maxsurf hullspeed resistance analysis - why is a gap between transom and stern wave?

Discussion in 'Software' started by Ngenjuh, Jul 27, 2017.

  1. Ngenjuh
    Joined: Jul 2017
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    Ngenjuh Junior Member

    capture-20170727-075927.png
    btw i am student. i am using maxsurf hullspeed to solving my resistance analysis, at above is pict when i take boat in speed 10 Knots... i use Savitsky Planning method, the result is : capture-20170727-204149.png

    but, when i take it in speed 20 Knots, why in wave visualization there is a gap between transom and stern wave like this pict below : ?????? is the method i used is not compatible with my design? or is any error with my design? have you ever like this? thank you
    capture-20170727-203858.png
     
  2. vkstratis
    Joined: Feb 2015
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    vkstratis Marine Designer

    The following are from the software's manual. I doubt your hull fits into the method's boundary conditions.

    The slender body method assumes the vessel to be slender (i.e. high length:beam or slenderness ratios). Ideally the slenderness ratio should be as high as possible, but in practice, good results can be obtained for slenderness ratios of around 5.0 to 6.0. If the vessel’s Froude number is reduced, the minimum slenderness ratio to which the method is applicable also reduces. The maximum Froude number for which sensible results can be obtained depends on the vessel’s slenderness ratio. It has been found that, for very slender vessels (slenderness ratios greater than 7.0), the slender body method may give sensible results for Froude numbers as high as 1.0. The slender body method may be applied equally well to round bilge and chine hull forms. Hulls with transom sterns are dealt with by automatically adding a “virtual appendage”. This method is described in detail in Couser et al (1996).
     
  3. Ngenjuh
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    Ngenjuh Junior Member

    thank you for your answer, i will check and refit it.
     
  4. vkstratis
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    vkstratis Marine Designer

    I don't know your design details but I assume is a semi-planing or planing hull. In such a case stick to the Savitsky method. MaxSurf Hullspeed uses the Savitsky's "short-form" method. If you use Rhino for your modeling work you can use planingRhino plugin (planingRhino http://www.food4rhino.com/app/planingrhino) that features the "long-form" method.
     
  5. Ngenjuh
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    Ngenjuh Junior Member

    yes it is.. Thanks for your answer, let me make a conclusion from it : for my long-form hull it would be better to use slender body method in maxsurf, or using planingRhino plugin in Rhino, because of Savitskyplaning is less compatible for my long-form hull. Is it right?...btw to solving the resistance of this hull, which one is technicaly better do you think, using maxsurf with SB method or using rhino? I never use rhino, if you think rhino is better for this case, is it means that rhino is more precise/ accurately/wins in this resistance analysis case/purpose?
     
    Last edited: Jul 31, 2017
  6. vkstratis
    Joined: Feb 2015
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    vkstratis Marine Designer

    Rhino3D has nothing to do with resistance prediction or its accuracy. It is a 3D cad modeler that can be extended with plugins such as planingRhino, Orca3D or others and perform different tasks such as resistance prediction. If you model in Rhino3D you can use one of these plugins to make calculations directly in Rhino, rather than exporting your hull to other software such as MaxSurf.

    In any case though, all methods have limitations and certain boundary limits. You have to check those before attempting to get any results. Otherwise output will probably be not very realistic. Savitsky method is semi-empirical and thus there are certain limits but covers most planing hulls (at planing speeds). Short-form or long-form Savitsky in irrelevant to your hull's L/B ratio. These are just two variations of the same resistance prediction method. MaxSurf's Savitsky method is the "short-form" variation.

    Regarding Slender-Body method, as stated in MaxSurf manual, will work for slender vessels (SR 5-6) for speeds up to Fr=1.

    I suggest to take a look at pg. 53 of Hull Speed manual. There is a nice presentation of all maxsurf included methods and their limits.
     

  7. Ngenjuh
    Joined: Jul 2017
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    Ngenjuh Junior Member

    okay, thank you so much for all your answer, these are very usefull answer for me. Terima kasih.
     
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