[PSUBS-MAILIST] Minn Kotta 101 - Thruster Reliability

Tue Jul 25 15:22:18 EDT 2023

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 wrote:

> The point of the thread is Psubbers like to use the Minn-Kotta 101 lower units as a starting point for thrusters because they are cheap, simple to control, quiet and simple to work on.For my boat I have used these with both air and oil compensation and have now lost a thruster using each of these pressure compensation strategies.Typical run lives of trolling motors are on the order of 5-10 years for boaters. This thruster had less than 20 hours of run time.How can we boost the reliability of these 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, support excellent.Dives were great!That’s the good news.The bad news is that after a submerged two mile transect when I surface, I lost the port stern horizonal thruster. Alec’s son Treavor was the safety diver for the expedition.I asked him to swim over and inspect.There were no obvious issues like had occurred last year at Lake Charlevoix when a limb got lodged between the prop and ducted nozzle and lockrf up rotation.After recovering the boat, I disassembled the thruster.These are Minn-Kotta 101 lower units that have been modified by adding hydraulic pressure compensation with WD-40 for the fluid and a small bellow style bladder for thermal expansion.Before disassembly, I noticed that the bladder for this unit was completely compressed.The bladders on the three remaining thrusters were expanded almost to the point of rupture and were black in appearance.Also, before I disassembly, I pushed radially on the prop shaft and was rewarded with a squirt of black 10WD-40.The shaft had a lot more radial play than normal.From this I could tell the shaft bushing was worn and that both the thruster lip seals had failed.Upon disassembly, I drained the contents of the 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 which was a portion of the brushes.Trolling motors are typically made of a blend of carbon and graphite also known as carbon-graphite.Upon pulling off the bow cap and brush end of the trolling motor I found that the surfaces were caked with black sludge.See picture.Inspecting the brushes showed the cause of failure.Both brushes were about half the thickness of a new brush set.One of the brushes springs had bottomed out thus no spring force was being applied to the brush and thus loss of electrical contact.The WD-40 fluid had been in the thruster since last year’s Psub convention in Lake Charlevoix.According to the manufacturer MSDS sheet, WD-40 consist of 30-60% petroleum distillates, 10-30% petroleum base oils and 5-15% Naptha.My working hypothesis is that one or more of the components in the WD-40 reacted with the binding agent in the carbon-graphite brushes and reduced the mechanical strength of the brush thus leading to accelerate wear.Over the two years period (17,500 hours), the thrusters had two main dive events with a total of no more than 20 hours on the units.The balance of the time, the thrusters were sitting on the boat in my shop soaking in this WD-40 at elevated Texas temperatures.BTW, the driver for using WD-40 is that is a very low kinematic viscosity (2.8 cSt at 100 F or 38C). Note water is about 1 cSt.
>
> One other observation on the failure was the wear on the armature shaft.It has a visible wear ring and the shaft bushing went from a snug fit to a loose fit.Working hypothesis is that the carbon-graphite particles in suspension were acting like an abrasive polish.
>
> The question is how can we improve the reliability? Should we investigate a different seal and try to get by with 1-ATM operation or investigate a different oil or go back to air compensation? What Alec and I discussed at the Expedition was to try a single mechanical carbon seal or a high pressure-rated lip seal. If we can come up with something to try, I am willing to put a Minn-Kotta 101 in my test chamber, power it up so that the seal in a dynamic mode and increase pressure to failure. A control would be to run an off-the-shelf MK 101 with no pressure compensation to failure.
>
> Any thoughts?I would like to hear what experience others have had with oil compensation on MK 101’s.
>
> Cliff
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