Examples of wave piercing hulls, please?

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

  1. groper
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    groper Senior Member

    I never said the data was realistic for this size ship or type, only that the data was consistent between models, the only difference was the hull topsides, nothing else. Im not a naval architect and have no business knowing much of anything designing a 90m cat. The most unrealistic number i input was a VCG of +10m... should have been much lower... and everything else was thrown together *Ad hoc*pardon the pun :D

    This is all beside the point however, the point was to illustrate that the topsides alone can be shown to effect the ship motions when below the waterline, things are held constant. So can we agree on that before moving on?
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    They are not only, not realistic, but also not consistent!

    If the hull topsides are different they are not the same hull, save for the WP bow. Change in topsides only affects the restoring force. The "c" part of the equation.

    Why, what for??..you have not posted your own equations of motions since you reject those which have been posted.
     
  3. Ulf
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    Ulf Junior Member

    I’m trying to understand this discussion…

    As far as I understand a WP-bow is a bow that is extended and has no or negative flare.

    Ad Hoc has the opinion that this does not affect the ship pitch or heave response to waves.

    And groper thinks it does.

    Is this correct??? :confused:
     
  4. groper
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    groper Senior Member

    If you dont like my data and feel that i have made a mistake somewhere, or that unrealisitc data has compromised the results, why dont YOU offer a similar comparison using whatever data you feel is relevant? Im sure as a professional you have access to much better tools than i do and would be able to come up with something in short order?

    So theres 1 difference right there...

    Incorrect,
    I did post an equation earlier in the thread, you ignored it and started on with something else about a ships natural period... go figure...
     
  5. TANSL
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    TANSL Senior Member

    Common sense tells us that everything that touches the water, especially if it is on center line and forward of the ship, affects the pitching motion of the ship. I can not know whether much or little affected because it depends mainly on the volume of the WP, but Certainly affects!
    This is what my humble brain tells me.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    If you elect to debate by plugging in numbers into a program that you don’t fully understand, how can you discuss the nuances of the theory? I am not the one that requires understanding the differences, you are the one since your anecdotal evidence noted here:

    See this is where your lack of understand is showing through again.
    In your zeal to claim “something” that is related to your feelings rather than facts you’re flip-flopping to suit your end argument. Here you say:

    Yet here you say:

    You can’t have it both ways! Either it is or it isnt :confused:

    I did not ignore it. I informed you what that equation reduces to when assumptions are made to make it “manageable” for establishing motions by approximations to linear rather than the more complex second and third order equations. You can look it up in any text book. Yet you reject it! So why bother? :(

    Even the program you have used uses this assumption and you are totally unaware of it:

    seakeeper.jpg

    Thus, if you wish to further your understanding you need to be more rational and do further background reading on the subject you like to be opinionated and emotive about, yet unable to be supported by facts other than your anecdotal viewings.

    It is, or was, a simple exploration of the factors that influence the seakeeping and hence motions of vessels. One trying to use their limited observations as fact another exploring the theory behind the factors that actually have an influence and go beyond personal observations/thoughts. Thus, never the twain shall meet :eek::(
     
  7. DCockey
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    DCockey Senior Member

    What are the limits of applicability of the equations that result "when assumptions are made to make it “manageable” for establish motions by approximations to linear rather than the more complex second and third order equations"?

    Questions about what happens when a significant portion of the topsides are immersed during pitching are not invalid because the "manageable" equations are not applicable for that situation.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    Linear.

    When the boat goes beyond a datum that is level trim or its initial equilibrium.
     
    Last edited: Sep 26, 2012
  9. DCockey
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    DCockey Senior Member

    So you consider questions about what happens when a significant portion of the topsides are immersed during pitching are invalid if they exceed the limits of the linear equations?
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Not me, per se.

    It is the generally accepted method. As also noted here, in case you didn't read it above, from the manual of seakeeper:

    seakeeper.jpg
     
  11. groper
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    groper Senior Member

    Sounds like we are getting closer to the point...

    So here you are adhoc, trying to ram some linear math theory down my throat when the problem is in reality, truly non-linear. - this is what i was referring to in terms of simplified, i didnt mean mathematically simplified, but a simplified approach to the problem.

    Instead of this math theory, why dont you show us some model testing or such like, where the results suggest there is no significant difference to ship motions, when considering a forward flared bow vs WP bow on an otherwise identical hull... surely an esteemed NA would have tested such theories and have something a little more tangible than a linear equation to go off?
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    I am not trying to ram anything down your throat. Merely showing you where and how to apply the theory, if you’re interested; but clearly not, as you’re being myopic and flip-flopping with each post.

    See, if you bothered to read my posts, that is exactly what you have presented to you. But, yet again, in your zeal to say otherwise, you’re throwing the baby out with the bath water. Why..no idea?!! You have even rejected such “simplification”. Go figure…

    Sure, see a small selection here :

    WP-1.jpg WP-2.jpg WP-3.jpg

    There are plenty of others out there if you take time and do the research, in order to understand the mechanisms that influence the behaviour and response, rather than just resort to subjective feelings and thoughts. But that doesn't come with the pressing of buttons into a program. It takes time and patience. :eek:
     
  13. groper
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    groper Senior Member

    Ok theres one example thats shows little difference - thanks for that. But by looking at the model shapes, there doesnt appear to be a large difference in forward buoyancy, Sure they are a different shape, but the volumes look very similar, more like a bulbous bow rather than a true WP bow...

    This is the boat i used to work aboard;
    [​IMG]

    Its LOA is 39 meters. Looking at the shape of this boat, you can see that the WP bows have reduced a significant amount of forward buoyancy. You can see from the picture that the WP bow is around 1/5th of the total LWL. The engines are located in the hulls where the giant Q is painted on the sides. You can also see from the general layout of the decks etc that is has a very AFT CoG, and the CoB is probably even further aft - around 33% from the transoms, whatever it is its most likely aft of the CoG which is at odds with conventional designs where its in front of the CoG. Do you have any data on these more extreme / more true WP type designs? ie. there is a large reduction in forward buoyancy...
     
  14. BMcF
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    BMcF Senior Member

    Having headed up the development and managed the fielding of the ride control systems for some of the first, and most subsequent wavepiercers, and having personally run all the earlier simulation work, I can assure you that linear code is quite adequate to predict both the nauseaus resonant pitching motions that plague wavepiecers, and also the performance of the ride control solutions used on them. From the Condor 9, ca 1991, and onward...so many I lost count long ago.

    In the beginning, it was a lot of retrofit work to install the ride control systems..and then they became essentially standard equipment.
     
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  15. Dave Gudeman
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    Dave Gudeman Senior Member

    Ad Hoc, I'm not sure I understand what your point is, so let me propose a thought experiment to see what you say about it. Let's assume you have a boat hull of whatever shape and you extend out from it a very narrow rectangular bow. Think of the boat as a switchblade handle and you are extending the blade out as a bow. The weight and shape of this "blade" have neutral buoyancy at, say, 5 feet depth.

    Now as you move this boat forward and encounter on-coming waves, you would expect the blade to not rise when it encounters a wave until the wave reaches the main part of the hull where there is buoyancy or until the height of the wave is such that the blade is more than 5 feet deep. The effect would be that the wave splits around the blade. I think it would be reasonable to describe this behavior as "wave piercing".

    Now, start thickening the blade to make it very slightly more buoyant. Now the bow will start to rise as soon as the wave hits it, but not very much. It is still going to mostly pierce the wave. As you get thicker and thicker, the bow will rise faster and faster on an incoming wave causing it to split the water more shallowly.

    So, isn't a wave-piercing hull one with a bow that is a bit closer to the blade shape than a normal bow is? And if so, why isn't it correct to describe what it does as "piercing"?
     
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