What causes lower unit failure?

OP wants to quickly drive the lower unit, he didn't write how long it should last.

 

I think his pitch to gear leg ratio math went the wrong way. Unless this is some really balsy motor being hooked up to the 2.5 ratio leg.

 

Yeah. The duty cycle will only be about 10 minutes per hour, and I really don't care if I have to change the oil every 2 hours or something.

I'm aiming for 4o kn, but I would be satisfied with 30. If that has to be done by spinning the input shaft at 10k rpm -- we will see

Actually I'm hoping the vessel is just really low drag. Definitely possible that there won't be enough torque -- I haven't finished the calcs yet.

I can't really find any hard figures on what the torque curve for such a small outboard looks like. Using some spitballing, I figure that the 15 hp (which is the same leg I have) puts out about 21 newton-meter (15.5 lb-ft) peak. My motor has the power output drop precipitously at 21 N-m, and does about 12 N-m at maximum power. That is assuming I go direct drive. If there is an overdrive, the torque values will be even lower.

So, I don't think I'll blow up the lower by exceeding torque limits, but possibly from overspeeding the bearings, or maybe vibrational/harmonic issues.

 

"Thinking" is a vague indication of reality.
Knowing is physics and data.

You're going to have to give up a lot more information before this thread can move from speculation to quantifying what you have and need.
Until then, good luck with your mystery project.
Post pictures and feedback.

 

9.9/15hp OMC they have different lower units.

 

This is an unnecessarily combative reply. I did a calculation right there in the post and then qualified it with an "I think" because I acknowledge the weakness of the calculation in the absence of more precise data.

 

Mathematically, horsepower equals torque multiplied by rpm. H = T x rpm/5252, where H is horsepower, T is pound-feet, rpm is how fast the engine is spinning, and 5252 is a constant. If you have the HP at what RPM, then torque can be easily calculated. That will let you know exactly.

 

Yes, indeed. But it is my understanding that peak power is usually rated at or near max max RPM (5500 or so) while peak torque is usually quite a bit lower (4000-4500). Since I can't find a dynamometer graph for any small outboards, I just did what you described to find torque at peak power and guessed that peak torque was ~20% higher than that based on the proportions of the dynamometer data that I could find for larger engines.

 

Lower unit failures are often caused by a combination of chronic wear and acute stress, with TBS bearings playing a central role in longevity and reliability. Chronic causes include gradual bearing wear from age, contamination, or insufficient lubrication, which can lead to increased friction and eventual seizure. Acute causes like shock loading from sudden impacts, ventilation, or breaching the surface can overstress the gears and bearings even if the unit isn’t exceeding RPM limits, while overspeed generally stresses torque and gear teeth more than the bearings themselves. Proper lubrication is critical: filling the lower unit to the recommended level with fresh, marine-rated oil—often anti-foaming to prevent aeration—ensures that needle roller bearings remain coated and protected, while oil foaming or low levels can starve bearings and rapidly accelerate wear. Monitoring bearing condition and maintaining clean, correctly filled oil are the most effective ways to mitigate lower unit failures under higher-than-normal operating conditions.

 

Hi coperlio ! Welcome to the forum .
"marine-rated oil " and anti-foaming , can you recommend some products ?

 

All modern gear oils have anti-foaming additives. Any of the major brands is good. I only use synthetic lubricants because they have a lower coefficient of friction and higher temperature rating. Running lower viscosity oils shortens the life of the gears and bearings. Also, gears are set up with clearance for a specified viscosity. A lower viscosity lubricant is equivanlent to setting gears with too much clearance.

 

I see @ziper1221 has checked out.
I wonder how his emotional project went...

 

We got third place out of about 30 entries. We did the 2 mile course in 8.5 minutes, first place was around 6 minutes. We built everything in only about 6 weeks because the funds were delayed, and didn't have enough time to do sufficient testing. The weak link was that the motor mount was too compliant. The steering would end up with a feedback loop causing chine walking above about 15mph. The lower unit and propeller never came close to their limits.

A very stupid cause of failure, unfortunately.

 

Good analysis

 

If you weld some flat bar as far around your mounting plate edge as you can, or screw/bolt some shaped and bent 90 deg. angle to follow the perimeter, that will stiffen it, and reduce vibrations, without a lot of extra weight. And you'll have somewhere to easily screw a belt cover onto. Now you have a benchmark, you could try different pitch or diam propellers, and try different hull trims for going faster.
Your steering arm seems a bit short (may be the photo angle); a longer lever will possibly reduce large steering vibrations.