Sterndrive bellows vulnerability

Discussion in 'Powerboats' started by erikgreen, Dec 23, 2009.

  1. erikgreen
    Joined: Mar 2008
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    erikgreen Junior Member

    Has anyone out there come up with a good solution to the problem of sterndrive bellows?

    I'm not talking about the elegance of design, maintenance requirements, or anything other than just the "what if" scenario of the bellows tearing on the water.

    A broken U-joint, debris damage, or whatever could slash a hole in them, and all of a sudden your 20 foot boat has water coming in at many gallons a minute.

    About the only thing I've seen to plan for this is to include multiple bilge pumps and make sure you maintain/service your bellows (all of 'em). If you service the U-joints, seals, and other parts the chance of something "sneaking" in on your transom to lacerate the bellows is low.

    Has anyone tried to design a boat with an isolated engine compartment that's watertight? So if water came in the transom it would fill around the engine rather than filling the whole boat's bilge? Obviously not good for the engine, but you'd stay afloat. Plus I'd think the sealed compartment could have nice sound deadening properties and with some work, good airflow.

    Anyone have a solution other than "Don't use a sterndrive?" :)

    Erik
     
  2. marshmat
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    marshmat Senior Member

    How about, don't use a Merc sterndrive? Or, just follow a good preventive maintenance program?

    It is my understanding (this may not be entirely accurate) that it is only the Merc drives that are vulnerable to catastrophic flooding if the bellows fails. Volvos have a bearing assembly that, while not watertight, will allow only a slow drip from a failed bellows into the boat. Some of the newer Volvo units (Ocean X) have a sensor inside the bellows to alert the captain if water gets in; the boat won't sink because of this, but the drive could be damaged.

    An isolated engine bay, separated from the rest of the boat by watertight bulkheads, is a Very Good Idea on any boat of any size. The below-waterline volume of this compartment should be kept to a minimum, and it should have two good, independent bilge pumps. Of course, separate bilge pumps are then needed for the other compartments.

    See " http://www.boatdesign.net/forums/powerboats/do-sterndrives-bellows-sink-boats-26971.html " for more on this issue.
     
  3. erikgreen
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    erikgreen Junior Member

    Ah, interesting. Sounds like Volvo has a leg up on Mercruiser in more than just reliability. Thanks for the link.

    I'm going to have to see how hard it is to design an engine compartment on a small boat to be sealed... obviously it'll have to be removable on at least three sides in order to do maintenance, since it can't be big enough to fit a person inside without taking up half the boat.

    I'm liking the idea more and more because of the sound insulation and air flow advantages too... extra bilge pumps aren't too much of a problem either.

    Erik
     
  4. CDK
    Joined: Aug 2007
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    CDK retired engineer

    The OMC stern drive did without bellows. Two submerged light alloy discs with small hemispheres along the circumference conveyed the engine power at any drive angle. The restriction was, that with high tilt angles, the contact area became very small, so only operation with low power settings was allowed.
     
  5. Red Tide
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    Red Tide Junior Member

    Not much different though than the low-power at high tilt angles best for not killing your u-joints, right? Or was it more limiting?
     
  6. Red Tide
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    Red Tide Junior Member

    What's the HP loss through a typical stuffing box? With the merc, the driveshaft is already passing through the gimbal bearing. Why did they decide not to make that a secondary protection? I would have thought that most boat buyers would choose safety (if presented as a selling feature by dealers) over 10 hp any day... There must be some advantage to the bellows (danger) design. I can't believe it is cost savings alone to have this single point of failure on a $10,000 engine/drive package.
     
  7. Red Tide
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    Red Tide Junior Member

    P.S. Do arneson drives have the same bellows vulnerability?
     
  8. CDK
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    CDK retired engineer

    It was more limiting.
    In normal position the two discs were parallel to each other and the torque was distributed over dozens of hemispheres, when fully tilted there was only one contact point.

    A Mercuiser drive will protest loudly with high rpm in tilted position and the joints will wear rapidly, but unlike the OMC there will be no slivers of metal in the water.

    The Arneson drive is a different story. Their rubber bellows just protects the CV joint from water. A tear will cause damage to the joint and bearing, but the water doesn't enter the boat.

    Even more important is the location. In a Merc the bellows are at the back of the drive and gimbal housing, hidden from view but not from the oysters and clams with knife-sharp edges. They are firmly attached to the drive in a location without anti-fouling, waiting for the moment the drive is turned or lifted.
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Lets say you've engineered the stern drive bellows not to die a sunken death in the unlikely event a U joint craps the bed. Do you now place a piece of armor plate below the engine to prevent an engine tossing one or more of it's rods through the bottom of the boat?

    It would be nice if we could engineer all of the potential hazards out of going farther from shore then we can swim back to, it is imposable. Assuming reasonable maintenance, bellows aren't a problem, nor are exploding U joints. The only real problem is owners, which we also can't engineer out of the equation either.
     
  10. apex1

    apex1 Guest

    I´m not absolutely sure Paul.

    Or, say, I feel a steadily increasing desire to do exactly that! Just have not managed to develop a proper business model of it.
    But roughly it should be like:
    watch the boat at the dock: ---free of charge
    watch the boat from the inside: ---15$
    watch me sailing the boat: ---45$
    Bring your own food and beverage and PFD (to watch me sailing only!).


    Regards
    Richard
     
  11. erikgreen
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    erikgreen Junior Member

    It's a bad design

    It's true that with reasonable maintenance bellows won't blow up on their own (nor will U-joints, generally). It's also true you can't engineer a boat to handle every problem that can happen. By the way, armor plate isn't needed to stop a rod going through the hull.... high density PU foam with a glass coat will do fine. A block of the stuff will "catch" bullets.

    My original reason for asking about this issue was due to the fact that bellows on the Mercruiser drives are an unusually bad design as far as failure mode goes. Pretty much if something cuts the bellows the boat sinks. Even a muskrat can send a boat to the bottom. I haven't seen too many muskrats that cause engines to throw rods through the block.

    Thanks for the discussion, all... I've decided for my own re-fit I'm going to enclose the engine in a sealed compartment of its own with ventilation, dual bilge pumps, and sound insulation. At the very least this will get me a quieter boat, plus if the bellows should fail the engine compartment will flood instead of the whole boat. I can also add a fire suppression system that requires a sealed compartment. Lights on my helm will display flood warnings, fire warnings, etc.

    For the future, due to maintenance and complexity I may avoid sterndrives altogether... we'll see.

    Erik
     
    Last edited: Dec 28, 2009
  12. CDK
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    CDK retired engineer

    As far as I know, a watertight engine bay with a bulkhead is common practice for cabin cruisers. My Draco twincab has one, yet it is just a 26 ft hull.
    When a clam shell had cut one of the bellows and the harbor master phoned, the boat was deep in the water but didn't sink and the cabins were dry. The air intakes were still above water, yet there was considerable damage caused by electrolysis. What used to be cables were empty hoses, so the complete wiring had to be replaced, plus starters, alternators, distributors, relays and hydraulic pumps. And of course the automatic bilge pump that didn't work when it was most needed.

    After repairs, I installed a 2nd bilge pump with an industrial float switch and a "sustain" circuit in a junction box, mounted as high as possible. It keeps the pump working for approx. 20 seconds after the water level has dropped below the float switch. With only a switch, because the water from the hose flows back when the pump stops, the float is lifted and the pump starts again.
    The other one is an ordinary all-in-one pump I've replaced twice because it stopped working, once because of ice and once because it was filled with water and the motor was rusted solid.
     
  13. Aviator5
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    Aviator5 New Member

    Even with Mercruiser drive water won't be coming in at many gallong per min.
    There is an oil seal rigt behind the gimbal bearing. So the water will be coming thru a small hole in the bearing housing. Average bilge pump should be able to handle it, for a reasonable time, until battery dies. So I don't see a big potential disaster here.
     
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  14. gonzo
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    gonzo Senior Member

    That's correct, unless a bonehead mechaninc or owner took our or broke the seal and two o-rings.
     

  15. Rik
    Joined: Dec 2007
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    Rik Senior Member

    WRONG. The boots on the Arnesons do not seal out any water at all. In fact they are not even water tight.

    A tear, rip or removal will not cause any water intrusion into the drive at all.
     
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