Mercury 300XS multiple lower unit failures

[bep078]

Hey guys,

A friend of mine has a 2009 Mercury 300XS 20" Shaft rigged on a 25ft Chaudron hull.

When the motor had just 20 hours, it blew it's original Sportmaster lower unit. This was replaced under warranty and sold to the second owner. After about another 50 hours, again, the motor destroyed the replaced brand new Sportmaster lower unit. The 2nd owner bought a brand new Torquemaster (fat shaft) lower unit from Merc and after about 50 hours of use, you guessed it.. It blew this brand new lower unit too. The 2nd owner repaired the lower unit (changed all gears, prop shaft, bearings, seals etc) basically everything brand new except for housing, main shaft and water pump. After just another 20 hours, inevitably, the motor claimed victim no 4..

Driving conditions:
Boat is used for recreational use (no racing)
Both boat owners usually cruised at around 3500 rpm to 4500 rpm with maybe some WOT blasts lasting about 30-45 seconds max here and there. We rarely have flat seas to run WOT for long periods of time.
Neither lower unit ever hit any rock or debris when running.
Prop (tempest 27) never hit anything neither

The failures:

1st Original Sportmaster lower unit: Reason unknown as lower unit was replaced under warranty.
2nd Brand new warranty replacement Sportmaster: Destroyed pinion gear
3rd Brand new Torquemaster, Split pinion gear and forward gear into about 8 small pieces.
4th repaired Torquemaster, broke the main shaft on the pinion side and upon inspection, saw major wear in the forward gear and pinion. So if the prop shaft didn't break, the forward gear and pinion were soon going to let go again.

Has anyone ever encountered a problem like this? I think that breaking a lower unit can happen, maybe even 2 in a short time if you are really unlucky considering that the boat and motor were never abused throttle wise. But to break 4 lower units 3 of which where brand new in a matter of about 140 hours of moderate use seems very odd. Oil was always changed as per service requirements using Mercury high performance gear lube.

Could it be that there is a problem coming from the power unit or mid section? All lower units fit in perfectly fine in place.

Any help will be greatly appreciated.

Thanks!

Beppe.

[davemvegas]

i would throw the prop in the ocean. even if it is not out of balance. or by chance has a out of pitch blade. it has to the unluckiest prop ever run.

[ghost28]

Water intrusion....its the only thing that kills 3.0 liter lowers....the factory upper seals fall short of doing the job they were designed to do...you meed to replace the driveshaft seal carrier with a billet quad seal carrier and double the seals in the shift perch or add a Lakeland vent kit shift perch....after that change the lube every 10 to 15 hours and they will not fail....i have 300xs motors out there that are run hard with no lower unit failures and over 200hours on em....

[bep078]

Oil after 20 hours does not show any sign of water intrusion.

Here are some pics.

[RikuY]

At that point, I would think something else is wrong. Is the driver leaving throttle in position as the boat catches air and re-enters, or are they throttling back?

Try a lower pitched prop. You will be able to use more of the rev range, and whatever force is causing the failures will be reduced by the mechanical advantage gained from the smaller pitch on the prop. I hope this helps!

[bep078]

There are about 20 exact same boats with the exact same motor, setup, props etc and none have this problem. There are way more aggressive drivers in different boats with the same setup who race offshore and never had a lower unit failure like this. Could this particular Tempest's square bore not be aligned perfectly and perhaps causing such failures? Or perhaps there be a problem with the output side of the powerhead where the main shaft connects to the powerhead? But there again all lower units fit normally without any hesitation.

I would like to correct an error i made when saying the 4th lower broke the prop shaft. In fact, it was the main shaft that failed on the pinion side and not the prop shaft. (im writing on behalf of a friend on mine and I had understood him incorrectly.)

Any more ideas?

[ghost28]

your friend needs to learn how to drive then...agressive driving is one thing...Careless and poor throttle control is another...also is he shifting this thing with the throttle above idle?? that will snap drive shafts and pinions....I will also say that the oil in that pic looks cooked like its got alot more than 20 hours on it....The oil is super important in these cases....Its the only cushion they have...If its dead then it cant do its job...these cases only hold a quart of oil and it dont take much to destroy that oil....Also the oil should have been changed after 10 hours to remove any particles from the break in process...

[vnemous]

I dont remember seeing anywhere what pitch prop he is running. Most if not all of us with high pitch props shut the motor down when shifting and start it in gear to avoid the shock on the gears. It doesnt take much to hurt them shifting from forward to reverse JMO

[racervboat]

27 tempest