prop nut failure and prevention

[Home Cookin']

my prop came off when I was finally able to get a restoration project in the water! Luckily it was at the pier, and at high tide, and later a diver retrieved the prop, but I still lost the nuts, bushings, etc. and had to put the boat away another 2 weeks.

When shopping online for the parts (1972 motor) I was surprised to see that the nut kit had one of those nylon-threaded lock nuts rather than a castle nut, so I was planning to get a castle nut anyway. Couldn't find everything at the boat junkyard. On advice I double-checked and sure enough my shaft is not drilled out so I have to go with the nylon, and have ordered the kit.

Obviously the old one failed and the consequences could have been a whole lot worse--certainly more expensive. When it first happened I blamed myself for removing the cotter pin several months ago and forgetting to replace--but it now appears that the nylon failed--maybe b/c i didn't use the motor for at least a year maybe two?

I've never lost a prop. I've also never had one with a nylon lock nut and I'm not happy about it. Even my old evinrude's plastic cap was secured by a cotter pin.

I also am not one who will inspect every nut and bolt literally from stem to stern before, during and after every boat trip. I use most of my boats frequently and expect more out of them; they are there to serve me and not the other way around.

Is this a common problem? A design failure? any solutions? maybe a double nut? Not sure I want to drill my own hole, but...?

1972 Merc 200 (20 HP) but it could be any motor rigged this way. i don't know if I could find a second one at the hardware store threaded for the shaft anyway.

 

[boobie]

Re: prop nut failure and prevention

I don't know the spec but did you have it torqued properly ?? Another thing, once you use a lock nut and take it off you don't use it over again. Need a new one.

 

[Bob_VT]

Re: prop nut failure and prevention

Is there a hole in the prop shaft? You can pick up castle nuts at most good hardware stores...... OR make a lock similar to the ones used on I/O props from light metal. Folling your restoration I know you have the skills. Yup, nylock nuts are single use only.

 

[Home Cookin']

Re: prop nut failure and prevention

thanks guys. I didn't think about nylocks being single use and I bet this happened b/c I removed it for inspection and replaced it. Fairly expensive lesson over a POS nut and could have been worse.

So I guess I should carry at least one spare nut in case I get a line wrap or something and have to pull the prop at sea? I hope it's a standard size/thread but knowing kliekhoffer not likely.

Sounds like an inferior means to secure a prop, and that's not just from my OMC/Yam prejudice. Merc should know better. Maybe I'll put two on if there's room. No hole drilled in the shaft for a cotter pin and I don't know if I want to risk drilling one in myself and causing other problems.

Is this something that happens or am I just the unlucky exception?

 

[dingbat]

Re: prop nut failure and prevention

I ran a Merc 175 with a nyloc nut for 5 years. Never had a problem.

Reusing a nyloc three or four times typically isn't a problem. Problems arise the 5th or 6th time

 

[Home Cookin']

Re: prop nut failure and prevention

Problem is--how do you know???

Never lost a prop in 55 years of boating until now.
Never used nyloc prop nut until now, either.
Coincidence? I think not!
(OK to be fair it's a 1972 motor with an original prop but that motor was "on ice" in a shed somewhere for 30 years or so, I think)

I'm still mystified by the forces that would unscrew a nut in the first place. On a splined prop shaft the whole unit turns as one; there is no "screwing" force, just stright back pressure (or isn't it forward?) And I was only in gear for a few seconds when this happened. But that's a boat for you!

Live and learn!

 

[java230]

Re: prop nut failure and prevention

Does yours not have a tab washer? Mine does as a back up to the nylock. Does the prop have a little nub either side of the nut?

like so?
Sierra - 18-3204, Parts, Sierra Tab Washer #18-3204 - FirstChoiceMarine.com

 

[dingbat]

Re: prop nut failure and prevention

Does yours not have a tab washer?

Ding, Ding, Ding.....

I'm guessing his mechanic neglected to install the tab washer when he put the prop on. Properly installed, the nut can't come off

 

[Home Cookin']

Re: prop nut failure and prevention

no tab washer on this model (Merc 200 (20 hp) 1972)

but thanks for showing me what those things are; the boat junk yard had a whole box of them

 

[Chris1956]

Re: prop nut failure and prevention

The Merc nut locking system is a very effective one. You could get by with just the nylon lock nut, or a regular nut and the tab washer, IMO, however, a standard Merc prop uses both. It is easily as good as a nut and cotter pin.

You do not need to change the locknut everytime you remove the prop. I have used the same one for decades. Occasionally you need to use a new tab washer.

 

[Home Cookin']

Re: prop nut failure and prevention

Prop Nut Kit, Mercury, Mariner older 20 HP

Part Number: MAL9-73962
Price: $10.95


Prop Nut Kit
Replaces: Mercury 11-64075A1
Fits: Older 20 hp

Prop Nut Kit | Mercury | Mariner older 20 HP

this set-up doesn't have the tab washer nor does it look like one would fit; I'm not familiar with that device so when I get my kit I'll see if one can be added

 

[fishrdan]

Re: prop nut failure and prevention

this set-up doesn't have the tab washer nor does it look like one would fit; I'm not familiar with that device so when I get my kit I'll see if one can be added

A tab washer wouldn't work with that kit as there isn't anything for the tab washer to lock into, that's fixed to the prop shaft. (IE: bronze sleeve in dingbat's pic)

I think you'll be safe with just the new nylock nut, if not.............

:watermelon:

 

[robert graham]

Re: prop nut failure and prevention

You could easily drill and tap your new prop nut for a small Allen head set screw....and when installed it wouldn't show up much at all and maintain that "original" look....

 

[Scaaty]

Re: prop nut failure and prevention

OK, time to chine in this. Been running fast "stacks", Merc's, with a surface slapping 2 blade Stainless, and they aint cheap either. Ann been at this 50 plus years too. OK, on the convoluted splines cap end prop, then seat the tab washer with the nut on, is old school, but a normal boat, fine. ME? (been Journeyman Machinist my life, as a wrench too). I simple on my 15 foot Flatty, cannot do any that stuff with the prop on it now.. What do I do? Simple. Thrust washer on first, prop, then a lock washer, and a Nyloc.
BUT...take a "Dreamel" with a cut off blade, and simple cut grooves on the prop, as the nut, for the lock washer to NOT spin off. Also Torque the crap out it. Not gonna hurt anything. Now next time off? (do it, and see what I mean), as works.
And I got a "priceless" blade my 150hp, 15 flatty..(not over powered either, boat designed for it). And I pull the prop anytime out water. Yearly, depending on use..(I run salt here too), I simple replace often, if needed.
Now, on your Merc shaft? Its either..(probably), a NF thread. Mine a 1 inch, 12 thread TPI, (or a NC which is 1-8, TPI) Nylocs sold everywhere. Been doing this since my old 70's Switzer, XS back! Only lost one blade then, (14x28 SS) when I got distracted putting in on to launch, and didn't torque the damn nut! Somewhere the Fox Lake Chain near "Blarneys, Thursday nite drags" is a $600 prop there! Now they are..price?..(that was the 70's!)
Pic my prop the Flatty...(oh yes, NEVER DO RED LOCTITE if ya ever want to get off again, as its a chemical bond, that solidifies the mating surface..like welding it if ya will)

 

[Scaaty]

Re: prop nut failure and prevention

You could easily drill and tap your new prop nut for a small Allen head set screw....and when installed it wouldn't show up much at all and maintain that "original" look....

Thats a good idea too! But do it after the prop nut tight. Then drill and tap a 1/4 20 set screw. Also never take a prop shaft without a pre drilled hole for a cotter pin, heat treated making it, chamfered, to relive "strees risers", metal. Do yourself? Well, hit a floter, a hard beach, and ya bust the end off the prop shaft...

 

[Chris1956]

Re: prop nut failure and prevention

Gee, I never saw a Merc propshaft with a hole for a cotter pin. Was it something they had in the 50's?

Scaaty, I once ran a 4-1/4 prop hub on a V6 Merc (4-3/4" hub) and got too much ventilation. Motor would hit 3K RPM, at idle speed. She would pick up a bit after that, but ran many RPMs higher than normal. How is it you can run that hubless prop? Doe she ventilate real bad on hole shot?

 

[Scaaty]

Re: prop nut failure and prevention

Gee, I never saw a Merc propshaft with a hole for a cotter pin. Was it something they had in the 50's?

Scaaty, I once ran a 4-1/4 prop hub on a V6 Merc (4-3/4" hub) and got too much ventilation. Motor would hit 3K RPM, at idle speed. She would pick up a bit after that, but ran many RPMs higher than normal. How is it you can run that hubless prop? Doe she ventilate real bad on hole shot?

Its a very light boat, maybe 350, then gas, batt, 330 for the merc, me 200, so under 1000. Actually of all the props I tried when building /rigging it, that blade works just fine. Its set up for acceleration fun. And ya nail it, spins to 3000, grabs, settles down to 2000, and then takes off. Lotta fun! Every set up is different so finding the right prop is a stroke of luck I guess!

 

[Home Cookin']

Re: prop nut failure and prevention

this adds to the frustration: the parts arrived here in town Friday night at Fed Ex but they won't deliver until Tuesday. So my boats sits in the driveway with the parts 5 miles away, out of commission for the long weekend ( I called and they won't let me come get it). Third weekend in a row out of commission since the launch and losing the prop after finally finishing restoration!