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Thread: What bent my prop shaft? Reply to Thread
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05-24-2012 07:41 AM
REW It does sound like you need to make an appointment with a person to get the prop shaft changed on your lower unit.
Hopefully the person could have a new lower unit shipped into them, so that you could just drive over, wait for the folks to drop your current lower unit, and put the new lower unit on the boat.

With the twisted prop shaft and the ground up backside of the case, it would seem to be the best thing to do to replace the lower unit and move on.

If you wanted to save the 12 hour drive, possibly the folks could drop ship the lower unit to you and you could drop the lower unit and put the new one on your motor so it would save you the drive. You could then ship the old lower unit in for core credit.

As I said before, your insurance should pick up the entire cost, since you had a bottom strike that caused the issue and that is what insurance is for.

Good luck
REW
05-23-2012 08:40 PM
Albertan
Quote:
Originally Posted by staylor View Post
Sometimes a gearcase design will have axial play in the shaft against a loading spring. This style of gearcase requires that you push in on the shaft when you try and spin the installed prop. I had an OMC V-6 at one time that had 1/8 inch axial play and if you didn't have at least 1/4 inch gap between the case and the stepped forward part of the prop outer hub you would risk gearcase contact. Two other possibilities relate to the prop itself and the thrust washer.

1. Most hubs have the prop's inner hub ring contacting a step on the prop washer to limit forward motion of the prop on the shaft. Typically when you torque the prop you will feel when the step is contacted because the nut stops turning and you just build torque against the metal to metal contact. If the thrust washer was not correct, you could properly torque the prop nut and end up with the soft rubber or plastic hub material carrying the axial load. Under running conditions this would allow the prop to come forward under load. If the prop was not torqued to spec this same problem could show up if the prop thrust washer was incorrect.

2. If the prop was cast or machined wrong with respect to the axial position of the inner and outer hub rings, then you could also end up with the soft hub material being forced to carry the axial loads, allowing the prop to come forward.
Doug
Thanks Doug. I can't even check for play now with the prop sitting on the gearcase. I checked it when I first installed it, realizing that it was close to the gearcase. The prop spun freely and there was no play that I could discern. I'll be interested to see what PowerTech says about the thrust washer they sent me. I think that's the problem. It's a spacer in the end after all, and the prop obviously wasn't spaced properly from the gearcase.
05-23-2012 08:36 PM
Albertan
Quote:
Originally Posted by REW View Post
Albertan,
You can't very well run your motor with the correct prop with a twisted prop shaft.
The lower unit will have to be removed, disassembled, and the prop shaft replaced.

It is possible that the prop shaft that was put in the motor was not tempered correctly as well.

I have never heard of a prop shaft twisting.

Good luck
REW
Yeah I understand the prop shaft needs replacing, and the disassembly required to complete that. I can't even mount the Suze props at this point. I'm more concerned if any other parts in the leg would likely be damaged. I'm just a little lost since I've never had this issue before with the Suze props. I'll have to find a place to have a look at it for me. Even though there are no Suzuki dealerships around, I should be able to find someone to assess the lower unit for me. I have the shop manual. Thanks for the feedback.
05-23-2012 06:21 PM
REW Albertan,
You can't very well run your motor with the correct prop with a twisted prop shaft.
The lower unit will have to be removed, disassembled, and the prop shaft replaced.

It is possible that the prop shaft that was put in the motor was not tempered correctly as well.

I have never heard of a prop shaft twisting.

Good luck
REW
05-23-2012 03:41 PM
staylor
Albertan...

Sometimes a gearcase design will have axial play in the shaft against a loading spring. This style of gearcase requires that you push in on the shaft when you try and spin the installed prop. I had an OMC V-6 at one time that had 1/8 inch axial play and if you didn't have at least 1/4 inch gap between the case and the stepped forward part of the prop outer hub you would risk gearcase contact. Two other possibilities relate to the prop itself and the thrust washer.

1. Most hubs have the prop's inner hub ring contacting a step on the prop washer to limit forward motion of the prop on the shaft. Typically when you torque the prop you will feel when the step is contacted because the nut stops turning and you just build torque against the metal to metal contact. If the thrust washer was not correct, you could properly torque the prop nut and end up with the soft rubber or plastic hub material carrying the axial load. Under running conditions this would allow the prop to come forward under load. If the prop was not torqued to spec this same problem could show up if the prop thrust washer was incorrect.

2. If the prop was cast or machined wrong with respect to the axial position of the inner and outer hub rings, then you could also end up with the soft hub material being forced to carry the axial loads, allowing the prop to come forward.
Doug
05-23-2012 10:25 AM
Albertan
Quote:
Originally Posted by staylor View Post
1. Prop shaft is failed in torsion- thus it was overloaded in torsion
2. Loading on this particular propshaft consisted of three different input sources as follows:
a. Normal running torque to push prop thru water
b. A continuous heavy torque overload from an improper thrust washer literally grinding the prop into
the gearcase. This alone should have melted the hub- but I suspect it didn't because the improper
thrust washer may have allowed a secondary load path to be formed by some of the hub parts
being bottomed metal to metal because of the improper washer.
c. A shock load coming in as both a bending and a torsion load to the propshaft- and this adds to
the loads from a. and b. above.
3. The short spline engagement is not unusual- many high performance boats run OMC props on Merc motors with only around 1/2 inch engagement with no problems- I have done this myself on many occasions running Raker props on Mercs with an aftermarket set of washers. Before they started selling interchangable hub kits if you wanted to run a Merc Prop on an OMC you would simply put a big drill into the hub and start drilling out the splines until you only had an inch or so left to carry the drive loads.
4. SS props are big and tough- thats why we buy them- but the prop shaft is also stainless of similar strength- except that its small and wimpy compared to the rugged high cross sections of the stainless prop

So, as the old song went- something's got to give- and in this case it was a simple torque overload of the prop shaft. And as a practicing mechanical engineer, I can honestly say that the pics show a near perfect classical torsional failure- no real out of plane bending- just a nice torsional twist past the yield of the material. The short splines did not shear off- so they had plenty of engagement because they still managed to twist the shaft past it's yield strength.

In conclusion- the load from a. above is normal. The load from c. above happens all the time and unless the boat grounds on the shoal the prop shaft doesn't fail because the hub spins. Thus, the load from b. above is the one that broke the camel's back- and the culprit is the incorrect spacing of the prop from the lower unit most likely caused by the wrong thrust bearing. When this prop was installed the last step should have been to simply spin the prop by hand with the motor in neutral and the kill switch pulled. This would have shown the problem right away- since you would not have been able to spin that prop by hand since it was jammed into the rear of the gearcase bullet. The loss of top end rpm was most likely being caused by much of the motor's torque being used to grind the gearcase away.
Doug
Thanks Doug. That's exactly the sort of info I was looking for. I am going to throw a wrench into your theory though...the prop spun perfectly freely when it was initially mounted. Somehow, either in operation or after hitting the rocks, the tolerance decreased to a point where rubbing occurred. My theory is that it happened under load during normal operation, which is why my motor stalled out at 5700 rpm a couple of times. But again, I don't know alot about outboards. Thanks again for your feedback/expertise.
05-23-2012 09:18 AM
Rippin Bite I have heard this before on power tech props as they dont go all the way on the spilne shaft, Your not the first on this,
05-23-2012 09:15 AM
Albertan
Quote:
Originally Posted by REW View Post
The good news is that all of the issues will be covered by insurance.
That is what insurance is for.

Replace the lower unit.
Get rid of the power tech prop.
If this is a Suzuki engine, just use Suzuki matching props and move on.

Have a great day and enjoy your rig.

Be safe
REW
Do you really think there will be issues with the lower unit? The nearest Suzuki dealer is over 12 hours away...
05-23-2012 09:14 AM
Albertan
Quote:
Originally Posted by dakotahunter12 View Post
First off I'm not familiar with Power tec or suzies but it sure looks like you are missing something.... did you have the right thrust washer for the Power tec?
A little more info on this matter, as it was late last night when I replied before bed.

I used the parts that were shipped to me by PowerTech with the prop. I also took pictures to confirm assembly with the vendor who sold me the prop. There isn't a thrust washer made that will change the fact that the PowerTech prop only has a little over an inch of splines engaging the shaft. But I'll discuss that with them as well when I contact them. I'm just trying to get some opinions from people with more knowledge than myself.

Just as an aside, this is a picture of the Suzuki thrust washer, with the arrow pointing to it, and the PowerTech washer sitting on top of it. They look completely different. But then the hubs on the two props are completely different spacing-wise as well.

http://albertan.smugmug.com/Other/Wa...ronMotor-M.jpg

And a picture of the PowerTech bolted on. Doesn't look out in left field to me. A larger thrust washer would have resulted in the prop nut being off the shaft/beyond the hole for the cotter pin IMO.

http://albertan.smugmug.com/Other/Wa...IMG0461-X3.jpg
05-23-2012 09:12 AM
Albertan
Quote:
Originally Posted by Shellback View Post
Props should be designed for the hub to fail long before twisting the shaft. Would the manufacturer use a softer material for the shaft so you don't destroy the whole lower unit? I'd send those pictures to Powertech and find out if the hub should have failed before twisting the shaft. Don't know what to say about the big difference in spline contact area. You would think they should have designed the hub to fail first if reducing spline area.
PowerTech props have a cushion lock hub consisting of a bunch of ceramic(?) and rubber rods that are supposed to fail to prevent damage to the lower unit...you can see it here.

The vendor is helping me out. Pictures are being sent to PowerTech to see what they say.
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