Reserve Buoyancy

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

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Is there any other way?? :p
     
  2. Timothy
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    Timothy Senior Member

    I am sorry if this question is uninformed or has been asked and answered already but are not "wave piercing" bows a function of narrow high length to displacement hulls where despite the reserve buoyancy provided by a conventional bow it will submerge ? I thought the shape of a "wave piercing bow" had been developed to shed water and avoid the tendency of conventional bows to trip over themselves.
     
  3. cthippo
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    cthippo Senior Member

    That could be part of the problem. nobody seems to be able to agree as to what a wave piercing hull is or what it's supposed to do.
     
  4. Poida
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    Poida Senior Member

    Perhaps I shouldn't say anything at all since the only experience I have with wave peircing bows is being on one.

    The opinion I have formed from the "on one" experience is that the only advantage of a wave peircing bow is speed and can be likened to cutting through a potato with a sharp knife rather than a blunt one.

    The problem I experienced was with the ride. With a standard hull I would have expected the a boat to ride on the waves but because this was a wave peircing hull it peirced the wave rather than lift the hull. However the bouyancy of the hull eventually takes over and lifts. The bow then rides over the top of the next twp waves and then due to gravity sinks back down into the waves to peirce the next one.

    This can be similated to holding a ball under water, the further you hold it down the higher it will shoot out of the water when you let it go.

    So I experienced a larger action of being in the wave and then above the wave.

    Now this being a catamaran the action is worse when the craft is not travelling 90 deg. to the waves, one side's up and one side's down.

    This is only using my observations and common sense.

    Now getting into an area where I really shouldn't, could the idea that a wave peircing hull has less bouyancy stem from the fact that a hull looks like it has less bouyancy because it is lower in the water due to the fact that it has because of gravity, has plunged back into the water after being thrust upwards?
     
  5. Tevens
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    Tevens Junior Member

  6. Pericles
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    Pericles Senior Member

    As there is lttle flare to the upper bow I suspect HMS Caroline might have been a wet ship in heavy weather. Nevertheless, she is there to inspire and overawe.

    http://www.battleships-cruisers.co.uk/caroline_class.htm

    bernd1072 posted photographs on a thread that complements this thread.

    http://www.boatdesign.net/forums/at...length-displacement-hulls-dsc07074-small-.jpg

    It's worth reading in conjunction with this one. Reserve buoyancy is not quite the same for full displacement vessels limited to their waterline speeds, compared with long slim hulls such as those on the Gunboat 66. It's a boat where all the parts work as one to deliver outstanding performance.

    http://www.thedailysail.com/offshore/11/57961/0/video-of-gunboat-66-rorc-caribbean-600-preparations

    P
     

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  7. MikeJohns
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    MikeJohns Senior Member

    P
    There's a lot of added unnecessary resistance in that bow wave climbing up the hull. Was the design supposed to reduce pitching for a more stable weapons platform?
     
  8. Poida
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    Poida Senior Member

    Hi Mike it's me again.
    Probably the artist who drew it didn't realy know what the real waveform was like.
     
  9. Pericles
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    Pericles Senior Member

    Mike,

    Look at the earlier photos, #22. A knife through butter springs to mind and it is a design launched in 1914. As a light cruiser, HMS Caroline scouted ahead of the fleet and when the enemy was sighted, she had a good chance of reporting back to the Admiral-in-Command.

    She was 420 feet overall and only 41.5 feet in width, no wonder she was fast.

    http://en.wikipedia.org/wiki/HMS_Caroline_(1914)

    P

    Addendum.

    Poida, it's a photograph from an original negative. http://www.battleships-cruisers.co.uk/caroline_class.htm
     
  10. MikeJohns
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    MikeJohns Senior Member

    Poida
    It looks like an authentic photograph I doubt an artist would have been quite that creative. The poor stokers would be working overtime:)
    The bow shapes of slender high speed displacement vessels changed considerably not long after this period. She probably had a fairly clean wave cut otherwise.

    Pericles
    Thanks
     
  11. Poida
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    Poida Senior Member

    When I said an artist, that was what could be considered our version of photoshop. Photographs because of the restriction of photography in those days, photos were enhanced by artists. When photos were taken everything had to be still that is what made me consider that the spray was added by an artist.

    The Navy may have had more advanced photographic equipment than available to mere mortals.

    Consideration must also be given to the fact I know less about photography than I do about boats.
     

  12. Pericles
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    Pericles Senior Member

    Mike,

    It's the pronounced forefoot that is lifting the bow wave and the convex curve that allows it to rise to that height and slide down the hull, but without slowing the ship. The stokers were not too taxed as HMS Caroline was oil fired. :)

    It is interesting to compare profiles etc.

    http://www.worldwar1.co.uk/lightcru.htm

    The Caroline class are half way down the page.

    P
     
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