Reasonably universal hull resistance prediction formula?

Discussion in 'Hydrodynamics and Aerodynamics' started by laukejas, Dec 17, 2018.

  1. laukejas
    Joined: Feb 2012
    Posts: 720
    Likes: 16, Points: 18, Legacy Rep: 128
    Location: Lithuania

    laukejas Senior Member

    Hi,

    I am improving on a small commercial application that I made, used for calculating various hydrostatics of hulls designed within CAD software Solidworks. I thought that adding resistance predictions at various speeds would be a nice feature to have in my application. I am relatively new to naval engineering, and self-taught I find this topic a bit overwhelming.

    So far, I have found that there are dozens of different methods to predict resistance, and most of them are regression-based (relying on data of similar hulls), and therefore are rarely consistent with one another. In my case, I am trying to build a versatile software that would work with any hull (or multi hulls) of any shape, not restricted to specific ranges of tested hull data.

    The frictional resistance part seems relatively easy, I already implemented that part, basing calculations on KAPER method (resistanceF= 0.99525 x coefficientOfFriction x wettedSurface x speed^2), which, seems to me, should work with any kind of hull. The residual resistance, however, seems incomparably more complicated, and I didn't use KAPER there, because it only applies to a very narrow range of hulls, and requires lots of eye-balled corrections. I also investigated Delft, Taylor, Holtrop... But in each case, these methods apply only to a very limited range of hulls. Not good.

    Even if I did include every possible method in my application (which is an incredible amount of work), the results would be inconsistent where the methods overlap, and there would be gaps. Due to the limitations of the software that my program is based on, I cannot make a fluid simulation either (and I'm not THAT good at programming anyway). Therefore, I am searching for the most universal method out there (not necessarily the most accurate for a given case), but something that would work with any kind of hull - from a bathtub toy boat to an oil supertanker. Ambitious, I know, and maybe not entirely achievable, but I know that there are some applications that did this withing some degree of success in the past (like Michlet).

    Can anyone give any suggestions, please? Of all the numerous hull resistance prediction methods out there, which one is worth the time to implement in my application?
    Thank you for your time.
     
  2. TANSL
    Joined: Sep 2011
    Posts: 6,218
    Likes: 295, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    There is no resistance estimation procedure that works for any type of boat. A well-known method, applicable to many types of displacement boats, and that is very easy to program is the Holtrop-Mennen.
     

    Attached Files:

    Niclas Vestman likes this.
  3. laukejas
    Joined: Feb 2012
    Posts: 720
    Likes: 16, Points: 18, Legacy Rep: 128
    Location: Lithuania

    laukejas Senior Member

    TANSL, thank you, I implemented nearly every part of this method, except one - the Rf (friction resistance), because the formula is omitted in this document. I took that bit from here, quoted as:
    Frictional Resistance Rf= 1/2ρwater * SV^2 * X * Cf. I hope this is correct.
     
  4. TANSL
    Joined: Sep 2011
    Posts: 6,218
    Likes: 295, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Rf = 1/2*ρwater * V ^ 2 * S * Cf

    V = speed m/s
    S = wet surface m2
    Cf = 0.075 / ((Log(Rn) / Log(10) - 2) ^ 2)
    Rn is Reynolds number
     
  5. laukejas
    Joined: Feb 2012
    Posts: 720
    Likes: 16, Points: 18, Legacy Rep: 128
    Location: Lithuania

    laukejas Senior Member

    Thank you. Do you know the limitations of this Holtrop method, meaning what are the boundaries of the hull parameters beyond which the calculations will become inaccurate? I thought I should make a check in my software to warn the user if his hull is outside what Holtrop was meant for. But I have not been able to find, definitively, what these boundaries are.
     
  6. TANSL
    Joined: Sep 2011
    Posts: 6,218
    Likes: 295, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

  7. laukejas
    Joined: Feb 2012
    Posts: 720
    Likes: 16, Points: 18, Legacy Rep: 128
    Location: Lithuania

    laukejas Senior Member

    Damn... I didn't expect that these ranges will be so narrow. Take Cp for example. 0.55 is the minimum..? Half of the sailboats/dinghies I ever studied had Cp lower than that. It appears that the Holtrop method is nowhere as universal as I thought. I guess I'm back to square one, searching for a better method :(
     
  8. TANSL
    Joined: Sep 2011
    Posts: 6,218
    Likes: 295, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    It is clear that this method does not work, nobody had said, for sailboats / dinghies. Neither does it work, for example, for multihulls or gliders. As I said at the beginning, there is no valid method for the generality of hulls. There is no better or worse method than another, there is the right method for each type of floating object. Sorry.
     
  9. laukejas
    Joined: Feb 2012
    Posts: 720
    Likes: 16, Points: 18, Legacy Rep: 128
    Location: Lithuania

    laukejas Senior Member

    Well, then how can I avoid the inconsistency between different methods where their ranges overlap? And what if a certain hull has parameters that are in between the ranges of different methods, and as a result, does not meet the requirements of any of the method? As I said in my initial post, this is the reason why I want to avoid using multiple methods in my software... The other reason being an extreme amount of work needed to reliably implement multiple methods. I'd rather focus on one method, and make sure it works properly. I am aware that no method can cover absolutely any kind of hull shape, but I'm hoping there is one that covers at least 90% of the boats/ships out there... Meaning it wouldn't exclude something as typical as a sailboat, canoe, or a cargo ship. At least in displacement mode. Multihulls are a whole other matter, I understand, so I would call it "good enough" if I managed to implement the method that covers at least most of the monohulls.
     
  10. TANSL
    Joined: Sep 2011
    Posts: 6,218
    Likes: 295, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Check, if you have time and desire, the various naval CAD programs and you can prove that they all offer the user various methods for estimating power. Why do they do so?. I am sure that they do not do it to complicate the life of the designer, but because it is the most correct, and the only, thing to do.
    I am sure that none of the existing methods, some of which are defined as "suitable for Type XXX vessels", do not even get to be correct for 80% of vessels of type XXX. Asking for a method to cover 90% of all boats/ships is pure fiction. Do not insist on that path, which has no way out. In any case, the only thing we can offer are approximations, not the total solution. The designer, based on his experience, is the one who must adopt his final solution.
     
  11. laukejas
    Joined: Feb 2012
    Posts: 720
    Likes: 16, Points: 18, Legacy Rep: 128
    Location: Lithuania

    laukejas Senior Member

    I see... What about that little application, Michlet? It seems to work with pretty much any hull I've tried so far. Of course, I wouldn't know if the resistance predictions are accurate, as I don't have any real life data to compare. Do you happen to know, by any chance, how that application solves this problem? Does it use these methods we're talking about, or is it making a fluids simulation? I went through the documentation, but I'm still not sure.
     
  12. TANSL
    Joined: Sep 2011
    Posts: 6,218
    Likes: 295, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    I know of Michlet's existence but I do not know its calculation procedures or the type of boats for which it can apply. Apparently it is very useful in the multi hulls case, which makes it a very valuable tool, but I can not say anything else. Contact Leo Lazaukas , creator of the software.
     
  13. laukejas
    Joined: Feb 2012
    Posts: 720
    Likes: 16, Points: 18, Legacy Rep: 128
    Location: Lithuania

    laukejas Senior Member

    I will. Thank you for all your help, TANSL.

    And if anyone else has any suggestions to my original post, this topic is still open. :)
     

  14. Dolfiman
    Joined: Aug 2017
    Posts: 1,208
    Likes: 510, Points: 113
    Location: France

    Dolfiman Senior Member

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.