Break joints, safety clutches or whatever

Discussion in 'Propulsion' started by daiquiri, Mar 25, 2009.

  1. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Time for me to ask few questions.
    Travelling in a shoal water, the prop hits a solid rock. What will break first, the prop, the shaft key, the shaft itself or the transmission gear?
    It never really happened to me but a guy here has asked me a similar question, plus how to prevent shaft breaking, falling off and leaving a nice hole in the hull.
    Since the prop is made of bronze, it appears obvious to me that it will break first. But if the shaft is heavily loaded, it might also happen that the shaft would fail first. So, without making a complex structural analysis, how do you assure that shaft and transmission remain intact? I was thinking about placing a break joint or a safety clutch between the shaft log and the prop but I was unable to find out the proper one, suitable for use in water.
    Can you guys indicate me some manufacturer of that stuff or do you have some better solutions to suggest? The boat is a 42', the shaft is 1"1/2.
    Thanks in advance.
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    First comes to mind what's common with outboards. They propably have proper names in english which I'm not aware of. Anyway the other one is break pin and the other is volcanized propeller coupling. Accordingly first one breaks (and usually new one is cutted from a regular nail) and the second one either flexes a bit or, in case of harder impact or entanglement, looses it's holding power more or less totally and needs to be revolcanized..
  3. marshmat
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    marshmat Senior Member

    I'm more familiar with outboards and sterndrives, which use one of the two systems Teddy describes. Either you have a shear pin, that can handle the torque of the engine but will break before the shaft does if you hit something; once it breaks, the prop will spin freely and you need to replace the pin to get propulsion back. Or, you have a rubber prop hub, that flexes a bit under minor impacts, or will slip a spline or two in a big hit. These usually find their grip again within a few revolutions, and once you stop panicking, you have power back (albeit at low speeds only, because your dented prop will shake like crazy if you try to go fast now.) A hit that's brutal enough to severely damage the slip hub will usually take half a prop blade off, so since the prop has to be totally rebuilt anyway, it's only an extra few dollars to replace the slip hub at the same time.

    For an inboard, conventional shaft drive, I'd expect most rock-vs-prop fights to end with a dent in the prop, maybe a blade folded half over. To damage the shaft from pure torque alone would be very difficult, and I'd think the gearbox would usually be worse off than the shaft in this situation. Now, if the shaft is sticking far enough down that the rock actually contacts the shaft, you're in a bit more trouble.

    In any case, if you're going to include some kind of "this breaks first" joint, I'd think it would most likely be between the transmission and the shaft- in the dry, where you can easily replace it.
  4. PAR
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    PAR Yacht Designer/Builder

    There is a part you can purchase that fits on the trans flange and mates to the shaft. It has a rubber center which is intended to sheer when the prop hits something. It can also tolerate some misalignment of the two flanges.
  5. kroberts
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    kroberts Senior Member


    I think you're talking about a flex coupling. I have exactly that on my hovercraft, and that's what we landlubbers call it.

    Going through the book you recommended, I'm learning that a lot of things I know the name for when they're on land have some other name when you're on water.
  6. Ad Hoc
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    Ad Hoc Naval Architect


    "..So, without making a complex structural analysis.."

    It is actually not that difficult at all. Every part of the drive train, from the prop all the way to the engine. Each item, keys, bearings, bolts etc, all will have either a manufactured SWL or just easily calculate it.

    The one item which has the lowest value for failure, be it shear or otherwise, will be you weak link.

    The the value you calculate/obtain for each item then needs to be put into context.

    Such as, say the bearing is bolted onto the frame. These blots, again assume, are the weakest link. The force required to pull these bolts out is say 100kg. Clearly a knock against a rock will dislodge these very quickly. However, if the bolts, again are the weakest link, but the value is now say 10tonne. Clearly this requires a major knock, as such this is a large force on a small boat it is not worth worrying about, since a force of that magnitude will create significant damage all over, not just pulling the bolts free.

    It is just asimple process of elimination and then being put into context.
  7. Landlubber
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    Landlubber Senior Member

    In real life, what normally happens is that the prop will be severly bent, the boat will bounce over the rock and then the driver will reduce speed, go overboard, say"#*^*" and then worry about getting home.

    On a rarer occassion the bronze skeg will bend as will the shaft, normally though there are no breakages as such from a "weak "point. This can of course rip out of the boat but not likely if the boat is built OK.

    This is from my yard doing repairs......
  8. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Thank you all very much for the replies.

    As I wrote in another thread on this issue, the ultimate goal of this solution would be to:
    1) prevent shaft breaking and/or
    2) prevent the transmission via shaft of the impulsive force and torque which could damage the drive system or (worse) the shaft log, opening the path for the water into the hull.
    So, a protecion of the costly transmissions and hull structures are the objective of this.

    I think you are talking about safety clutches, or torque limiters, kind of thise:
    Well, that's what actually inspired me for this talk. :) But I would like to have it behind the shaft log, so something suitable for use in salt water...

  9. PAR
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    PAR Yacht Designer/Builder

    The big Lubber has it right. In practice, when you hit a submerged something or other, the prop bends, nicks, folds up or tosses a blade. Often a skeg or strut can get bent, even ripped out. I've seen shafts severely bent, but other then the horrible sounds and violent vibrations coming from the belly of the boat (and skipper), most don't seriously jeopardize the ship.

    A lot of this has to do with the speed the boat was making during the mishap. Even a very light weight boat traveling at a good clip can stuff it's gear up into it's belly on a rock, pretty easily. On the other hand a slow moving boat can bounce over things with minor damage.


    This is the coupler I mentioned.
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