Hull Asymmetry and Minimum Wave Drag

Discussion in 'Hydrodynamics and Aerodynamics' started by DCockey, May 28, 2011.

  1. Ad Hoc
    Joined: Oct 2008
    Posts: 7,114
    Likes: 1,056, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    The point of the question is the old adage of the gulf between theory and practice.

    Whilst I understand your motivation to garner a better understanding of what the LCB actually does, the way you describe this, in detail at times, indicates the exploration is purely a theoretical one and is somewhat “obscure”. In the sense that one can explore and play with theory but bears no relationship with reality, a non sequitur as such.

    For example, taking one of endless papers on such subjects..

    L. Doctors “The foil-stabilised super-slender monohull” , In which he concludes:

    “The analysis and the numerical experiments presented here demonstrates vividly that there is much room for improvement in the design of displacement vessels if one were free to radically alter the principal dimensions of the vessel..."

    He goes on

    "The plotted results for such longer vessels strongly suggest that the monohull appears to be much more attractive than the catamaran, at least from the point of view of hydrodynamic resistance”.

    And finally

    “It is further shown here that there would be considerable further gains if one could construct perfectly smooth hulls and to maintain their surface condition”.

    So hydrodynamically, he has explored the theories behind resistance even further. But, to what end?

    Being free from constraints is wonderful, especially in the theoretical realm of explorations, it serves to push the boundaries of knowledge and I whole heartily applaud such efforts. However, it falls short when in summing up with statements that say a very long slender monohull better than a less long one..of course it is, basic hydrodynamics will tell you that, this is not rocket science. Keep the hulls “hydrodynamically smooth”, again wonderful statement, but is this ground breaking…no.

    And looking at the hull forms produced, not one would be of any practical use, except for further research.

    And that is my point about have you designed a hull before. Since once one goes through the iterations of designing a hull, with a real world application, many of the “what if” questions so often posed by theoreticians that seem to have merit, have no real world applications. It becomes research for the sake of it, and a self fulfilling prophecy. Isolating one parameter to further one knowledge is to be applauded. But to understand this mechanism, one really requires to design a hull to fully appreciate why certain things actually occur because then it becomes obvious that isolating one constraint of a 3D shape, a hull, in reality makes no sense at all.

    I think Leo will be the first to admit he is not a naval architect. But he does come to the obvious conclusion far quicker than most:-


    I’ve been a naval architect for nearly 25 years…asymmetry has only ever been referred to in the transverse plane of hulls. To refer to asymmetry in the longitudinal plane, is ostensibly just academic speak, ie a datum for comparative research only. I may well be corrected by others, but I have never seen a hull/lines plan with asymmetry being referred to in the longitudinal plane...only transverse. Since a hull other than a canoe/kayak, is by its own definition not symmetrical (long.t)..it is a “ship shape” this is implicitly understood..so why refer to it as such?
     
  2. DCockey
    Joined: Oct 2009
    Posts: 5,023
    Likes: 512, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Thanks for the explanation and insight.
     
  3. DCockey
    Joined: Oct 2009
    Posts: 5,023
    Likes: 512, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    (Disclaimer, this question is theoretical and does not concern the design of real boats.)

    Leo or anyone else interested,

    How general is the mathematical phenomena that wave resistance given by analysis stays the same if the direction of motion is reversed? Leo mentioned it in conjunction with Mitchell's Thin Ship Theory. Obviously the attitude of the hull would have to be invarient to direction of motion. I have a vague recollection that it is more general, and applies at least to the more general case of a linearized free surface boundary condition applied at the nominal surface but with the kinematic boundary condition applied at the hull surface. I think it may also apply to a full potential flow analysis with a deforming free surface and the exact free surface boundary conditions. This would be consistent the Japanese experiments Leo mentioned.

    I've had a quick look at the sections in Marine Hydrodynamics by Newman on three-dimensional waves and wave resistance but haven't gone through it in detail. These are the sections prior to any linerization assumptions. The final result of resistance/drag as the integral of the amplitude squared multiplied by the cos cubed implies that drag would be the negative opposite when direction is reversed IF the amplitude is the same.
     
  4. NoEyeDeer
    Joined: Jun 2010
    Posts: 983
    Likes: 32, Points: 28, Legacy Rep: 300
    Location: Australia

    NoEyeDeer Senior Member

    This may be a case of different fields habitually using different terminology. Although I am not a qualified naval architect I have always been interested in boats, and have read just about everything I can get my hands on over the last several decades. Since my interest is primarily in small or smallish rowing and sailing craft those are mostly what I choose to read about.

    Usually, unless we are talking about exceptions like the old Hobie cats, such craft will have hulls that are transversely symmetrical. They may or may not be longitudinally symmetrical. However, when writing about them the general practice seems to be the opposite of what you have encountered as a professional NA. It seems to be assumed that the hulls will be transversely symmetrical (unless specifically noted as not) and the term "symmetrical" is usually applied to hulls that are longitudinally symmetrical. Hulls that are broader aft are often referred to as "wedge shaped" in the case of yachts and as "Swede form" in the case of kayaks. Hulls that are broader for'd are usually called "cod's head" or "fish form" respectively, but in all cases these hulls would be classified as not symmetrical.

    Anyway, regarding this question of the LCB, it may be more useful to phrase the question another way. Rather than trying to isolate the LCB it is probably more useful to ask something like: "In the case of studies (like the Delft series, etc) which claim that an LCB aft of midships is beneficial, what are the relevant factors which give such hulls lower resistance?"
     
  5. Ad Hoc
    Joined: Oct 2008
    Posts: 7,114
    Likes: 1,056, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    There is a very good reason for this.

    Academics rarely have any real hulls to use for their research, secondly keeping the hull symmetrical, simplifies the analysis, and helps to draw much easier conclusions. As I noted before, the symmetry is only prevalent, in the sense you’re referring to, in the academic realms. Academics rarely design hulls and so keep things simple, otherwise the list of variables is simply too much.
     
  6. NoEyeDeer
    Joined: Jun 2010
    Posts: 983
    Likes: 32, Points: 28, Legacy Rep: 300
    Location: Australia

    NoEyeDeer Senior Member

    I am aware of why academics simplify things. I'm reminded of what some scientist said: "Technically, all models are wrong, but some of them are useful."*

    I'm not talking about academics though. I'm talking about real design and construction of real small craft, done by people who may not or may not be trained NA's as such. In just about everything I have ever read on yacht and small craft design over the last forty years, the term "symmetric" would refer to a hull that was symmetrical longitudinally.

    Note that in practice there is a bit of leeway here. A hull described as such may not be precisely symmetrical longitudinally but only almost so.

    You are thinking of things from the perspective of commercial naval architecture. In that world, it would indeed be very rare to find a hull that was longitudinally symmterical. This is why you assume that such hulls only appear in academic studies.

    However, even in commercial naval architecture there are rare cases of real hulls that are longitudinally symmetrical. The old Sydney Harbour ferries are an example I'm familiar with, having spent several years sitting right on the bow of them when crossing the harbour (back in the days when it was possible and legal to sit right on the bow with your feet dangling outside :D ).

    In the case of smaller craft it is a lot more common to find hulls that are longitudinally symmetrical, or are very close to it and are called "symmetrical" anyway. There is no automatic assumption that hulls will not be longitudinally symmterical, but there is an automatic assumption that they will be laterally symmetrical. As I said, the general usage of the term "symmetrical" in the case of small craft seems to be the exact opposite of what you are used to.

    Note that I'm not arguing about who is right and who is wrong in their usage of the term. I'm just pointing out something I have actually observed.

    *ETA: looked up the quote out of curiosity. It's by George E. P. Box.
     
  7. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 151, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    Fore-aft symmetry for minimum wave resistance certainly applies to pressure distributions acting from above the free-surface. See:
    "Free-surface pressure distributions with minimum wave resistance" at
    http://www.cyberiad.net/library/pdf/tl01.pdf

    I have not seen any studies that systematically vary LCB. The excellent NPL experiments held LCB constant; other studies I have seen (e.g. Series 64 etc) vary the hull shape too much (e.g. transom area) to be able to see trends with respect to LCB only.

    If I was going to study the problem, I would start with something like a Wigley hull and add parallel middle body. After I understood what was happening in level trim, I would then include squat effects. The advantage of this approach being that many of the integrals can be done analytically.
    The disadvantage is that the hulls are not practical, but that wouldn't worry me. (Engineers and NAs were invented so mathematicians don't have to get their hands close to dangerous machinery :p)

    If I find something in the morass of Japanese studies I have I'll pass it on to you.

    Leo.
     
  8. NoEyeDeer
    Joined: Jun 2010
    Posts: 983
    Likes: 32, Points: 28, Legacy Rep: 300
    Location: Australia

    NoEyeDeer Senior Member

    Ok, question. If, theoretically, longitudinal symmetry is best for mimimum wave drag then why is it that, in practice, some boats have less wave drag at some speeds when they are not longitudinally symmetric?

    ETA: Or, to phrase it another way, what assumptions are being made in that model?
     
  9. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 254, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Very good point.
     
  10. Ad Hoc
    Joined: Oct 2008
    Posts: 7,114
    Likes: 1,056, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    Must be hard to wrap up the wife's present without scissors :p

    Does a knife and fork classify as "dangerous"...??? :eek:
     
  11. Ad Hoc
    Joined: Oct 2008
    Posts: 7,114
    Likes: 1,056, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    There is a paper "SUS A - The scope of the VWS Hard Chine Catamaran Hull Series '89" by Burkhard Muller-Graf. The shows the trends, but not enough for DCockey to understand perhaps.

    The paper that Daiquiri posted (can't remember where now)
    "Resitance Characteristics of Smei-displacement Mega Yacht Hull Forms" by Blount & McGrath, fig 5, does just that.

    Ive just checked my "Seris 60 - Methodical Experiments with Models of Single Screw Merchant Ships" By FH Todd.
    He devoted a whole chapter to this...if i get time i'll try and up load this.

    I'll see if i can scan in our tank test reports which shows this (without showing confidential stuff)....as this is called an LCG chase, when tank testing. Should always be done too.
     
  12. DCockey
    Joined: Oct 2009
    Posts: 5,023
    Likes: 512, Points: 113, Legacy Rep: 1485
    Location: Midcoast Maine

    DCockey Senior Member

    Comments suggesting I won't be able to understand something are tiresome and add nothing to the discussion.
     
  13. Leo Lazauskas
    Joined: Jan 2002
    Posts: 2,696
    Likes: 151, Points: 63, Legacy Rep: 2229
    Location: Adelaide, South Australia

    Leo Lazauskas Senior Member

    It could be because they are in a real seaway, or they are squatting, or they are fairly beamy so the thin-ship linear argument doesn't apply.

    Off-topic slightly...
    For catamarans, we have found that cambered hulls (another other type of asymmetry!) can reduce wave resistance, but only if the hulls are spaced at a non-optimal position. If the hulls are optimally-spaced, camber doesn't seem to reduce wave resistance further.
    All mathematical conclusions, of course. Nothing of use for practitioners :)
     
  14. Ad Hoc
    Joined: Oct 2008
    Posts: 7,114
    Likes: 1,056, Points: 113, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    No, not at all.

    It is clear you want a purely mathematical theoretical paper, not an empirical based one.
     

  15. daiquiri
    Joined: May 2004
    Posts: 5,373
    Likes: 254, Points: 93, Legacy Rep: 3380
    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    DCockey's post has forced me (damn...) to pull out some books from the shelf and check out what do prominent authors have to say about this issue. Not much info though. Some generic notes, like this one from "Introduction to Naval Architecture" by Tupper:
    "Longitudinal distribution of displacement
    Even when the main hull parameters have been fixed it is possible to vary the distribution of displacement along the ship length. This distribution can be characterized by the longitudinal position of the centre of buoyancy (LCB). For a given block coefficient the LCB position governs the fullness of the ends of the ship. As the LCB moves towards one end that end will become fuller and the other finer. There will be a position where the overall resistance will be minimized. This generally varies from just forward of amidships for slow ships to about 10 per cent of the length aft of amidships for fast ships. In considering the distribution of displacement along the length the curve of areas should be smooth. Sudden changes of curvature could denote regions wherewaves or eddies will be created."​

    So (considering the other papers seen before), the tendency of the LCB travel appears to be pretty much accepted, as it has been mentioned in more than one technical paper or book.

    I personally cannot but say, albeit with no realistic possibility to mathematically or experimentally prove my opinion, that it appears sound to get a change in resistance by (slightly) modifying volume distribution of the hull, without changing other parameters - until an optimum (in terms of minimum resistance) for a given speed is reached. Consider these pictures, taken from Chapter V of "Principles of Naval Architecture Resistance, Propulsion and Vibration, Volume II" (by Van Mannen, Van Oossanen):
    Wave profiles 2.gif Wave profiles.gif
    It shows a breakdown of the wave profile for two symmetric (both laterally and longitudinally) hulls at Fn=0.238.The first hull is a wedge-form, the second one is parabolic. As you can observe, though they are longitudinally symmetric the resulting wave systems are not. They show a marked increase in wave height towards the bow.

    Now, since hull-generated wave systems depend on Froude numbers, their shape will obviously change as Fn change. It follows that a hull should ideally continuously conform it's shape to these resulting wave systems, in order to get a minimum resistance for each Fn. After all, the Principle of Minimum Energy says that nature seeks it's positions of equilibrium where the total energy of a closed system (made of waves and hull in this case) is minimum. It can be interpreted as a hull and it's wave system shaped in such way to give wave heights and trim and squat such to minimize the potential energy of both waves and the hull). I repeat, the proof of this is beyond my mathematical expertize, so consider it just as my philosophical reflection on this issue, for the sake of discussion.

    One of the parameters which will change in this process will be, of course, the LCB - hence (imho^2) the observations from the previously cited papers.

    Remains the validity of all the previously expressed doubts regarding the possibility to effectively separate the LCB from the other parameters which might have a more important role in this process, as have been pointed out by Ad Hoc and Leo. Squat, trim and transom width and depth are the first few which come to my mind - and the first two will change the LCB without changing the geometric design of the hull

    At the end, I wouldn't dismiss the effects of viscosity either (friction) since it is predominant in low Fn regimes, where the literature mostly indicate that LCB should be set ahead of midship.

    Cheers!
     
Loading...
Similar Threads
  1. krawiec
    Replies:
    2
    Views:
    192
  2. dustman
    Replies:
    99
    Views:
    2,346
  3. Leo Ambtman
    Replies:
    24
    Views:
    1,634
  4. Bukmaster9
    Replies:
    32
    Views:
    1,808
  5. dustman
    Replies:
    70
    Views:
    2,889
  6. MoeZ
    Replies:
    0
    Views:
    863
  7. NoviceJoe
    Replies:
    20
    Views:
    3,118
  8. myszek
    Replies:
    5
    Views:
    1,019
  9. mitchgrunes
    Replies:
    30
    Views:
    3,476
  10. Alexanov
    Replies:
    12
    Views:
    1,352
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.