Wave piercers

Discussion in 'Hydrodynamics and Aerodynamics' started by ShaneK, Feb 21, 2018.

  1. BMcF
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    BMcF Senior Member

    Properly engineered active motion stabilization systems react quickly and aggressively to angular rate and acceleration, creating a force in opposition to undesired motion almost before any discernible motion has actually even occurred. Passive fins and cross-flow plates (bilge keels, for example) do not generate a damping force until the undesired motion has occurred to a significant degree. On a lightly-damped SWATH, for example, a 2 square meter active control fin will achieve damping levels and motion reductions far in excess of anything that could be achieved by a passive appendage 10x that size.

    Further, for properly designed SWATH vessels, the stabilization system provides a very high degree of underway platform stability, maintaining commanded list and trim values to withing very tight tolerances, even when subjected to "awkward" loading conditions related to crew movement, temporary deck loads, etc.
     
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  2. fastsailing
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    fastsailing Junior Member

    Does that mean it's typical to use active stabilization foils in both front and aft ends?
    Because if only aft ones are used and a wave lifts bows, pitch control would require upwards control force at the aft end and at the same time downwards control force to reduce heave response. The only way to counteract that contradiction would be to use down force at bow location. Just like canards are able to do in unstable european fighter aircrafts. That obviously can not be done in static stabilization foil control systems.
     
  3. fastsailing
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    fastsailing Junior Member

    When a vessel is moving against the wave propagation direction, and a wave lifts the bow to cause pitching, I would have described that as excitation moment, not restoring moment, that is proportional to GM_longitudinal. I find it confusing that you use the opposite definition.
    I also would have guessed that cats would have less GM_L than similar length monohulls, and therefore would not have call cats as high stiffness vessels in pitch. In roll it's obviously the opposite. Did you meant roll? I did't even consider active system trying to damp rolling in cats, only in monohulls and SWATHS. Glad to be informed otherwise and learning something new.
     
  4. BMcF
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    BMcF Senior Member

    Absolutely.
    It is not correct to say that damping pitch motion requires opposing forces on both ends. It does not. On may vessels, only aft effectors (trim tabs, fins, interceptors) are used for active pitch and roll damping, and that still works well in many cases (but never as well as having them both fore and aft). The extent of every system always involves, performance, cost, space/weight, and draft penalty trade-offs.


    Of course it's not unusual to see passive damping fins, foils or fin-like bulb installed forward too. Some damping results, of course, or they would not be fitted. Nothing like the results from an active system though.
     
  5. BMcF
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    BMcF Senior Member

    The typical long-slender-hull fast displacement cats (wavepiercing or other) are fairly lightly damped in pitch..a bit stiffer in roll. The first active ride control systems were invented to cure primarily the sickness-inducing motions resulting primarily from pitching motions, where resonant pitching occurred to a significant degree in many sea conditions and, worse, occurred at the frequency at which human beings are most susceptible to becoming sea sick. Were it not for the active control solutions introduced almost immediately after the first 47, 74, 80 etc meter fast catamaran ferries were built, they would never have survived as a viable transportation technology. They ALL have active motion stabilization systems.

    Roll is damping is included too, and some also include yaw-damping, long slender hulls with forward-shifted lateral center or areas being directional unstable as well.
     
  6. fastsailing
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    fastsailing Junior Member

    I didn't imply that damping pitch motion requires opposing forces on both ends. Instead I wrote that damping both pitch and heave would require opposing forces at the aft end at the same time when a wave lift bows, which is not possible.
    When the wave is lifting the aft end then a forward foil is unable to provide pitch and heave damping at the same time, because that would require opposing forces at the same time in the same foil.

    If you happen to know a well written PDF or a website describing these marine control & stabilization issues properly, would you share a link please?
    I would enjoy looking at the correct math too, not just descriptions.
     
  7. BMcF
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    BMcF Senior Member


    I didn't see you refer to heave motion and since I know that there is very little pure heave motion associated with a pitching catamaran, I didn't consider it a part of the problem. That's not always true though...we found that trying to actively control pitch motions of other types - surface effect ships in particular - resulted in the unsatisfactory result that the pitch motions were essentially being "converted" to heave motions, resulting in no beneficial ride quality improvement overall.

    There have been some good technical summaries written about the stabilization and control of various advanced marine vehicles...no good repositories of it though. You could check out the International Hydrofoil Society web site though; in addition to having an archive of various technical presentations, they sell, for a very reasonable cost, CDs containing entire libraries of tech publications and papers cover various AMV topics, including stabilization.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    The basic equation of motion is:

    aZ.. + bZ. +cZ = F

    Z.. = acceleration
    Z. = velocity
    Z = displacement

    The . (dot) referring to the differential eqn.
    and then
    a = virtual mass of the vessel (added mass + mass)
    b = damping constant
    c = restoring force
    and
    F = exciting moment.

    The wave is the exciting moment.

    If the eqn is = to zero, i.e. no wave, we can find the natural period of motion of the vessel.

    The "c" or restoring force, is a function of the vessel's geometry.

    For example, in heave c = (rho).g.Awp

    rho = density of sea water
    g = 9.81
    Awp = waterplane area

    So, depress a vessel how does it restore itself....??

    Similarly for roll and pitch....that's it.
     
    Last edited: Jul 10, 2018
  9. fastsailing
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    fastsailing Junior Member

    Is that how you define it, how it is commonly defined in naval architecture, or both?
    You seem to mix force (left hand side) and moment(right hand side) in the same equation.
    The same wave most certainly does not produce the same excitation force or moment for all vessels at all speeds.
    Doesn't make sense in the physical context to assume otherwise, since in real world excitation force is caused by the interaction between the wave and the vessel, and thus dependent on:
    1) properties of the wave including it's height, length, propagation speed, and waveform
    2) properties of vessel in areas where it is in contact with the wave including width of bottom/bow, deadrise of bottom/bow, trim angle of the contact area, etc.
    3) velocity of vessel including both speed and direction of motion relative to wave propagation direction.

    In case of pitching instead heave, one should also include moment of inertia of the vessel and the distance of wave interaction from the center of gravity of the vessel.
    And one must also include negative wave interaction as well, meaning reduced displacement at the trough of the wave, not just increased displacement at the crest based on reserve buoyancy.
    And there is also dynamic effect due to accelerating mass of the wave, not just buoyancy effects on the vessel it causes. These should considerably reduce the effect of a head wave in long ships far away from the initial impact area compared to small boats, because small boats are much lighter than the wave, while big ships are much heavier than the wave inside interaction volume. Same thing can be interpreted as conservation of energy effects, because assuming the wave to remain the same during interaction would result the ship gaining more energy than the wave originally had, an obvious flaw not present for small boat interacting with the same wave.
    And since we were talking about foil stabilization systems, I would expect the forces in those to be dependent on square of relative speeds, not linearly like damping constant b would do in your equation. Not sure how displacement hulls would respond, but at least planing hulls should likewise depend on square of speeds, not linearly.

    As you can hopefully see, it's not quite that simple from my perspective.

    The on the other hand if you are calculating RAOs (Respond amplitude operators), perhaps those effects are counted while using them rather than when calculating them?
     
  10. BMcF
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    BMcF Senior Member

    I've attached something published fairly recently that covers time-domain simulation results for cats with active motion control. For decades, we (I) relied almost entirely on frequency-domain models and solutions, but with the ever-increasing power of desktop PCs, time-domain solvers have become much more practical. That said, I still rely most heavily on the proven freq-domain programs I have for SWATH, catamarans and mono hulls (non-planing).
     

    Attached Files:

  11. Ad Hoc
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    Ad Hoc Naval Architect

    I would suggest you read naval architecture books, to understand the terminology and generally accepted conventions.

    Nope. I refer you to my reply above.

    If you were to read some of the vast literature (several my ex-lecturers/tutors) you would find many different ways of expressing the same, with regards to seakeeping:

    upload_2018-7-11_8-40-36.png
    or
    upload_2018-7-11_8-41-11.png
    or
    upload_2018-7-11_8-41-30.png
    or
    upload_2018-7-11_8-42-3.png
    or
    upload_2018-7-11_8-42-20.png

    etc...etc.

    Each represent the same, that being:
    Inertia moment + Damping moment + Restoring moment = Forcing moment
    ....but each with their own mathematical slaint....the more you read the more you'll see this.

    Indeed...because you are not a naval architect, nor well versed in the terminology.
     

    Attached Files:

  12. BlueBell
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    BlueBell Ahhhhh...

    NOW we're back on topic, thank you.
     
  13. fastsailing
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    fastsailing Junior Member

  14. fastsailing
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    fastsailing Junior Member

    Thank you. It will take me some time to properly read it through and understand it, but so far it looks like what I was looking for.
     

  15. fastsailing
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    fastsailing Junior Member

    If I manage to get my hands on one I most certainly will.

    Indeed, that is what I did, and this is what you wrote:
    Like I said: "You seem to mix force (left hand side) and moment(right hand side) in the same equation"
    Just a typo I guess, but it's yours, not mine. I changed the font to underlined type to make it more clear this time to help you to pick up the issue I was talking about.

    Exactly.
    Which is why I was asking questions here on the subject, and some other people teach the stuff at universities, not me.
     
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