What causes lower unit failure?

Discussion in 'Outboards' started by ziper1221, Jan 26, 2025.

  1. ziper1221
    Joined: May 2018
    Posts: 63
    Likes: 13, Points: 8
    Location: florida

    ziper1221 Junior Member

    Hi there,
    I'm trying to figure out what generally causes lower unit failure with the goal of running a lower unit at higher than rated RPM. I get the feeling that the internal bearings are probably rated for much higher than the operational RPM, and the issue is exceeding torque limits (not really an issue due to the low drag of the vessel) or shock loading (definitely possible). I'm curious if anyone can comment on common causes of failure and how likely they are and how to mitigate them.

    Chronic causes:
    • Gear wear due to age/use
    • Bearing failure due to age/use
    Acute causes:
    • Overspeed (probably not really an issue because powerheads typically cut spark over max RPM)
    • Shock loading due to ventilation/breaching the surface
    • Oil foaming? (How exactly does lubrication on a lower unit work? When you fill the oil until it comes out of the top hole, is the cavity 100% filled? 90%? Is using a fresh marine rated oil with anti-foaming properties enough to prevent all lubrication related failures?)
     
  2. BlueBell
    Joined: May 2017
    Posts: 2,980
    Likes: 1,111, Points: 113
    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    How are you going to overspeed the rated RPM?

    The short answer to your question is: fatigue.
     
  3. portacruise
    Joined: Jun 2009
    Posts: 1,519
    Likes: 191, Points: 63, Legacy Rep: 218
    Location: USA

    portacruise Senior Member

    Fatigue, defects, wear and tear , and skipping required maintenance, would be my guesses. Not sure anything will completely mitigate, but use of higher rated quality units than needed, may postpone it.
     
  4. kapnD
    Joined: Jan 2003
    Posts: 1,415
    Likes: 490, Points: 83, Legacy Rep: 40
    Location: hawaii, usa

    kapnD Senior Member

    I see a lot of lower unit failures due to water intrusion. It’s not weakness of gears and bearings, but lack of lubrication. Fishing line, even seagrass can open the seals to water intrusion.
     
    Tomsboatshed likes this.
  5. comfisherman
    Joined: Apr 2009
    Posts: 843
    Likes: 428, Points: 63, Legacy Rep: 10
    Location: Alaska

    comfisherman Senior Member

    Most catastrophic failure from impact of some kind, progressive failure from a myriad of seal failures. (Electrolysis, seal west, foreign object).

    What are you overspeeding that can't be better serves with proper gear and prop pitch selection.
     
  6. ziper1221
    Joined: May 2018
    Posts: 63
    Likes: 13, Points: 8
    Location: florida

    ziper1221 Junior Member

    I'm converting a small gas outboard to high speed electric. Yes, a proper gear ratio would do it, but the outboards in the size range I need usually have pretty dumpy gear ratios (2 or 2.5 to 1) since they are usually designed around pushing a dinghy or sailboat. I picked up two of these 5hp johnson/evinrudes that have a 2.44 to 1 ratio lower for basically nothing, and I'm not opposed to blowing up one for the sake of testing.

    Propeller selection is quite limited -- my options are 7 or 9 inch -- and I'd need something around 22 inches in pitch. I am working on designing a custom prop, which will be my first choice, but I was curious about the overspeed possibility in case that avenue fails.
     
  7. comfisherman
    Joined: Apr 2009
    Posts: 843
    Likes: 428, Points: 63, Legacy Rep: 10
    Location: Alaska

    comfisherman Senior Member

    22 inches of pitch, that must be some spectacular input torque.
     
  8. seasquirt
    Joined: Dec 2015
    Posts: 273
    Likes: 125, Points: 43, Legacy Rep: 10
    Location: South Australia

    seasquirt Senior Member

    You could rebuild the lower unit with the least internal wear, with new seals, and good quality bearings, (not from the land of instant land fill), and use high speed synthetic oil or mineral oil. High speeds cause shear in cheap oils, allowing more metal to metal contact. Or could you run a small pulley on the motor, and a larger pulley on the leg, a cheap simple ratio change, if belt / pulley side thrust loading is accounted for.
     
  9. BlueBell
    Joined: May 2017
    Posts: 2,980
    Likes: 1,111, Points: 113
    Location: Victoria BC Canada

    BlueBell . . . _ _ _ . . . _ _ _

    More info trickles in...
    So, the most common reason you're going to get a gear failure now is from over torque and over RPM.
    But, if you have such low hull resistance ( that's still a mystery ) you may not encounter over torque if you go easy on the throttle.
    Over speeding RPM may not be such an issue if it's low torque by the time you get to high RPM.
     
    Tomsboatshed and jehardiman like this.
  10. jehardiman
    Joined: Aug 2004
    Posts: 3,961
    Likes: 1,325, Points: 113, Legacy Rep: 2040
    Location: Port Orchard, Washington, USA

    jehardiman Senior Member

    Yep, I'd start with gear tooth design. Gears are designed with a specific torque to rpm profile. Get too far off that and you will have all sorts of issues.
     
  11. gonzo
    Joined: Aug 2002
    Posts: 17,547
    Likes: 2,061, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    A large percentage of the failures are from oil seals damage or deterioration.
     
  12. montero
    Joined: Nov 2024
    Posts: 288
    Likes: 34, Points: 28
    Location: Poland

    montero Senior Member

  13. seasquirt
    Joined: Dec 2015
    Posts: 273
    Likes: 125, Points: 43, Legacy Rep: 10
    Location: South Australia

    seasquirt Senior Member

    Since you have two units, try atf in the worst condition one first, and see how long it lasts, then you still have the option of using better oil in the second one. See what types are used in heavy transmissions, drag cars, and what the militarys use in their equipment, then choose. Since legs have cooling by default, I think a full synthetic oil would perform best, but unsure how they go after some salt water ingress messing with the chemistry.
     
  14. gonzo
    Joined: Aug 2002
    Posts: 17,547
    Likes: 2,061, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Racing applications don't translate directly if you want something to have a long life in usual use. Drag cars only run seconds at a time; so do drag boats.
     
    DogCavalry likes this.

  15. Rumars
    Joined: Mar 2013
    Posts: 1,951
    Likes: 1,233, Points: 113, Legacy Rep: 39
    Location: Germany

    Rumars Senior Member

    What exactly does "high speed" mean? These outboards run 5000rpm engines, so unless you plan to run the electric motor at a constant 10,000 rpm the gearbox should be fine. If you do plan on such high rpm the transmission ratio is to high, it will give you 4098rpm at the prop.
     
    jehardiman and comfisherman like this.
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.