Mercury 250 optimax breaking driveshafts.

[ljay]

Awesome thank you. I will look at that next

[Fishinmymission]

Post pictures of the fractured shaft. Post as detailed as possible pictures of the fracture surface. Also post detailed pictures of the splines.

I work failure analysis engineering for a helicopter company and have worked my fair share of broken shafts.
From just looking at the fracture surface you can see where the crack started. Sometimes it comes from a defect or wear (unlikely since you had two shafts break). Sometimes you can tell without a microscope if the shaft broke from being overloaded instantly or if it fatigued. Usually you need a microscope or SEM though.
I can't guarantee the photos will be good enough to learn anything but I might be able to see something.

Sometimes you can detect misalignment from spline wear patterns.

From the information provided it sounds like a misalignment.

[Dave S]

Send him, maybe that will help. Also could be a bad lower crank bearing.

[flabum1017]

If it's out of alignment, it might show on the upper driveshaft bearing....

[ljay]

I left it with a shop and gonna see what insurance will cover i will know something in a week or so maybe. Thanks guys

[ncgnick]

I have a similar problem. Perhaps the same. The 250 XS has twice broken the driveshaft at pinion. Both times under the same conditions. I was traveling without weight (2 persons on the boat) on a calm sea at 4000 rpm with a normal trim and a speed of about 35 knots. The lightest conditions an XS can experience. Suddenly over rpm buzzer and lost of propulsion. Before the first time I never had a problem even when driving to the limits and rough seas for many years. The first time it broke the debris broke all the gears and I changed everything (clutch, driveshaft, propeller shaft, gears, pinion, bearings) I only cept the sportmaster case. Now everything is intact but the driveshaft broke at pinion.This happened prior to complete 10 operating hours after the first repair.

[ljay]

Yeah the local mechanic said he has seen it several times. had 2 shafts he showed me broke about same spot says its from letting off throttle and hitting it again to fast. another said it was a series of bad shafts put out. who knows

[ncgnick]

"Who knows" is an easy wording. The hard part is the shaft's cost (about 350USD) and the mechanic's labor which is almost 300USD. If there is a defect number of parts out there Mercury racing has to take its responsibilities. But lets focus on the alignment issues I read in this thread. Is there any such chance or misalignment with crank is not an issue that can occur?

[ljay]

Yes. It can and will occur on lots of them. The 3 things i have found thru this forum and a couple of other forums is First where the powerhead meets the mid section if it was not seated correctly or bolts back out will cause this problem. 2 where the lower mounts to mid if its not seated right will cause a it. 3rd the shaft bearing all cause the same action which is a slight bend which causes a vibration and thus breaking after time. all are supposed to be a easy tell but i never seen it on mine.

[Fishinmymission]

I have done quite a few drive shaft fatigue crack investigations but not on outboards. Showing the best macro pictures you can take of the splines and fracture surfaces can help identify the cause. Basically its either torsional fatigue or bending fatigue from misalignment (as long as you didn't hit anything or had two defective shafts). Sources of misalignment are power head mounting surface to crankshaft orientation, upper and lower mounting surfaces of the mid and plates, mounting surface of the lower unit to the drive shaft orientation, dowel locations. I'm notfamiliar with the Merc lower unit layout but ensure setup can't be throwing it off angle or position Shimming off? If there is misalignment the splines sometimes show a taper in the Top land between upper and lower portions of the driveshaft splines since they will see different stress and wear patterns if there is a misalignment. A two piece drive shaft or even better in intermediate vespel OD ID splined adapter will eat up misalignment on many applications where it's to expensive to design or machine perfectly aligned systems.
If you cycle too high a load through a shaft (a certain manuever with a high pitch propeller that resists acceleration) and do it repeatedly it could fail in torsional fatigue. Might need microscopes or SEM to see the small surfaces needed to make the conclusion.

[ncgnick]

Thank you Ijay. Thank you Fishinmymission. Before the first damage I used the boat in very tough seas and I had many times the propeller out of the water at high rpm. Then into the water again and so one. After that I used the boat for my vacation very gently. After one week of very gent use the shaft broke when in calm sea at 4000 RPM without any load without hitting of anything. (Maybe the cause was already there from the tough sea driving). The second time I never stressed the engine and the gearbox. I was in the first ten hours after the repair. One day when trying to park the boat the propeller engaged with an anchor rope in a port when run at zero throttle (just engaged). The engine stopped. I removed the rope and continued without any problem. After one week the shaft broke at pinion when run at 4000 rpm.I checked the oil. No water in it. Everything fine. No markings on the gear teeth. Perfect simming. Before the first failure I used the engine for 400 hours without any problem. Most of these hours at high rpm and heavy loads. We are talking for the sportmaster racing gearbox. Not any gearbox. Can you suspect any reason for misalignment gained after all these? The engine power head has never been disassembled. Thanks

[Fishinmymission]

Very interesting. Your thinking about this the right way though you have two exact failures at the same location. Fatigue doesn't always have to take the same mount of time but think what changed about 5 to 20 hours before the first drive shaft went. I can almost guarantee you will snap a third shaft if you install one without addressing the issue. Post some good pictures it might help. I don't believe an engine stall at idle from a rope wrap is enough stress to cause any ill effects. Did anything get bent or lose in the lower unit on that rough trip hard to believe but a possibility. Were all the lower unit parts reused if so look for any sort of wear near the fracture. The smallest amount of wear can create a rough surface which could start a fatigue crack. Something in the lower unit could be wearing the shaft at the fracture.

[ncgnick]

Thanks fore once more. All the parts are completely new. As have explained earlier I kept only the aluminum case. Failing at only 4000 rpm twice to the same point can lead me in two possibilities. 1. Harmonic vibration with peak at 4000 rpm. In this case I will find the driveshaft bearing at least slightly affected. 2. Misalignment forcing the shaft to bent. I will ask my mechanic for careful inspection. If it is harmonic problem then I will not be that lucky. Its very hard to find it as it is a function of RPM and load on the propeller shaft. In that case I have to think of some trick. Maybe by tightening a small inox clamp on the shaft I could destroy the vibration and hopefully to tune to a harmonic outside the operating RPM

[Fishinmymission]

Thanks fore once more. All the parts are completely new. As have explained earlier I kept only the aluminum case. Failing at only 4000 rpm twice to the same point can lead me in two possibilities. 1. Harmonic vibration with peak at 4000 rpm. In this case I will find the driveshaft bearing at least slightly affected. 2. Misalignment forcing the shaft to bent. I will ask my mechanic for careful inspection. If it is harmonic problem then I will not be that lucky. Its very hard to find it as it is a function of RPM and load on the propeller shaft. In that case I have to think of some trick. Maybe by tightening a small inox clamp on the shaft I could destroy the vibration and hopefully to tune to a harmonic outside the operating RPM

I don't under stand the harmonic vibration as vibrations are some complex ****. But something would have had to have changed to start the vibration after your first 400 hours if that is your theory. did you change props?

[powerabout]

my 2c, usually a bent drive shaft housing causing misalignment will break the shaft somewhere around the water pump