Tunnel Section Lost Buoyancy

Discussion in 'Stability' started by Abhilash Satheesan, Feb 15, 2018.

  1. Abhilash Satheesan
    Joined: Jan 2018
    Posts: 20
    Likes: 0, Points: 1
    Location: Rhode Island

    Abhilash Satheesan Junior Member

    Hi Everyone,

    I would like to share a concern with you all. I have attached two pictures with and without tunnel cut outs. Im working with the new model picture and now i want to include the lost buoyancy due to tunnel cut outs. I dont want to go back in rhino and model the tunnel section. Is there any way i could add lost buoyancy in maxsurf stability module directly? Please correct me if im wrong in my understanding.

    Thank you
     

    Attached Files:

  2. TANSL
    Joined: Sep 2011
    Posts: 5,699
    Likes: 175, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    You can add, in each load conditions, a negative weight of value and center of gravity equivalent to the volume lost with the tunnels.
     
    Abhilash Satheesan likes this.
  3. Abhilash Satheesan
    Joined: Jan 2018
    Posts: 20
    Likes: 0, Points: 1
    Location: Rhode Island

    Abhilash Satheesan Junior Member

    Thank You TANSL! But does it not affect my hydrostatics?
     
  4. TANSL
    Joined: Sep 2011
    Posts: 5,699
    Likes: 175, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Of course, the hydrostatics will not be those of the hull with tunnels.
    If you use the hydrostatic values of the hull without tunnels, the weights indicated above must be positive. The hull has more buoyancy than the real hull so you must compensate with some additional weights.
     
  5. Abhilash Satheesan
    Joined: Jan 2018
    Posts: 20
    Likes: 0, Points: 1
    Location: Rhode Island

    Abhilash Satheesan Junior Member

    That is right! Thanks
    So, can you also advise what is the best option to add that weight to get my hydros right?
     
  6. TANSL
    Joined: Sep 2011
    Posts: 5,699
    Likes: 175, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    You have to calculate the hydrostatics, and all the other calculations, with the hull without tunnels. Then, you calculate the volume (v) and the CoG of the tunnels and, in the weights table of each load condition, you place an additional weight of value = v * water density and its CoG at the tunnels CoG.
    If you do not know how to calculate the volume of the tunnels, send me the model with them and I'll help you do it.
     
    Last edited: Feb 16, 2018
  7. Abhilash Satheesan
    Joined: Jan 2018
    Posts: 20
    Likes: 0, Points: 1
    Location: Rhode Island

    Abhilash Satheesan Junior Member

    Thanks! But i have figured the total volume and CoG from RHINO MODEL. But i still have the concern about adding weight in each loading condition. How does it compensate for the buoyancy? Also, can in ignore tunnel section if its total volume enclosed is less than a certain percentage say, %0.5 or %1 of design WL displacement?
     
  8. TANSL
    Joined: Sep 2011
    Posts: 5,699
    Likes: 175, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Adding a weight is equivalent to decreasing the thrust, although it is true that sometimes a certain error can occur. For example, the flotation area, its second moment of inertia and, therefore, the metacentric radius, for several angles of list will not be the same with tunnels that without them.
    An error of up to 2% may be admissible in volume calculations. In fact, the appendices are often ignored (bow thruster tunnels, large rudders, domes, propellers, propeller shafts, ...). It is simply indicated that the calculations have been made without appendices. Another example, in metallic boats, the calculations are made without taking into account the thickness of the lining.
    In summary, you can forget the tunnels without problem.
     
  9. vv3k4i
    Joined: Mar 2008
    Posts: 40
    Likes: 2, Points: 18, Legacy Rep: 10
    Location: Gdansk

    vv3k4i Junior Member

    This is not the best idea to do any calculations this way. This proposition of adding negative weight sounds like weak workaround. Why don't you add a surfaces in Maxsurf, which will simulate the tunnels? And what is the reason you don't want to go back to Rhino any more, sine on one of the sreenshots you already have a rhino model?
     
  10. TANSL
    Joined: Sep 2011
    Posts: 5,699
    Likes: 175, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Probably I did not explain myself correctly in my first post because I did not interpret correctly what the OP needed. Let's forget the negative weights, if that's okay.
    It is totally normal to perform all the calculations of naval architecture without taking into account some (not all) appendices. Therefore, if the OP wishes, for whatever reason, not to take into account the tunnels, my advice is to forget them and that's it.
    @vv3k4i, how accurate do you get in your calculations? You know it?. From there we could start talking about serious things.
     
  11. Raffaele Frontera
    Joined: Feb 2018
    Posts: 31
    Likes: 1, Points: 8
    Location: Amsterdam, The Netherlands

    Raffaele Frontera Naval Architect

    Good evening gentlemen,
    I think that it is really important to know the purpose of the calculations. If It is a didactical calculations, then fully agree with TANSL that you can skip without problem. If we are in a professional level, then I don t agree anymore, because with all respect but skip the appendices was done 20 years ago when the ship design software were not user friendly like now. But being now so easy to use there is no reason to don t create the model accurate much that is possible. In particolar for inland waterway which play important rules like the new ADN, where the distance of the emergency exit from the waterline in damage stability is important. In that case you play on the cm, and having or not appendices like sea chest, bowthruster etc make a lot of difference to have the vessel complies..and the owner can never be happy if you tell to change a goosneck in wynel cup if he knows that the model was not accurate enough...
    Or unhappy because making more accurate the vessel doesn-t comply anymore but this is another story...

    Regards,

    Raffaele Frontera
     
    Last edited: Feb 17, 2018
  12. TANSL
    Joined: Sep 2011
    Posts: 5,699
    Likes: 175, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    When a project is made to obtain the permission to build the boat, not a project for academic purposes, it is completely normal not to take into account, for example, the thickness of the shell (in metal boats, not some PRF boats), or rudders, or the propellers and their axles or of the tunnels of the transverse bow propellers, or of those propellers, or of the dome for the sonar, or of ..... other diverse appendices.
    Have you made any calculation in your professional life taking into account the propellers ?.
    If you have adequate CAD tools you can take into account everything your patience allows but the normal practice is to forget all those details and I, frankly, would not say that those projects are not correct. As I have already asked on another occasion, do you know how accurately you are calculating in order to know if disregarding the tunnels of the OP ship is an "unacceptable error"? Do you know that the well-known method of Simpson for integration, in ships of polygonal forms, can give errors greater than 3 or 4% ?. Have you checked if your CAD package correctly calculates the wet surface ?, because it is very possible that you will be surprised to know how much it differs from its real value.
    If you want to be exquisitely accurate in your calculations, I praise you, but I am sure you are committing (you or your program) more inaccuracies than you think.
    Another reflection: you can calculate with all the biggest precision you like but then, do you prove that the real ship has been built with as much accuracy as your calculations?. What weight have you estimated and how much does the boat really weigh? I could ask you a lot more things but I think that is enough.
     
  13. Raffaele Frontera
    Joined: Feb 2018
    Posts: 31
    Likes: 1, Points: 8
    Location: Amsterdam, The Netherlands

    Raffaele Frontera Naval Architect

    Good evening Tansl,

    Yes, I did.

    Yes, In my case also up the 2% regarding the LCG...
    Depends on your software how impose the Simpson equations (calculating based from frame or waterline..), and in this case I agree with you because there are still some case where the hydrostatics are calculated with just first stopping at the first order in the formula of Simpson (like we are in the 1960) which can give the errors that you are saying..
    I agree and replied partially with above.

    Here I am sorry I don t agree, you make the model more accurate that you can in accordance with the reality, end of the story.

    Fully agree, for inland waterway for instance often is more important the experience that you have with that kind of vessels...the drawings not always match the reality , but again the model have to be close with the reality.

    As you prefer, for me it is a pleasure to discuss about stability with you and in general I like very much this forum.

    Thanks and regards,

    Raffaele Frontera
     
  14. TANSL
    Joined: Sep 2011
    Posts: 5,699
    Likes: 175, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Naturally I will believe everything you say but, truthfully, according to my experience, it is not worth as much accuracy as what you, apparently, apply to your calculations.
    I am surprised that you say, although I believe you, that you have been able to check the accuracy of your CAD program calculations because it is not easy to do it for the normal user.
    It is normal for the inspectors of the Administration of each country to check the main dimensions of a ship because it is understood that they may not be the same as the project. This is used, for example, to calculate the ship's tonnage, but it is not required that the initial project's drawings be modified.
    Every builder, boat or whatever, rigorous has to draw the plans "as built" because the initial project usually differ from the final result.
    I mean that each one of us decides to make his models as accurate as he thinks, but to say that they are exactly the same as reality, imo, is pure fantasy.
    I will also be happy to exchange opinions with you on any subject you want, if that is within my knowledge and experience.
    BR
     

  15. Raffaele Frontera
    Joined: Feb 2018
    Posts: 31
    Likes: 1, Points: 8
    Location: Amsterdam, The Netherlands

    Raffaele Frontera Naval Architect

    Good sunday Tanls,

    That's perhaps because I also work with loading computer software, where it is really important to match the results of the stability booklet. Anyway using my experience I hardly saw with ADN 2015 a model of vessel without bowthruster, sea chest like it was done in the past, where indeed this kind of details were completely skipped. Also the definition of the accomodation was often not modeled before, and now if you want to model that for existing vessels you can make damage or not the accomodation. If you don't make damage then you have to prove the strength of the accomodation, and not every class has the same opinion about that.. sometimes they asked me to comply with linear deformation, sometimes with plastic deformation.. Using the FEM method for a stability issue seems a bit over kill but anyway...

    Fully agree, a normal user cannot see easily so in depth, but in my case I am not only a user but a developer as well. But for instance, the accurancy also comes from the mathematical background of the CAD, often for CAD like Rhino you cannot reach accurate results, not because the designer is not capable, but just because modelling with NURBS surface is not the right way.

    Modelling area of the vessel like the tunnel or bow, can be extremely complicated even impossible sometimes to get in the end what in reality will be build.

    It is strange, because the computational ship design should follow the old school to draw a lines plan, so a correct way is to work with curve Methods, such as 3D NURBS.

    Fully agree, imagine for calculations of tonnage for inland waterway the procedure of Ginevra of the '60 period is still alive, which is absurd, it is still based on the first order of the Simpson formula, but apart from that, the method to calculate the waterline for different trim is really old fashioned, giving a difference of accurancy also of 3 or 4 %.

    I agree, also about this there is a lot of to say.

    :)
     
Loading...
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.