If a shaft breaks what usually happens?

Discussion in 'Inboards' started by captndon, Jun 10, 2014.

  1. captndon
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    captndon Junior Member

    Not long ago in this part of the world a dive charter boat wrapped a line around a prop and it pulled the shaft right out of the boat. The boat went down in short order. I was told that the 3" shaft in my boat would need to be replaced by now. It passes through a thrust bearing fore and aft. It is 18 feet long. Is it likely that it would simply slide out of the tube if it broke?
     
  2. Petros
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    Petros Senior Member

    yes, if it broke it would be very bad. It is more common to have it removed during other heavy maintenance when in dry dock and have it checked for cracks, repaired if necessary and rebalanced by a drive shaft shop, rather than to replace it. Usually a compatent shop can rebuild the driveshaft for less cost than replacing it. This should be part of routine heavy maintenance, just like having the engine overhauled, or the hull stripped, inspected, repaired and repainted.
     
  3. sdowney717
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    sdowney717 Senior Member

    If you can put a collar on the shaft inside the boat. like a c-donut zinc.

    or use a hose clamp.
     
  4. gonzo
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    gonzo Senior Member

    That was a pretty poor dive charter that couldn't stick something in the hole to prevent the boat from sinking. It will take quite a while for a boat to go down. It is possible to stop most of the water from the inside with a plug.
     
  5. PAR
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    PAR Yacht Designer/Builder

    You'd be surprised how quickly a boat can sink with just a 1" shaft hole, open to the sea.
     
  6. gonzo
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    gonzo Senior Member

    With a full crew of divers onboard?
     
  7. PAR
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    PAR Yacht Designer/Builder

    In a panic, even a boat load of divers might not get a handle on things in time, though admittedly, it's suspicious to say the least.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Bilge pump is not much good if in can't keep up with the inflow till a stop gap is organized.
     
  9. PAR
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    PAR Yacht Designer/Builder

    I've been aboard several boats, that have sprung a leak for one reason or another and panic can set in very quickly with some folks, which tends to make everyone else panic too. This is especially true if you don't fully understand or can't quickly identify the leak. In every case (for me) a cool head prevailed and simple stop gap measures would slow the leak enough to ease most concerns. A clever skipper will mark the bilge, so a nervous crew can see the water go down, reassuring them the pump(s) are doing their job. I've also been aboard when the pumps couldn't keep up (and we had a fire too), so additional measures where taken. On this particular case, we almost lost her, but we fought and eventually gained control and got to a stable state again.

    I can easily see an inexperienced skipper and crew scratching their collective heads, while hundreds of gallons of water rushed aboard. Something as simple as tossing a bimini cover, sail or tarp over the side and letting suck into the hole, just isn't something they would think about, so they sink.
     
  10. AndySGray
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    AndySGray Senior Member

    That's actually one hell of a good question/comment!

    I guess very few boat owners have ever considered if their pumps would be able to deal with loss of a shaft (myself included).

    I did a little research and came up with a formule for velocity vs depth

    v = Sqrt(2gh) where h is the water column height (meters) and g = 9.8 = grav constant

    My propshaft is a little over 2' below the waterline

    70cm or 0.7m

    so v= Sqrt(2 x 9.8 x 0.7)

    gives me 3.7 meters per second


    Part 2
    My shaft is 1.5" so 0.75" radius or 1.9 cm or 0.019m

    pi R squared gives an area of 0.00113411 square meters....

    Multiply by the velocity gives 0.004196 cubic meters per second

    264.172 gallons per m3 gives 1.108 gallons per second.

    about 66 gallons a minute or 3990 gallons an hour.

    Or a ton of water every 4 minutes!

    I think the above formulae is for Fresh Water...

    Hmmm - my 2000gph bilge pump is looking somewhat too small on its own, BUT there is a second one in the next section with only an 8" lip before it floods over, so I reckon the pumps would cope but not make headway!

    Sobering maths!


    BTW Don't expect any help from the divers - Sure, they'll be putting the dive gear on -
    But only because it's really cool to be the first to dive a new wreck !:eek:

    :p:p:p;)
     
  11. PAR
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    PAR Yacht Designer/Builder

    Most bilge pumps can't even keep up with a summer thunderstorm, let alone a leak.
     
  12. gonzo
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    gonzo Senior Member

    Yes, the numbers are correct. However, a t-shirt and a broomstick will lower the numbers significantly. A diver can stop it even more since the water pressure will help hold the plug
     
  13. SamSam
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    SamSam Senior Member

    Water comes in an open hole at an amazing volume. Easy access to the hole is a large factor in stopping it quickly. Visibility is another, you might have to turn your head away to be able to stretch and reach the hole or the water is murky and you end up blindly trying to feel around to locate the hole and then do the same to plug it.

    The average (Rule) bilge pump is a flimsy contraption that cannot be counted on to work when needed. If the connections aren't bad, or the plastic motor seized up, it's liable to plug up in a heartbeat, especially when the bilge is flooded and more junk than ever is floating around loose. Plus any capacity ratings are more than likely determined under the most ideal conditions, possibly they are a hopeful guess. This is from West Marine
    This is from David Pascal
     
  14. gonzo
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    gonzo Senior Member

    We are talking about a rather small vessel. Anyone in fair shape can jump overboard and plug the hole. With a crew of divers, and a divemaster onboard, it should've been an easy fix.
     

  15. PAR
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    PAR Yacht Designer/Builder

    Most folks don't think like us. You and me, well we'd quickly figure out what was up and jump over the side to address the problem. The vast majority of folks lose logical and rational capabilities pretty quickly, when threatened like this. I can easily see a bunch of recreational divers getting a good panic going, after just a few minutes, watching the water level rise so quickly. These are the same type of folks, that have the Volvo dealer replace the windshield wipers on their cars. You and I are in a NAPA store looking for the best deal on a good set of wiper blades that'll fit. Simply put, do you know how many times I've made a service call, just to flip a tripped, over sensitive GFCI breaker? Of course the owner's upon seeing this remark that they could have done that, to which I quickly reply, "yeah, but would you have known why it tripped and if this was a problem or just a splash from the hot tub?"

    We're an odd lot Gonzo. There's us and those that end up calling us. We are grossly outnumbered by them, so we'll still be booked, several years after our deaths.
     
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