Examples of wave piercing hulls, please?

Discussion in 'Boat Design' started by solitaire, Sep 22, 2012.

  1. solitaire
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    solitaire Junior Member

    Thanx a lot, so far, good people. Seems I have a lot to read through.

    I had a smaller motor yacht in mind, but I'm curious abot any design to run the bow through the wave rather than bumping on top of the waves or rolling with the waves.

    Since I am a bit limited in height the hull/s has to cater for good leg room and probably some kind of gangway.
     
  2. solitaire
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    solitaire Junior Member

    That might be so. I can imagine you'd need a certain degree of velocity and mass in order to fully pierce a wave. http://www.youtube.com/watch?v=dxJOJDGchTs&feature=related

    I also have caught the glimpse of some bows where the leading edge have a negative angle to the water, thus forcing water over it/ the boat under the water, but I suppose that's only applicable at certain speed and mass.
     
  3. groper
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    groper Senior Member

    The motions of a ship, are dependant on the LWL and period and size of the waves, and wave encounter frequency influenced by vessel speed. In certain circumstances hull A might perform better in terms of vessel pitch amplitude and accelerations than hull B of equal size, just by changing the reserve bouyancy characteristics. Then of course, in different sea states, hull B might perform better.

    So im going to have to agree with Ad Hoc in this instance and to summarize quite simply, "it depends".

    And you quite simply SHOULD NOT make assumptions like whether its "better" to go "thru waves" or "over them"... Why - because waves are always different sizes and periods and 1 blanket assumption doesnt hold true for "every" situation.
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    I'm sorry, but you're missing the point.

    Take a box of dimensions 1.0m x 1.0m x 1.0m.

    If the box weighed 500kg, would it float?
    If the box weighed 2000 kg, would it float?

    Once you have decided or worked out the answer, please tell me where speed plays a part in your answer.
     
  5. groper
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    groper Senior Member

    Speed, the size of the wave, and the inertia of the vessel all plays a pivotal role in whether you "observe the bow go through a wave" with little pitch variation Ad Hoc? - But thats the point isnt it, who cares what the very tip of the bow does, its the overall bouyancy and its distribution that affects the ships motion...
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Well, look at the question I posed above.

    What is the answer?
     
  7. groper
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    groper Senior Member

    Speed affects the encounter frequency and coupled with the vessels inertia and buoyancy distribution will vary the vessels response.

    How far into the waves the bow is pushed, is of course a simple question of buoyancy alone.

    I guess you could call most true wave peircing multihulls, semi-swaths now couldnt you? I mean where do you draw the line in the sand? To me a wave peircing design has a reduced water plane area - so why not improve the vessels motions in certain sea states?
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    No, not one bit.

    Once you address the simple question above, and take things sequentially..it all becomes much easier to understand.
     
  9. groper
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    groper Senior Member

    Ok, lets play Adhoc`s game... again...

    Referring to your simple question above.

    Yes the box floats
    No the box sinks

    Where does speed enter the equation, nowhere...

    Ok now whats the next sequential step?
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Again.??..if you have "played" this before, I'm curious why you're making the leaping statement above. Since they are not wholly correct.

    However, ok, one floats one sinks ergo buoyancy. And independent of speed. Simple enough. If so, why this constant "it perices the wave" reference, either it floats or it does not. Ergo, a hull floats or it does not, irrespective of its shape.

    So, since you're referring to motions, what factors effect say the pitch motion?
     
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  11. groper
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    groper Senior Member

    [​IMG]

    Proably missed a few, but heres start;

    moment of inertia for pitch
    vessel mass
    heave exciting force
    heave displacement
    pitch exciting moment
    heave and pitch velocity
    heave and pitch acceleration

    These factors are influenced by;
    bouyancy
    encounter frequency
    wave amplitudes
    vessel weight distribution
    vessel dimensions such as LWL etc
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    hmm...ok i've gone 1 step too far.

    What factors effect the natural period of pitch.
     
  13. groper
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    groper Senior Member

    Well all of the above?
     
  14. Ad Hoc
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    Ad Hoc Naval Architect

    Well yes and no.

    The formula for the natural period of pitch is reduced to (from all those noted above) to:

    2.pi x sq.rt[( IL + ILavm)/GML displacement}]

    IL = the mass moment of interia about the transverse axis
    ILavm = the added virtual mass, about the transverse axis
    GMl = the vessels GM in the longitudinal axis
    Displacement =- self explanatory.

    Thus from this we can see having a low GML or low GMLxdispl product shall yield a high natural period of pitch. In other words a long slow period, rather than a short sharp one.

    Similarly increasing the IL and ILavm shall yield the same result.

    Knowing this, lets explore the wave piercing bow.

    So, take a standard 40m hull of say 2.50m wide midships and say 2.0m stern and then gradually reduces to zero at the FP. Hulls spaced at say 9.0m apart Cl to Cl. Then add a WP bow of say 3.0m, of fine angle of entry for the comparisons.

    So, given the above what is the percentage change of the product of the numerator (IL + ILavm) when adding a wave piercing bow with no WP bow?

    Also do the same for the denominator. What effect does a wave piercing bow have on:
    1) displacement
    2) GML?

    Using the simple values above?
     

  15. groper
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    groper Senior Member

    Im not sure what the relevance of this question is?

    If you are considering 2 similar vessels, therefore we can assume the GML is the same, and also the displacement... is this what your after? So no change... Now what?

    If we are considering an example of a high speed catamaran where the ship has a relatively small GML, this results in a long natural pitch period; consequently, in certain typical sea states and in head waves, the ship can operates in the super critical range. The frequencies of encounter with the waves are much greater than the natural pitch period where the pitch motion response is low. Now in a following sea, the frequencies of encounter with the waves may concentrate more wave energy near the natural pitch period and this may often result in higher pitch motion response...

    So going back to your original post, "it depends"... which is where we agreed with one another...
     
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