[PSUBS-MAILIST] Minn Kota 101 - thread spec

Mon Jul 21 21:47:17 EDT 2014

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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