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<div dir="ltr" data-setdir="false"><div><p class="ydp84f85499MsoNormal">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.<span> </span>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.<span> </span>Typical run lives of trolling
motors are on the order of 5-10 years for boaters. <span> </span>This thruster had less than 20 hours of run
time.<span> </span>How can we boost the reliability of
these thrusters?</p>
<p class="ydp84f85499MsoNormal">R300 Thruster Failure, Beaver Island Expedition July 15,
2023</p>
<p class="ydp84f85499MsoNormal">We had a great IS Expedition at Beaver Island on Lake Michigan.<span> </span>Water was blue, visibility was great, support
excellent.<span> </span>Dives were great!<span> </span>That’s the good news.<span> </span>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.<span> </span>I asked him to swim over and inspect.<span> </span>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.<span> </span>After recovering
the boat, I disassembled the thruster.<span> </span>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.<span> </span>Before
disassembly, I noticed that the bladder for this unit was completely compressed.<span> </span>The bladders on the three remaining thrusters
were expanded almost to the point of rupture and were black in appearance.<span> </span>Also, before I disassembly, I pushed radially
on the prop shaft and was rewarded with a squirt of black 10WD-40.<span> </span>The shaft had a lot more radial play than
normal.<span> </span>From this I could tell the shaft
bushing was worn and that both the thruster lip seals had failed.<span> </span>Upon disassembly, I drained the contents of
the remaining fluid into a plastic pail.<span>
</span>See picture at the Psubs web site.<span>
</span>What came out was black WD-40 fluid and a lot of loose black sludge which
was a portion of the brushes.<span> </span>Trolling
motors are typically made of a blend of carbon and graphite also known as
carbon-graphite.<span> </span>Upon pulling off the
bow cap and brush end of the trolling motor I found that the surfaces were caked
with black sludge.<span> </span>See picture.<span> </span>Inspecting the brushes showed the cause of failure.<span> </span>Both brushes were about half the thickness of
a new brush set.<span> </span>One of the brushes
springs had bottomed out thus no spring force was being applied to the brush
and thus loss of electrical contact.<span> </span>The
WD-40 fluid had been in the thruster since last year’s Psub convention in Lake Charlevoix.<span> </span>According to the manufacturer MSDS sheet, WD-40
consist of 30-60% petroleum distillates, 10-30% petroleum base oils and 5-15%
Naptha.<span> </span>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.<span> </span>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.<span> </span>The balance of the
time, the thrusters were sitting on the boat in my shop soaking in this WD-40
at elevated Texas temperatures.<span> </span>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. </p>
<p class="ydp84f85499MsoNormal">One other observation on the failure was the wear on the armature
shaft.<span> </span>It has a visible wear ring and
the shaft bushing went from a snug fit to a loose fit.<span> </span>Working hypothesis is that the carbon-graphite
particles in suspension were acting like an abrasive polish.<span> </span></p>
<p class="ydp84f85499MsoNormal">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.<br></p>
<p class="ydp84f85499MsoNormal">Any thoughts?<span> </span>I would
like to hear what experience others have had with oil compensation on MK 101’s.</p>
<p class="ydp84f85499MsoNormal">Cliff</p></div><br></div></div></body></html>