[PSUBS-MAILIST] Minn Kotta 101 - Thruster Reliability

Tue Jul 25 16:02:55 EDT 2023

 Minn Kota is very tight lipped about any of the design aspects of their products.  I guess they are fending off cheap knockoffs.  As such I can find no real data on materials such as make-up of the brushes.  I am looking at transformer dielectric oil as a possible replacement like https://phillips66lubricants.com/wp-content/uploads/2019/12/Transformer-Oil.pdf. As to arcing being the possible cause of the blackened fluid, it is possible, but my guess is a suspension of brush particles given the amount of sludge left on all the parts and the wear pattern on the shaft.  What would support arcing hypothesis is that all of these thrusters were filled using a vacuum pump.  Yet all the bladders experienced gas pressure build-up.  Likewise, Alec filled all his thrusters with WD-40 and saw similar blackened WD-40 in bladders and similar gas built up past what you would expect from thermal expansion of fluid.
Cliff

    On Tuesday, July 25, 2023 at 02:23:16 PM CDT, Sean T. Stevenson via Personal_Submersibles <personal_submersibles at psubs.org> wrote:  

 I have no specific experience with using the WD40 product for pressure compensating thrusters (or any other volumes, for that matter), but I do question its efficacy for that purpose on the basis of its known properties.

WD40 is a proprietary mixture of lubricants and solvents known to have strong dielectric properties. While the exact formulation is a trade secret, in general terms the product is an emulsion of oil and alcohol. Marketed as a penetrant, it has a low viscosity afforded by the substantial fraction of volatile, lightweight components, which help to allow capillary action to draw the emulsified product into crevices. In unencapsulated service (open to atmosphere), typically the lightweight components will evaporate into the surrounding air, temporarily serving their function as a solvent to dissolve certain solid or high viscosity residues before leaving behind the heavier oils which serve the lubrication function.

When you encapsulate WD40 in a closed volume, the lightweight components do not evaporate, leaving them to perform their solvent function on any soluble material in contact for as long as the equilibrium concentration gradient supports that. Additionally, you hold these lightweight components with low viscosity / high volatility (remember, this product is marketed as a penetrant) in contact with whatever sealing arrangements are designed to keep it contained, where these seals would otherwise perform much better in contact with a higher viscosity oil or grease which augments the seal performance.

In short, for any compensation of electronics in static applications, I would consider only the dielectric properties and dimensional stability (bulk modulus) of the compensating fluid. In a dynamic application, such as with a drive shaft seal, I would also pay attention to viscosity and lubricity. Material compatibility, however, is critically important in both cases, and I would be much more comfortable with a product for which the chemical makeup (and thus the material compatibility matrix) is known and available on a technical datasheet, versus a proprietary product that has no such publically available information.

Similarly, with regard to compensating a Minn Kota motor specifically, I would want to know what all of the internal materials that may be in contact with the compensation fluid are, to make sure I was specifying a compatible fluid. I have no idea what information is available in that respect.

Finally, I do wonder how much fluid breakdown is occurring not as a result of chemical incompatibility, but rather as a result of the brushes lifting from the armature due to the journal effect of the compensating fluid forming a boundary layer in between the two parts as they rotate, and a consequent electric arc forming which jumps the gap and cooks the fluid in the process. The combination of fluid breakdown and ablation of the brushes from such arcing could possibly be the source of the blackened fluid, as opposed to dissolution of the components due to chemical incompatibility.

I find myself wondering if there might be a sweet spot with respect to the compensating fluid viscosity where the rotor is actually slowed by the fluid to the extent that the brush contact is actually better than it would be when rotating faster in a lesser viscosity fluid.

Sean

-------- Original Message --------
On Jul. 25, 2023, 12:11, Cliff Redus via Personal_Submersibles < personal_submersibles at psubs.org> wrote:

The point of the thread is Psubbers like to use theMinn-Kotta 101 lower units as a starting point for thrusters because they arecheap, simple to control, quiet and simple to work on.  For my boat I have used these with both airand oil compensation and have now lost a thruster using each of these pressure compensationstrategies.  Typical run lives of trollingmotors are on the order of 5-10 years for boaters.  This thruster had less than 20 hours of runtime.  How can we boost the reliability ofthese thrusters?

R300 Thruster Failure, Beaver Island Expedition July 15,2023

We had a great IS Expedition at Beaver Island on Lake Michigan.  Water was blue, visibility was great, supportexcellent.  Dives were great!  That’s the good news.  The bad news is that after a submerged twomile transect when I surface, I lost the port stern horizonal thruster. Alec’sson Treavor was the safety diver for the expedition.  I asked him to swim over and inspect.  There were no obvious issues like had occurredlast year at Lake Charlevoix when a limb got lodged between the prop and ductednozzle and lockrf up rotation.  After recoveringthe boat, I disassembled the thruster.  Theseare Minn-Kotta 101 lower units that have been modified by adding hydraulicpressure compensation with WD-40 for the fluid and a small bellow style bladderfor thermal expansion.  Beforedisassembly, I noticed that the bladder for this unit was completely compressed.  The bladders on the three remaining thrusterswere expanded almost to the point of rupture and were black in appearance.  Also, before I disassembly, I pushed radiallyon the prop shaft and was rewarded with a squirt of black 10WD-40.  The shaft had a lot more radial play thannormal.  From this I could tell the shaftbushing was worn and that both the thruster lip seals had failed.  Upon disassembly, I drained the contents ofthe remaining fluid into a plastic pail. See picture at the Psubs web site. What came out was black WD-40 fluid and a lot of loose black sludge whichwas a portion of the brushes.  Trollingmotors are typically made of a blend of carbon and graphite also known ascarbon-graphite.  Upon pulling off thebow cap and brush end of the trolling motor I found that the surfaces were cakedwith black sludge.  See picture.  Inspecting the brushes showed the cause of failure.  Both brushes were about half the thickness ofa new brush set.  One of the brushessprings had bottomed out thus no spring force was being applied to the brushand thus loss of electrical contact.  TheWD-40 fluid had been in the thruster since last year’s Psub convention in Lake Charlevoix.  According to the manufacturer MSDS sheet, WD-40consist of 30-60% petroleum distillates, 10-30% petroleum base oils and 5-15%Naptha.  My working hypothesis is that oneor more of the components in the WD-40 reacted with the binding agent in the carbon-graphitebrushes and reduced the mechanical strength of the brush thus leading to acceleratewear.  Over the two years period (17,500hours), the thrusters had two main dive events with a total of no more than 20hours on the units.  The balance of thetime, the thrusters were sitting on the boat in my shop soaking in this WD-40at elevated Texas temperatures.   BTW,the driver for using WD-40 is that is a very low kinematic viscosity (2.8 cStat 100 F or 38C). Note water is about 1 cSt. 

One other observation on the failure was the wear on the armatureshaft.  It has a visible wear ring andthe shaft bushing went from a snug fit to a loose fit.  Working hypothesis is that the carbon-graphiteparticles in suspension were acting like an abrasive polish.  

The question is how can we improve the reliability?  Should we investigate a different seal andtry to get by with 1-ATM operation or investigate a different oil or go back toair compensation?  What Alec and I discussedat the Expedition was to try a single mechanical carbon seal or a high pressure-ratedlip seal.  If we can come up with somethingto try, I am willing to put a Minn-Kotta 101 in my test chamber, power it up sothat the seal in a dynamic mode and increase pressure to failure.  A control would be to run an off-the-shelf MK101 with no pressure compensation to failure.

Any thoughts?  I wouldlike to hear what experience others have had with oil compensation on MK 101’s.

Cliff

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