Self-righting hard chined boat

Discussion in 'Stability' started by Trixini, Sep 1, 2016.

  1. Trixini
    Joined: Aug 2016
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    Trixini Junior Member

    Hello everyone,

    I'm currently designing a small boat (around 8m length). The boat is relatively wide (3m) and is designed as a hard chine hull. In the fwd part there is a closed cabin and aft is an open area with some sort of "spoiler".

    The boat has to be self-righting and I have currently two ideas:
    a) construct some kind of watertight tarpaulin or hard cover for the aft part to have additional buoyancy volume
    b) put an expanding bag onto the spoiler

    Obviously such boat is the opposite of optimal for self-righting, but the breadth cannot be reduced.

    I'm calculating heeling lever curves and when the boat is swimming upright this is a stable position because if the boat heels to port or starboard I have a positive heeling lever. Regarding the design of the heeling lever curve I would therefore try to avoid any negative heeling levers between 0 and 180 for port and starboard side - in theory this should be sufficient because there would only be one stable swimming position: the intact one.

    My question is: Am I missing something here? Is it okay to simply apply "simple" hydrostatics?

    Note: Since I'm not allowed to put the project online yet, this is a good idea of what I'm talking about in general: http://www.bsu-bund.de/SharedDocs/p...07/Ubericht_415_06.pdf?__blob=publicationFile (page 24)
     
  2. The Q
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    The Q Senior Member

  3. daiquiri
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    daiquiri Engineering and Design

    Hi Trixini,
    as TheQ has said, simply closing the aft deck might not be sufficient, unless the stability curve demonstrates that it is AND if you can assure that all the enclosed volumes will be watertight at all times (for example, no open doors or hatches).
    IMO, your best bet is to go for the second option - the buoyancy volume on the aft spoiler. The one over the forward cabin alone might not be sufficient in case the aft part (where the CoG is) gets flooded for any reason. In that case your boat would just float nose-up.

    You can look at the work done for fast rescue boat, to get an idea of possible solutions:
    http://www.norsafe.com/Products/Rescue/Mako-655-jet/
    http://www.fassmer.de/lifeboats/rescue-and-fast-rescue-boats/
    https://www.harding.no/products/rescue-boats/frb-2

    Cheers
     
  4. Trixini
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    Trixini Junior Member

    Hi,

    thank you for your quick response!

    I'm aware of the floating-bag-spoiler-solution many boats use, I just don't think those look very nice (but I'm already in contact with a supplier and the design of the bag storage can be customized, so that might be okay).

    I'm more interested in the hydrostatic problem since I do not want to trust the results of my stability software blindly since this software was not designed to calculate capsizing.

    To my understanding, the boat only has to be self-righting in intact conditions - if damaged, the buoyancy foam integrated in the hull should do its job to prevent sinking.
     
  5. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    It is generally understood that the ship must be "self-righting in intact conditions" or without water entered the hull.
    The foam can prevent water in some areas but not all spaces may be filled with foam so the use of seals must be taken into account. If the boat shipped water, its properties of "self-righting" can be greatly diminished ..
     
  6. Trixini
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    Trixini Junior Member

    Hi again,
    I've calculated heeling arm curves for the open and closed version with and without a floating bag.

    I've uploaded the heeling lever curves for both hull versions, each with and without additional buoyancy on top. Shown are heeling levers [m] over heeling angle [-180 to +180 degrees].


    Self-righting hard chined boat.jpg

    It can be seen that the closed boat (bottom graph) has a better stability but is still not self-righting (blue curve).

    What I'm not sure with: At 180deg the heeling lever obviously is 0 since the boat is symmetric. However, to my understanding this should be a unstable position and it should start righting itself back up. Am I misreading the graph in any way?
     
    Last edited by a moderator: Sep 1, 2016
  7. Gary Brown
    Joined: Mar 2012
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    Gary Brown Junior Member

    Trixini:
    At 180 degree heel, the blue curve ship is in a metastable position. https://en.wikipedia.org/wiki/Metastability It is stuck in a small potential energy well. You have a small symmetric energy hump on either side of the 180 degree heel point due to the GZ crossing the zero value. You will have to supply energy to the ship to get over the hump and then righting can occur back to the more stable upright position.
     
  8. Trixini
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    Trixini Junior Member

    Understood. That's why the 2nd curve shows the boat including additional buoyancy to avoid crossing h=0 ;-)
     
  9. Heimfried
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    Heimfried Senior Member

    Each point GZ = 0 in a righting lever curve is stable (or metastable) if in a small interval around this point the GZ changes from negative values into positive ones (read in the direction of growing heel angles).
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't see any mention of the occupants of the boat while all this up-ending is happening !
     
  11. Trixini
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    Trixini Junior Member

    That's quite a good point. However, according to the classification only the boat needs to be self-righting without paxe on board as it is estimated that everybody can swim out of the capsized boat.
     
  12. Trixini
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    Trixini Junior Member

    Model

    Alright, I've created a very simplified 3D model to show you guys so you could understand better what I am talking about.

    If anybody needs the file (created in Rhino 3D) I can send it to you.

    rhino.jpg

    The model shows the ship without stability bag (this would be mounted on top of the frame).

    The resulting heeling levers look like this:

    heel.jpg

    So the boat is obviously far from self-righting.

    Before calculating the required bag-size I would like to confirm the stability curve. What I'm not understanding: somewhere around 45 degrees heel, the inner part of the boat gets flooded since the side of the deck is underwater. To my understanding, this should result in a visible bump/change in the heeling levers curve? I'm not sure if this is an error in my stability software or if I am thinking wrong.

    Thanks again for all your help guys, much appreciated.
     
  13. Gary Brown
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    Gary Brown Junior Member

    Trixini:
    You didn't indicate what software you are using but if you are using Orca within Rhino, or many other programs, the lack of a hump indicates that the software is flooding that internal cockpit before it reaches the flooding angle. The cockpit forms an internal well and the software will fill it up to the waterline at any heel angle. This should be in your software manual. The walls you used to create the cockpit have surface normals pointing outward and to your software, it probably thinks it needs to be wet. Just remove the cockpit, and any other internal walls, and get a curve up to the flooding angle. Beyond the flooding angle use the curve with the cockpit.
     
  14. Trixini
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    Trixini Junior Member

    Hi Gary Brown,

    that sounds logic, I'll try that later and post the results.

    I'm using Autohydro by the way. Unfortunately, the manual is not optimal...
     

  15. Heimfried
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    Heimfried Senior Member

    I don't know the abilities of your software. May be it is unable to deal with downflooding, may be you missed to set a special adjustment for this case.

    When the heel angle grows, at the downflooding angle the water will start to flow in the cockpit and that means a loss of buoyancy as you said. The draft increases and the rate of downflooding increases. This should be visible at the graph.

    I did a simple software to calculate and show basic hydrostatics of boats and floating bodies online.

    http://www.bootsphysik.de/rechner/bootxp3.php

    There are millions and millions of calculation steps necessary to find the new floating position of the boat e. g. after the heel angle grows 0.1 degree.
    That leads to a server response time which kills the connection between server and browser. So I had to compromise. The sampling frames all are decked and no entry of water is possible.

    To consider downflooding a lot of additional conditions is to determine.
     
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