity effect on the motors are measurable but
 from
       practical application it doesn't appear to be much
 of an issue
       when two or more motors are used.  There's enough
 power to move
       the subs as desired even with any viscosity effect
 that is
       present.

       Jon

       On 7/21/2014 10:22 AM, Cliff Redus via
 Personal_Submersibles
       wrote:

                James, a couple of points.  First, to
 me the fewer
                 the leak paths the better so I would not
 install the
                 added plug.  The issue is how to get traped
 air out of
                 the Md-101 when using oil compensation.  
 I like Alec
                 and Hanks ideas for removing trapped air due
 to nipple
                 protruding into body.  The other point is
 the wrap
                 around tube volume can compensate for the
 small amount
                 of air that remains trapped.  To deal with
 thermal
                 expansion of the oil, first of all you are
 dealing with
                 a small volume to start with so the
 tube/reservoir does
                 not have to be all that large.  If you do a
 quick back
                 of the envelope calc on the required volume
                 to compensate for only thermal expansion of
 the oil you
                 about need 3 US teaspoons for a MK 101 (
 Assume oil has
                 a thermal expansion coefficient of 0.00056
 1/F and that
                 there is one US pint of oil in the body of
 the 101 and
                 that the temperature swing is 70F to 130F. 
 Delta volume
                 is 0.125 gal * 0.00056 1/F * 60F = 0.0042
 gal*128 OZ/gal
                 *6 US TSP/OZ = 3.2 teaspoons).  
               To me the design pressure inside the ME
 101 should be
                 ambient pressure as they have lip seals on
 shafts.  Lip
                 seals are design to take external
 pressure.  They re not
                 designed to take internal pressure.  So a
 simple wrap
                 around tube for oil compensation with say a
 volume of 5
                 US teaspoons should work just fine as this
 would allow
                 for thermal expansion of the oil and a small
 volume of
                 trapped air and because the tube is
 flexible, the
                 pressure inside the 101 is ambient which
 makes the lip
                 seal happy.  As to Alan's suggestion on
 omitting all
                 pressure compensation and only relaying on
 the lip seal
                 without any pressure compensation, I am not
 wild about
                 this idea unless the boat is only designed
 for shallow
                 water.  MK designers when they speced the
 lip seals for
                 MK were designing shallow submergence of a
 trolling
                 motor with a factor of safety.  So as you
 get deeper and
                 deeper, you are starting to expose these
 lips seals to a
                 significant differential pressure which
 causes them to
                 overheat and fail at some point.  Is this
 10ft or 50 ft
                 or 100 ft.  Don't know but to me this
 exposes the boat
                 to some risk particularly if use the
 101's for depth
                 stability rather than a VBT and dive the
 boat negatively
                 buoyant, i.e., vertical thruster fails,
                 boat starts to descend and pilot is
 forced into dropping
                 ballast.  

               To me a bigger question on air vs oil
 compensation is
                 how much power are you giving up with oil
 compensation
                 due to viscosity difference between oil and
 air.  

               As both Alec and Vance point out, there
 has been a
                 lot of bottom time on MD-101s with oil
 compensation
                 without a lot documented failures.  

               I have not decided in my own mind which
 compensation
                 method I will use on my MD-101's for
 future boats.  

               Cliff

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