[PSUBS-MAILIST] Minn Kota 101 - thread spec

Mon Jul 21 22:02:14 EDT 2014

Cliff,
Messy, not hard.
Vance

-----Original Message-----
From: Cliff Redus via Personal_Submersibles <personal_submersibles at psubs.org>
To: Personal Submersibles General Discussion <personal_submersibles at psubs.org>
Sent: Mon, Jul 21, 2014 9:47 pm
Subject: Re: [PSUBS-MAILIST] Minn Kota 101 - thread spec

Concur Jon that water will act as thermal sink and that the duty cycle on the thrusters are not severe but I don't agree that the thermal expansion in negligible. If you assume that the temperature change I estimated was high by a factor of two, then you would still end up with 3 teaspoons of oil that would be forced into the ambient tube.  Small but not negligible.  There will be a change in the internal temperature of the motor from the off position to on and given the amount of oil in the 101 body, there will be  oil that has to escape.  

Granted, the primary purpose of the compensation is to keep from exposing the housing to a high differential pressure but thermal expansion still has to be accounted for in sizing the ambient reservoir.

These MK 101 are powerful at almost 1.8 HP at 47 amps when air filled  but there has to be some degradation in power when filled with oil due to the viscosity difference with air. At 75F, light oil is over 1800 times as viscous as air. IS there enough power to push a psub about, sure.   I am just trying to come up with what kind of HP loss you get with oil compensation. 

Does anyone know what rpm these motors max out at in the water?

Good point Vance on trying to make the armature as hydraulically smooth as possible to maximize power when oil compensated.  I wonder how hard this would be to do?

Cliff

   From: Jon Wallace via Personal_Submersibles <personal_submersibles at psubs.org>
 To: Personal Submersibles General Discussion <personal_submersibles at psubs.org> 
 Sent: Monday, July 21, 2014 7:55 PM
 Subject: Re: [PSUBS-MAILIST] Minn Kota 101 - thread spec

      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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