Examples of wave piercing hulls, please?

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

  1. Ad Hoc
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    Ad Hoc Naval Architect

    It is the difference between a simple noun and whether this new noun is a true reflection of what it represents, like an onomatopoeic. That is the issue.

    You could easily have any shape with near or zero buoyancy, that you like. If i take my knife out, and place that onto a 40m vessel, my knife pierces the wave...so what? There is no argument there. A plank of wood in a similar fashion, shall to do the same. So, to give this "bit of add on" a nomenclature, the term wave piercing has been attributed.

    The trouble comes when one uses this noun as a reason to define or explain what is assumed an improvement in motions or accelerations, owing to its own definition!. How does a noun do this??....if that very same WP bow is called a Wave Slamming bow...the visual imagery in ones mind is radically different. One instinctively thinks of a bow slamming into waves, why??..because it is called a wave slamming bow! Does changing its name only suddenly change the motions of the vessel?!!

    Thus either you are debating a simple definition of a noun..or, you are debating the verifiable quantitative effects of appendages and hull shape on a vessels motions, which has nothing to do with the noun, other than it may be an onomatopoeic word.

    Thus does the bow perice through the wave, and that is all. And as such, it is called a "WP bow".
    OR
    Does the bow peirce through the wave, and by doing this radically improve the motions of the vessel, and yup, it is still called a WP bow
    OR
    Does the bow peirce through the wave and have no improvement on the vessels motions, and yup, it is still called a WP bow.

    It is as simple as that.
     
  2. groper
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    groper Senior Member

    The boat i pictured above, has no ride control system and neither does its slightly larger sister ship. Its SOR is a passenger ferry that operates in the partially smooth waters behind the great barrier reef. The largest seas we get here is about 3m in strong winds. The median sea state is 2-2.5m that result from our very predictable trade winds in this part of the world. This design works well here, due to the minimal forward buoyancy. A very blunt bow would otherwise cause bow slamming / vibrations from the very short period waves and high encounter frequency with the 33kt cruising speed. I would not expect this design to work well in more open waters, but for these conditions it seems to work very well.

    I think the design has alot more to it than just the bow design tho, such as the CoB vs CoG location etc...
     
  3. Ad Hoc
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    Ad Hoc Naval Architect


    Firstly define a short period wave.

    Then please explain to me how a bow blunt or WP or any, would cause slamming in short period waves?
     
  4. groper
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    groper Senior Member

    Im sure youll find problems with this ADHOC, but heres something on the INCAT 112m wave piercing ferry operating in Japan;
    http://www.marine.osakafu-u.ac.jp/~lab15/papersearch/papers/PDF/2009-009.pdf

    Ill quote the first 3 points in the conclusions;
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    It seemed to me that they were comparing a monohull with a WP cat. I'd prefer to see comparisons of a WP cat with a more conventional cat.
    Or did I miss something?
     
  6. groper
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    groper Senior Member

    no your correct, but if we only compare the pitch motions, you can still see a difference with a more bluff shaped hull...
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    Leo,
    No you didn’t. Just like comparing apples with fruit pie then :p

    The figures are, or rather their conclusions are wrong too. They are only look at RAOs, not the response in terms of motions. So X heave in Y waves. Great….look at a swath, it’s hopeless on those terms! What does that ratio relate to in terms of actual accelerations!

    It is hard to extra data from this, but looking at fig 2 the motions in a T=7s and Hw=3.2m. The WPC has a rough “RAO” of 0.15 in pitch and the mono of roughly 0.26. Given a speed of 30 knots for the WPC and 18knots for the mono, the acceleration for the WPC is 2.23 m/s^2 and for the mono 2.3m/s^2. In other words, no difference!

    But when looking at the T=9 and Hw= 4.4, for pitch.
    The WPC has an acceleration of 5.2m/s^2 and the mono of 3.5m/s^2. The mono is much lower!

    The conclusions in this paper are telling only half the story and an incorrect one at that!


    Just wondering if you’re going to answer the simple question..

     
  8. BMcF
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    BMcF Senior Member

    Nearly all of the many "wavepiercers" over 40m LOA wre built with ride control systems or had them retrofitted. Nearly all are/were, of course, ferries. The list of route locations is rather long.


    The waterplane area size and longiitudinal distribuution affects wave forcing and damping. At the size of most WP ferries (74-100m) that combination results in a lightly-damped natural pitch resonance that is spot on the most sensitive region of the human sensitivity for motion sickness (ref: ISO 2631, MSI).

    Ironically, for the laypersons that view the aesthetically pleasing bow that seems to fit the aura...if pitch motions on a 74m WPC are of sufficient magnitude that the center bow is even makiing routine but minimal contact with the water......too many passengers have already grabbed their sickness bag. I still have some of the news clippings with passenger horror stories..from the early English Channel debacles, the Nova Scotia run..etc.;)

    Quite a few years ago now, a systematic study was undertaken by ..I've forgotten whether it was Delft or Marin now..anyway, to try andd understand why the wavepiercer hull was superior to others. They included a typical round-bilge cat, a hard-chine cat, a wavepiercier and a Quigley. They came away from their model testing effort somewhat surprised by the results...:cool: If I recall correctly, their results were presented in one of the Fast Ferry or FAST conferences in the early 90s.
     
  9. groper
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    groper Senior Member

    Im glad your all actually seeing a difference now... we werent discussing whether the WP design is better or worse, just trying to show that the shape of the topsides does effect the ships behavior, for better or worse.

    The reason i went down this road is because adhoc would have me beleive the linear theory holds true regardless of changes to the hull topsides which basically assume a vertical wall from the DWL if im not mistaken? Thus accurate predictions for extreme increases in buoyancy from topside flare or reductions from extreme WP bows, cannot be reasonably compared in the same linear fashion. Thats the only point im trying to make here... im sure the "generally accepted method" works adequately for most typical designs out there, but surely the extreme WP designs im referring to, fall outside this general scope?
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Yup, you are mistaken. Again.

    Linear theory is used to make the calculations more manageable. The theory’s assumption, not yours, not mine, the assumption of the theory, just as in your program which you seem to enjoy plugging numbers into, is that the equations are reduced to a linear function. This simple fact you have failed repeatedly to understand, yet constantly pontificate to the contrary. Not sure why? Which means you cannot use your fancy program as a basis for any comparisons, it is beyond its limitations! :(

    As BMcF has noted above too, linear is more than adequate. (BTW, he is probably one of the world’s leading experts on such matters).

    So, yet again you’re flip-flopping.
    Here:

    Hmmm..interesting, since here, as you have pointed out:

    If there is minimal fwd buoyancy, there is no flare in the top sides, as there is no top side, it has no buoyancy because there is no hull there! Thus, either there is buoyancy above or there isn’t..which is it?? And if there is no buoyancy, what is there to integrate, in a linear or otherwise fashion?? :confused::confused:

    Oh btw, you are answering with a question, i.e., you are not even sure of your own reply.

    See what I mean? And finally, again:

    Well…are you going to define what you mean by short period waves and how a bluff, sharp or otherwise bow, is effected by such short period waves?
     
  11. BMcF
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    BMcF Senior Member

    If I might offer some insight/opinion as to what really are the more "extreme" examples of catamarans that "pierce waves", they are embodied in the designs like the USN SeaFighter or Stenna's HSSL family.

    The waterplane area and vertical-plane stability derivatives of those examples are more SWATH-like than the Incat and other WPC designs. A true SWATH being the "ultimate wavepiercing catamaran", of course, although I'm loath to use that kind of description except rarely because all of the designs under discussion here represent a "continuum" of volume and waterplane distribution characteristics and the labels tend to obscure that simple fact.

    So, using the USN Sea Fighter as an appropriate example, that vessel exhibits the usual unfortunate/uncomfortable human-unfriendly pitch resonance seen in other similarly-sized catamarans. But at higher speeds of operation, the active motion control does indeed dampen that resonance to a degree that the vessel does achieve near-platforming performance in some pretty high sea states, "piercing" waves in the process.

    But..and this is an important "but"..the Sea Fighter hull form is very much SWATH-like forward; a good deal more so than a typical WPC. Thus the effect, and effectiveness, of the stabilization system is also greater.
     
  12. daiquiri
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    daiquiri Engineering and Design

    Wave-piercing under some conditions: http://www.youtube.com/watch?v=mvxtqUvWIvc&feature=player_detailpage#t=20s

    Wave-hobby-horsing in some others: http://www.youtube.com/watch?v=mvxtqUvWIvc&feature=player_detailpage#t=120s

    Wave-diving in yet others: http://www.youtube.com/watch?v=mvxtqUvWIvc&feature=player_detailpage#t=70s

    Cheers
     
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  13. Dave Gudeman
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    Dave Gudeman Senior Member

    Ad Hoc, you know a lot about marine design and I know almost nothing, so I'm just trying to figure out what you are saying because the topic interests me and I want to learn. And frankly, your way of talking about this is just confusing the hell out of me.

    So you quoted Solitaire saying "the hull dives into the wave and then parts it" and you responded by calling the statement a myth. This confused me so my thought experiment was intended to get you to explain why it is a myth in terms that I could understand. Maybe I was just totally confused about what a wave-piercing hull is? But no, it seems that my understanding was correct and that wave piercing hulls do actually behave like Solitaire said --at least more so than normal hulls.

    So at this point I'm starting to think what all you mean is (1) wave-piercing hulls don't necessarily have better motion, and (2) people's intuitions about the motion you get from piercing waves are wrong. Both of these are interesting points to me because my own intuitions were apparently wrong about this. But you just confuse things by saying it is a myth that wave piercing hulls pierce waves.

    It seems to me that what you are doing is like complaining about calling some boats "double masted" because some people will assume that this makes the boat twice as fast. But the phrase "double masted" doesn't actually mean "twice as fast", it just means that there are two masts. Similarly, "wave piercing" doesn't actually mean that the boat has better motion, it just means that the bows pierce waves.
     
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  14. groper
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    groper Senior Member

    Thanks BMcF,

    Im glad you brought up these 2 designs as i had previously looked at them prior to this discussion... I had looked at them previously to try and guage what was the difference between a semi-swath and a normal catamaran - would semi-swath be an appropriate term for these 2 aforementioned vessels?

    So in continuance, do these semi-swatch (if i may?) vessels offer improved seakeeping compared with conventional / traditional catamarans despite help from their ride control systems, or because of them? - i was under the impression swath type vessels offered improved seakeeping regardless...

    And are you saying that by removing the reserve buoyancy, ride control systems can be made more effective? This makes sense to me, as the ships motion would then seem to me, somewhat decoupled from the uneven sea surface.

    Ive looked at several pictures like these of the USN`s Sea fighter, and i just cant see much difference to a normal / conventional / traditional catamaran other than what seems to be a considerable reduction in forward buoyancy? - from the midships aft i cant tell a difference... Or is there something else im missing here?

    [​IMG]

    [​IMG]
    seafighter.jpg
    And does the linear theory still make accurate predictions of semi-swath or swath type hulls behavior? If so, why or why not?

    Cheers
     

  15. BMcF
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    BMcF Senior Member

    You pretty much nailed it. And without some form of active stabilization, all such very lightly-damped hulls are going to behave badly when wave conditions cause resonant pitch motions..and might behave well otherwise.

    The motions of the Sea Fighter in a seaway, at all relative wave headings, with RCS off and on, were predicted within an astonishing plus/minus 10-15% on average when compared to measured values. Well within all the various sources of error involved in such a comparison.

    All with purely linear catmaran seakeeping simulation code.

    It's been my experience over the the decades I've been doing this "funny boat stuff" that those who most loudly clamor for the need for non-linear time-domain simulations to predict the motions of these vessels in even very extreme conditions (were we can, it is true, clearly observe/measure non-linear events)...were simply working with, or unable to successfully develop, good linear simulation tools. I know of not one exception to that personal observation. 'scuses are 'scuses.

    That's just my .02...its been blithely dismissed before so my feelings won't hurt. :)

    There certainly are many areas where the solution of time-domain models is very usefull and where frequency-domain solutions barely hint at the important physical results that are desired, and even then only in a statistical sense. But the seakeeping of ships is not one of those areas.
     
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