ELECTRIC MOTOR SHEAR PIN...

[Sea Antsy]

Looking for some guidance please .. Opinions or facts gratefully accepted…
A neighbor (really) got this brand new 3hp ELECTRIC motor.. Not a trolling motor, more suited for a dingy or a small - medium sailboat.. The 2 blade plastic prop broke off both blades the first time it hit the sand bar….

?.. I don’t know if there is a overcurrent protection circuit to shut down when “excessive” resistance is encountered.. as current increases.. Or if any circuit could react that fast… but

The fail safe device is olde school.. A shear pin…looks pretty standard.. Bout 1 ½” long and 3/16” or so.. But it’s non magnetic.. ? I suspect it’s stainless steel… it is also perfectly intact !
And as a point of reference the 800# gorilla in trolling motors uses a stainless steel shear pin as well but it 1” x ⅛” quite a bit lighter and Amazon rates the replacement for up to 65# of thrust.

I seem to recall, back in the day, only several decades ago that shear pins were ? brass..
At any rate, once he gets the 2 new propellers (not in stock, backordered, ugg) I don’t feel comfortable advising him to put that “stainless” pin back in for another go round..
I have him going to the hardware store and we will cut down a #10 brass machine screw and see if that will sacrifice it’s self rather than the prop blades.

I am unarmed in a battle of wits over comparing the 3hp electric to a gas outboard, or 65# of thrust.. But I am more than willing to sacrifice a few test shear pins vs the plastic props..

And VERY conveniently to replace the prop one only needs to carry a 17mm DEEP socket to get the NYLOCK nut off the prop shaft !!!!... no simple castle nut and cotter pin… ??

For a electric motor that has smart gps, and all the whistles and bells and costs over 3K$,
Might not be well thought out….IMHO

? Comments, observations welcome, thanks

 

[Al U Minium]

Comment

Don't run the prop into the ground

The pin is probably not a shear pin but a drive pin

If the prop broke, sounds like it was deigned to fail when striking the immovable earth

I like the idea of carrying spare parts and tools

 

[airshot]

You can always use a smaller dia pin so it will shear easier...add a dab of grease to hold n place during assembly.

 

[Pappy]

Every trolling motor I know of has or at least recommends you add over current protection. Usually in the form of a circuit breaker. Most circuit breakers are not designed to break quick enough to protect the prop from damage.
Your idea of a brass shear pin will hopefully accomplish what you want it to do but......the manufacturer may have designed the blades to go first and protect the motor. Brass will shear before SS will.
Your idea of the castle nut and cotter pin can be done pretty easily by the way. Drill your hole in the propshaft and then make sure to chamfer the hole at both sides to reduce shear load due to the sharp edges. Good thinking on your part!

 

[Still Afloat]

I agree that it’s drive pin rather than a shear pin. Even brass is likely too strong to save the prop.

You may want to try cutting the head a nylon bolt for use as a shear pin. I’m not sure how well it will hold up but, you need a material that’s softer than the plastic prop if it’s expected to function as a shear pin.

Back when I was into RC planes nylon bolts were commonLH used where decent shear strength was needed during flight but, you still wanted the bolts to break in the event of a minor crash or hard landing. A commonly use was to fasten the wings and landing gear. I never had an in air failure despite some high G maneuvers.

 

[Pappy]

I agree that it’s drive pin rather than a shear pin. Even brass is likely too strong to save the prop.

You may want to try cutting the head a nylon bolt for use as a shear pin. I’m not sure how well it will hold up but, you need a material that’s softer than the plastic prop if it’s expected to function as a shear pin.

Back when I was into RC planes nylon bolts were commonLH used where decent shear strength was needed during flight but, you still wanted the bolts to break in the event of a minor crash or hard landing. A commonly use was to fasten the wings and landing gear. I never had an in air failure despite some high G maneuvers.

LOL ....I did, and it was spectacular! Wing sheared the nylon bolts, the fuse buried itself in the ground and the wing came down intact after an additional 10 seconds or so. Got a standing ovation for that one.

 

[Still Afloat]

LOL ....I did, and it was spectacular! Wing sheared the nylon bolts, the fuse buried itself in the ground and the wing came down intact after an additional 10 seconds or so. Got a standing ovation for that one.

Oops! Maybe you got a bad one. Seriously, I used 1/4-20 pan head nylon bolts for hundreds of flights and never lost a wing. What’s the saying, your mileage may vary?

I did lose half of a wing in a midair collision once.

 

[GTS225]

In either case, gravity gets VERY unforgiving.

Roger

 

[Sea Antsy]

GR8 Ideas.. we got the juices flowin here.. I like the nylon screw idea.. will see if I can get the right length and try it out.. and report back of course..
thanks all for the brain share..

 

[Sea Antsy]

well.. I did find a nylon screw at LOWES.. it was 1/4-20 so it needed some trimming to fit.. they did not have anything in #12 screw size..
Weather here is crappy.. neighbor may not get to test till it clears a bit.. will report back..

 

[airshot]

Looking forward to how that works...

 

[Still Afloat]

well.. I did find a nylon screw at LOWES.. it was 1/4-20 so it needed some trimming to fit.. they did not have anything in #12 screw size..
Weather here is crappy.. neighbor may not get to test till it clears a bit.. will report back..

Better bring a steel pin along, just in case.