Reserve Buoyancy

Discussion in 'Boat Design' started by Tevens, Apr 11, 2011.

  1. Olav
    Joined: Dec 2003
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    Olav arch. nav.

    Ad Hoc,

    now I did read that thread and I think I get your point. ;)

    Actually it then comes to the aforementioned subtleties (well, maybe not so subtle at all...) in language as we have to clearly distinguish wave-piercing (i.e. complete immersion of the forebody) and wave-"slicing" with more or less reduced vertical accelerations due to lack of bow section flare. I didn't pay enough attention to this difference and thus was wrong. Speaking of "true" wavepiercers you are of course perfectly right in saying they don't have reserve bouyancy.

    I stand corrected. :eek:
     
  2. Submarine Tom

    Submarine Tom Previous Member

    No

    Tevens,

    To answer your question, no. But, more importantly, why do you ask? It's an intriguing question and I think I understand where you're coming from but want to be sure, so I ask the question.

    Perhaps instead of "extra reserve buoyancy" you meant "momentarily submerged buoyancy", I don't know.

    Don't be discouraged by the responses to your original post, they mean well but their delivery sucks.

    -Tom
     
  3. Tevens
    Joined: Feb 2009
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    Tevens Junior Member

    Okay forget about the wave piercers then, that wasn't really what my question was about. though i was meaning the wave piercing hull you find on a racing cat or tri.

    An AXE bow defiantly has reserve buoyancy, and if you think it doesn't just pretend it does ( because it defiantly does)

    Is there a way to calculate the advantage gained by the reserve bouyancy?
    In a circumstance where the bow is heading into head seas.
    seeing if the hull would dive through the wave or would be pushed up due to the available buoyancy. (sorry for the lack or a better phrase "pushed up")
     
  4. Tevens
    Joined: Feb 2009
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    Tevens Junior Member

    p.s.
    thanks for the support tom!
     
  5. Submarine Tom

    Submarine Tom Previous Member

    No problem. Once again, the answer is no, and, once again, I ask why?

    How would YOU define "reserve buoyancy"?

    How would YOU calculate the advantage of "reserve buoyancy"?

    What is your question really about?

    -Tom
     
  6. Ad Hoc
    Joined: Oct 2008
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    Location: Japan

    Ad Hoc Naval Architect

    Olav

    Here, i address the motions aspects in a bit more detail:
    http://www.boatdesign.net/forums/multihulls/proa-questions-33138-2.html#post372968

    As you can see, I also tried to explain (again), yet it seems despite the known laws of hydrodynamics and its effects, some prefer to debate about "feelings" and "thoughts" rather than actual facts and real known mechanisms that effect behaviour. Despite their claims of of understanding the subject matter.

    Thoughts and feelings fuel subjective debates and become a case of "my dog is bigger than your dog"; with no conclusions other than more words and emotions....rather than trying to really understand what is really occuring and learn.
     
  7. Pericles
    Joined: Sep 2006
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    Pericles Senior Member

    Attached Files:

  8. cthippo
    Joined: Sep 2010
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    cthippo Senior Member

    It's a function of what she was designed for. Light cruisers in the days before radar were designed to scout ahead of the battle fleet and find the enemy and then report back. To do this they needed great speed.
     
  9. CatBuilder

    CatBuilder Previous Member

    I think I know where that issue with reserve buoyancy came from in the beginning of the thread. When thinking about a wave piercing hull, there are two ways to conceptualize it:

    1) An addition of length at the bow is added to a standard LWL. This additional length is then made in such as way as to have low reserve buoyancy, but since it is added, it does add to the total reserve buoyancy of the vessel.

    2) Start with a given length boat and cut some of the bow off to achieve less reserve buoyancy at the bow. This reduces reserve buoyancy at the bow (remember - unchanging LWL in this case) and reduces overall reserve buoyancy of the overall boat.

    I think in these two different thoughts about how one takes a standard bow and creates a wave piercer, people get lost... especially a novice.

    Clearly, depending on how you imagine the boat to be modified from a standard bow, you can come up with differing ideas on reserve buoyancy. I'm going to have to guess Daiquiri's way of thinking about it is probably the way a NA or designer is formally trained to think about it (and hence is the correct way).
     
    Last edited by a moderator: Apr 13, 2011
  10. Submarine Tom

    Submarine Tom Previous Member

    cthippo,

    My four year old daughter and I would like to know what your picture is of beside your name.

    Is it an alien?

    -Tom
     
  11. CatBuilder

    CatBuilder Previous Member

    Tom: You haven't seen that one? There is a very frightened guy hanging on for dear life in Cthippo's avatar.

    I'll let him tell the story though :)
     
  12. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Thanks D..as always you’re too kind with words. I was only pointing out what, well to me, was obvious upon closer inspection.

    Success :)

    Credit to D for his persistence.

    If you read the exchange between myself and Will, here:
    http://www.boatdesign.net/forums/bo...g-myth-design-ingenuity-30296.html#post318738
    don’t worry only goes on for 2 pages…just ignore the chaff from those less informed but claimed to know better.

    This is pictorially illustrated here:

    WP adding bow length.jpg

    As it is also very very much related to why….that aspect is also covered here, regarding motions.
    http://www.boatdesign.net/forums/multihulls/proa-questions-33138-2.html#post372968
    just 2 posts to read, scroll down for the other.

    Sorry for hijacking the thread on a deviation.

    But success, lessons being learnt…the “myth” of wave piercers is being exposed for what it is, as most NAs know anyway. Just clever marketing. There are papers out there saying as such….but discreetly :p
     
    Last edited by a moderator: Apr 13, 2011
  13. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    My comment is the one from the post #7:
    You have to compare things with the clause "everything else being equal". By adding or cutting something to/from the LWL, you are cheating. :) You'd have to make the same change to the hull you are comparing to, or else you get the "apples to oranges" comparison, which leads to nothing...
    So, you could, say, add the same amount of LWL to the hull with the "conventional" bow, and shape it in such way that the bow remains of the conventional (flared, with an overhang) type. In that case you again get the result that the "wavepiercing" addition has less reserve buoyancy.

    Cheers!
     
  14. CatBuilder

    CatBuilder Previous Member


    True.

    Just wanted to point out that the two different ways to conceptualize the formation of a wave piercing bow could lead a novice (like myself) to incorrectly suggest an addition of reserve buoyancy by adding on length.

    I thought that might have something to do with the original confusion earlier in the thread.

    Here is the example picture. While it is incorrect from a NA/design standpoint, the layman or novice can see this done either by cutting off something or adding LWL. I think this is how they get confused into thinking reserve buoyancy is added.

    **Note the original length of the boat at the bow is approximately where we see a dark blue sail cover by the bow.

    [​IMG]
     

    Attached Files:


  15. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Yep, that's it! An illustrated guide of how to compare apples to apples and oranges to oranges. :)
     
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