Philosophy of Self-righting

Discussion in 'Stability' started by petlily, Nov 23, 2009.

  1. petlily
    Joined: Jul 2006
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    petlily Junior Member

    Does anyone care to explain how a boat can be made self-righting? I read a lot of lifeboat nowadays which has this self-righting capability and was truly amazed by the technology entailed in making these boat self-righting.
    Can someone please, perhaps provide a link or sources on where to get these information?

    Thank you very much.
     
  2. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    Self righting is easy, and always has been. The amazing thing is that they now keep the crew and passangers dry and the engines running while self righting.

    Lifeboats have been self righting since the late 1800's. The concept is to place a large amount of low waterplane buoyancy high and weight low. This will ensure that RM always remains positive. Normally this is done with buoyancy compartments high in the bow and stern with a strong sheer (along with buoyant coachroofs in modern MLBs). These buoyancy chambers are designed to float the overturned vessel high with low waterplane resulting in a "negative" GM in those conditions.

    Take a look at these sites.

    http://www.uslife-savingservice.org/boats_of_service

    or

    http://www.mlb44.com

    And notice the large watertight chambers fore and aft as well as the low sheer amidships.
     
  3. Submarine Tom

    Submarine Tom Previous Member

    petlily,

    (Nice handle) Self righting first needs to be defined.

    A large cat or trimaran is the opposite of self righting.

    One basically has to make the vessel highly unstable when inverted...

    Take a look at the buoyancy chambers high aloft on the Canadian and

    American 47' Coast Guard life boats. You should be able to find footage of

    one self righting.

    The most effective way to stay upright is not to roll over in the first place!

    -Tom
     
  4. Kay9
    Joined: Oct 2006
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    Kay9 1600T Master

    MLB School


    This is why the USCG sent us to Cape Dissapointment in WA. The boats were comming back with the crew dead inside from massive roll overs. So the USCG decided to send us to the Motor Lifeboat School to learn how to deal with Surf without Broaching the boat.

    K9
     
  5. Submarine Tom

    Submarine Tom Previous Member

    Yah baby!!

    -Tom
     
  6. hmattos
    Joined: Jun 2004
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    hmattos Senior Member

    UK RNLI Tamar Class self righting boat video.

    For those who have not seen these UK Lifeboats in action, here is a link to a Facebook video which shows a test roll in this 16 metre offshore all weather lifeboat.

    The design is supposed to minimise the chances of inversion due to breaking waves, with the precaution is taken to ensure that the boat will self right with engines still running, if it is upturned.

    The link is: http://www.facebook.com/video/video.php?v=1056467946658
     
  7. FAST FRED
    Joined: Oct 2002
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    FAST FRED Senior Member

    Piston aircraft engines (some) can operate inverted for very extended periods of time .

    Whats the big deal in operating inverted in a rollover?
    FF
     
  8. Kay9
    Joined: Oct 2006
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    Location: Central Coast Oregon US.

    Kay9 1600T Master

    Well, Im my case Fred. I cant get my head out of a-hole enough to see what Im doing when inverted. But I suspect your talking about the engines operating normally not the captain ;)

    K9
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Wet sump oiling for one . . .
     
  10. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    Yes, dry sumps are easy, but air supply without down flooding is another matter. The GM V6-53's in the 44' MLBs would suck vacuum on the compartment in 4 to 5 seconds if there wasnt a special way to supply inlet air. This is not a trivial problem, China lost the whole crew of a deisel sub a few years back when the poppet valve on the snorkle stuck.
     
  11. Knut Sand
    Joined: Apr 2003
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    Location: Kristiansand, Norway

    Knut Sand Senior Member

    Lifeboats designed for the MODU code shall have deluge/ sprinklers, and if equipped with these again; the LSA code requier the boat to be self supported by air for approx 10 minutes, running at 6 knots...(= air tanks with compressed air).

    So a rollower lasts normally shorter than 10 minutes.. hopefully.:cool:

    And the engine normally cuts, when boat is inverted.
    Makes it necessary to rekrank the engine again...

    BUT; I see the point by some "type" rescue boats beeing able to run the engine when inverted; shorter responce time, operater able to hit the throttle the sec he's aware of the direction to head... Rescue boats operating from shore would be one of the types I'd prefer having an engine able to run inverted...
     

  12. Submarine Tom

    Submarine Tom Previous Member

    A 7 - 10 meter RIB takes about 7 seconds to self right after buoyancy bag

    inflation is initiated. A 47' North American Coast Guard life boat rights in

    less than 15 seconds and takes on about 325 US gallon (~1200 litres).

    -Tom
     
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