[PSUBS-MAILIST] Minn Kotta 101 - Thruster Reliability

Tue Jul 25 14:37:57 EDT 2023

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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