250xs Lower Unit Catastrophic Falure

[Yiannis]

Hello everyone, long time reader - first time poster.

I have a RIB powered by two 2005 250xs Mercs with a fleet master lower unit. The engines have 100 hours. During my first outing for the season and after annual service and winterisation the port engine suffered catastrophic failure of the lower unit. The attached pictures are very... informative.

The failure happened after about an hour of running throughout the rpm and speed range, from low to WOT while at the specific time of the failure they were running at approximately 5000 rpm.

I would be very interested to hear any ideas on what happened here.

[EuropeanAM]

That "looks" like either a pinion gear failure, or the clutch dog "shattered".

Obviously, the end result was that the clutch dog broke into pieces, and at least part of it was expelled out the port side of the gearcase.

New gearcase time- no doubt.

[Yiannis]

Thank you for your answer! Since I saw no oil on the surface of the sea, do you think that this could be caused by lack of lubricant in the LU?

[EuropeanAM]

That is a definited possibility, as is metal fatigue or flaw.

Inspection of the bearings may give some idea of how long it "spun" without lubricant prior to the failure.

As far as "seeing" an oil slick... it would have been quite a ways back by my guess...

[John Jackson]

I see rust. So I'm thinking one of the seals went bad and you were running mostly with water in the lower. It will run longer that way than you think. (Done that myself but caught it before major damage. Had to have shaft turned & built up with stainless at the seal).

[EuropeanAM]

I would inspect the driveshaft bearings. They will tell the story..... can't necessarily blame light surface rust on a bad seal- 'case it has a "gaping hole" in the side of the case- there's no doubt everything was flooded with water immediately upon the failure.

If this was just a "metal part failure"... the bearings will have light surface rust, much like the gears, but not show signs of heat damage, spalling, or other "no lubricant tell-tale signs".

If the bearings/shafts show signs of heat damage, etc.... I'd suspect it was indeed a "no or low lubricant" failure (or water in case). Feel free to post or send pictures of the bearings.

[Bass Commander]

Wow!!! Glad you werent hurt. If that type of failure were to happen at me at wot, It wouldnt be pretty! Hope you get back on the water soon!

[Yiannis]

On the issue of seeing or not a possible oil sleek and its distance from where I stopped, well, after a U-turn I returned back home on one engine at 8kn, I guess I would have seen it... but still maybe I didn't.

On rust, it looks superficial to me, at least from what I can see. Once Merc's representative gets back from vacation I will have some developments and photos to post (naturally).

RobShaw1, thank you for your kind words, my boat is twin powered and I think this was helpful (at least getting back) and larger / heavier than the bass boats you usually run on in the US, so I didn't feel any real danger at the time. In addition, I was not at WOT at the time, only 5k rpm and about 50kn.

To all: Many thanks for your interest from very far. Amazing what the web can do!