[PSUBS-MAILIST] Minn Kota 101 - thread spec

Mon Jul 21 20:55:49 EDT 2014

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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